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Matt-itude
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Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:13 Go to next message
I spose you have all heard the saying, "Engines need backpressure"

im almost positive that this is a myth. i cannot see a reason why backpressure would help increase torque when it is just another force for the engine to overcome before it produces any power.

i guess i would like to know why putting an exhuast "too big" on your car would reduce torque in the low rpm range.

thanks for all your help.
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Kyosho
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:24 Go to previous messageGo to next message
Matt-itude wrote on Wed, 09 February 2005 19:13

I spose you have all heard the saying, "Engines need backpressure"

im almost positive that this is a myth. i cannot see a reason why backpressure would help increase torque when it is just another force for the engine to overcome before it produces any power.

i guess i would like to know why putting an exhuast "too big" on your car would reduce torque in the low rpm range.

thanks for all your help.
You are 100% correct...
It is a myth...

But the reason you loose torque down low on a HUGE exhaust...
As the exhaust diameter is made larger, more gas can fill it? (You following)
So your engine pumps out say 3L per minute at 2500RPM...
Your STANDARD exhaust holds say 3L of gas... So it takes 1 minute to remove the gas at 2500RPM...

Now if we increase the size of the exhaust, the exhaust now can take 6L of exhaust gas... So to get our exhaust fumes out, it now takes 2 minutes...

With this extra amount of time, the exhaust gas cools, as it cools, it becomes more dense, and takes up less room...
Now with it taking up less room, other gas can fill around it, so it slows down...

Now another factor, is that, as the exhaust cools, air travels slower...

So you have two factors slowing the gas down (Taking even LONGER then 2 minutes to get out)
Now as it slows down and becomes more dense, it actually creates MORE back pressure...

So the engine can't breath as easily at the low RPMS as well as it does at high RPMS (because the gas is moving quicker to start with and doesn't cool as much at higher RPMS)
When the engine can't breath as well, you have to do 2 things...
a) Retard ign timing
b) lean the mixture...
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Matt-itude
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:29 Go to previous messageGo to next message
thank you so much, that was the answer i was looking for.

it does make sense when the gas expands it cools down and simple thermodynamics will tell u that once the pressure has decreased, so will the flow rate.

now would the engine run its best with no exhaust pipe at all? or am i missing something?

thank you for ur quick reply

[Updated on: Wed, 09 February 2005 08:31]

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draven
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:37 Go to previous messageGo to next message
there's a reason F1 cars run tuned length extrators
and it's not to reduce the noise Smile
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Matt-itude
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:43 Go to previous messageGo to next message
can anyone give a proper explination of this?

im guessing its something to do with the pressure nodes and making them the correct lenght would increase flow?
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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:47 Go to previous messageGo to next message
mm, i follow that, but i always think of it in terms of cam/s..

i bet the first time you heard that "engine needs backpressure" it was from a V8 guy who had a really big cam...

the overlap of a cam works by allowing the intake and exhaust valves to be open at the same time. ideally, there is a low pressure pulse arriving at the exhaust valve (produced by length tuning of the manifold) as it is nearly closed, and this helps to, a) suck more waste gas from the cylinder, and b) lower the cylinder pressure when the intake valve opens.

the problem is that a given overlap works best in a fairly small rpm range. too high rpm means that the overlap is not large enough, and there is not enough time for the exhaust to assist cylinder filling. too low rpm means that the intake and exhaust valves are open at the same time for too long, such that there is little benefit from the exhaust, and also allows the fresh intake charge to be sucked out through the exhaust valve...

ideally, you want the exhaust negative pulse to hit the exhaust valve bwfore the intake valve opens, and a positive pulse to hit the exhaust valve _JUST_ as the fresh intake charge reaches the exhaust valve, as this maximises the benefit of the low exhaust pressure pulse, and minimises intake charge loss.

now.. if you have a motor that revs to 5000rpm.. and a cam with 300deg duration, you have a serious problem. a cam like that is _meant+ to work at sayyy.. >5000 or >6000rpm. if you never reach that rpm, then you will always lose fresh charge out thru the exhaust, and thus you lose the benefit of the exhaust pressure depression.

now, if you have a too large cam and you are losing fresh intake mixture out the exhaust, what do you do? of course you restrict the exhaust, such that you _INCREASE_ the pressure in the exhaust manifold, such that it is above the residual cylinder pressure, so that when the exhaust valve is open, the fresh charge will not escape as much past the exhaust valve.

this will be effective at increasing power and economy, as you are keeping more of the air/fuel where it should be..


so basically the long and the short of it is, if you have a free flowing exhaust, you should choose your cam carefully so that it matches your desired rpm range of operation....

OR, if you have chosen too large a cam for your desired rpm range, fit a restrictive exhaust so that you keep more mixture in the cylinder.

the former will give better power and torque overall, and the latter is a band-aid (elastoplast?) patch for a poor choice of camshaft Smile

so it is not really a myth.. it is based on experimental fact, but is is also based on poor engine design Very Happy




and to answer your next question:

the reason you have an exhaust (apart from not burning your engine bay) is so that you can have a negative pressure pulse reflected by the exhaust manifold so that it reaches the exhaust valve at the correct time to suck more exhaust out of the cylinder.

this occurs because when the positive pressure pulse (fromt he exhaust valve opening and letting exhaust into the manifold) reaches a junction, a negative pressure pulse is reflected bacwards.

by choosing the correct configuration (ie 4-2-1, 4-1, 6-2-1 6-1 etc) AND the correct pipe lenghts from primaries, secondaries and collectors, you can increase the VE significantly (around 10%?) over an untuned exhaust.. the other aspect is that as well as 'good' rpom regions, there will also be 'bad' rpm regions where it performs worse (but not much worse over untuned).
you can also tune for 1st reflection, or 2nd reflection etc...

read "the scientific design of intake and exhaust systems" by P.Smith.

Cya, Stewart

[Updated on: Wed, 09 February 2005 08:52]

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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:49 Go to previous messageGo to next message
Matt-itude wrote on Wed, 09 February 2005 19:13

i guess i would like to know why putting an exhuast "too big" on your car would reduce torque in the low rpm range.



it's actually the more sensible idea that

"putting a "too big" CAM on your car would reduce torque in the low rpm range."

which is remarkably sensible and obvious Wink

Cya, Stewart
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Matt-itude
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:51 Go to previous messageGo to next message
wow thanks, wouldn't it be nice if we could have vvt tuned for every rpm before redline?

or a way better idea, would be to keep the rpm at a steady point at max power, and just use gear ratio's to increase speed but still keep at the max power rpm.

worth looking into really.
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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 08:57 Go to previous messageGo to next message
Matt-itude wrote on Wed, 09 February 2005 19:51

wow thanks, wouldn't it be nice if we could have vvt tuned for every rpm before redline?

or a way better idea, would be to keep the rpm at a steady point at max power, and just use gear ratio's to increase speed but still keep at the max power rpm.

worth looking into really.


or continuously variable VVTi-L.

or use a motor that is tuned for a very specific rpm to turn sayyyy.... an electric generator, and then that power is transferred to eletric motors that drive the wheels and batteries to store power so you don't need to run the petrol motor as much....

and how about we call it a Prius Wink

there have been continuously variable gearboxes made.. i remember seeing a Honda one at some stage (and someone else will remember what it actually was), btu the limiting factor will be the amount of power such a variable gearbox can take..

or you could use an auto gearbox, and use an electronically controlled torque convertor to keep the engine in a specific rpm range perhaps?
Cya, Stewart
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Fr3aK
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:11 Go to previous messageGo to next message
Matt-itude wrote on Wed, 09 February 2005 19:29


Now, would the engine run its best with no exhaust pipe at all? or am i missing something?

Good question.
Look at a 4 second 1/4 mile dragster. They only have the extractor primaries.

Look at an old Spitfire WWII fighter jet. Only have the primaries!

It's a pity us mere mortals have road rules.
Today i took off my exhaust at the primaries. Started it. Got scared. It sounded like a farking BEAST! (too scared to drive it anywhere tho...)

Long story short: Yes. But big pipes make this difference negligible for road cars.
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Farkurnell
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:30 Go to previous messageGo to next message
So what is too big an exhaust pipe in your collective opinions?
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Kyosho
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:34 Go to previous messageGo to next message
Matt-itude wrote on Wed, 09 February 2005 19:29

thank you so much, that was the answer i was looking for.

it does make sense when the gas expands it cools down and simple thermodynamics will tell u that once the pressure has decreased, so will the flow rate.

now would the engine run its best with no exhaust pipe at all? or am i missing something?

thank you for ur quick reply

If you unbolt your exhaust and take headers off...
Richen it up sligthly and nock arond 5 degree of timing in across the board (If N/A) and you will have maximum power approximately... (Unless the car is rich already)

Even tuned length extractors create a small flow hindrance...

F1 cars run Tuned length headers so they can make the most of their power, whilst still meeting a sound level...

Can you imagine the sound of one of those engines would make at full ball with no exhaust? Well I can... Helps that my uncle used to race a Formula 5000...
Just dropping the Honey Combs off increased the exhaust noise around 5-8dBA...
Take the Loud exhaust off... It got even louder...
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Kyosho
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:36 Go to previous messageGo to next message
Farkurnell wrote on Wed, 09 February 2005 20:30

So what is too big an exhaust pipe in your collective opinions?


Depends on the engine
How it is built
How it is being driven

For a 2.0L, I'd go a MAXIMUM of a 2.5" really...
Prob run around a 2.25"
On my 2.4L, I'll put on a 2.5" system straight through if this one shits itself...
And I'm N/A

But turbo 2.0L cars can run a 3" exhaust from turbo back with NO problems
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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:36 Go to previous messageGo to next message
Fr3aK wrote on Wed, 09 February 2005 20:11

Matt-itude wrote on Wed, 09 February 2005 19:29


Now, would the engine run its best with no exhaust pipe at all? or am i missing something?

Good question.
Look at a 4 second 1/4 mile dragster. They only have the extractor primaries.

Look at an old Spitfire WWII fighter jet. Only have the primaries!
Long story short: Yes. But big pipes make this difference negligible for road cars.


actually.. no.. not necessarily..

the extractor primaries you speak of are TUNED to a specific length, and being only a primary will work in a very specific rpm range.. by adding other pulses to an extractor system, you widen the effective rpm range in which they help.

if you had similarly tuned pipes, your car would work in that small rpm range (maybe 500-1000rpm?), but outside that, it would only be benefiting from the reduced backpressure (Wink), whereas when you add the secondaries and collector, you are amplifying the exhaust assistance Very Happy

for the dragster, it's a packaging thing.. even if they had 2-1 exhaust pairs, it would be a packaging nightmare!! considering the volume of exhaust and the small rpm range the work in (and the physical thrust they produce), short tuned length primaries, pointing upwards, are the best solution.

same for the WW2 planes, the engines are bloody large enough already, and the pipes may not even be long enough to be tuned as such, but again, the packaging makes it difficult to have a more complicated exhaust.

for road cars, it is only the noise and toxicity of exhaust that requires the use of an exhaust system (extractor-back)..

i think the main thing with large exhausts (past the end of the tuned section) is that they sound terrible (imho).. AFTEr the tuned section, going larger may not decrease power as much as you would think, but will increase noise...

Cya, Stewart
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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:40 Go to previous messageGo to next message
Kyosho wrote on Wed, 09 February 2005 20:36

Farkurnell wrote on Wed, 09 February 2005 20:30

So what is too big an exhaust pipe in your collective opinions?


Depends on the engine
How it is built
How it is being driven

For a 2.0L, I'd go a MAXIMUM of a 2.5" really...
Prob run around a 2.25"
On my 2.4L, I'll put on a 2.5" system straight through if this one shits itself...
And I'm N/A

But turbo 2.0L cars can run a 3" exhaust from turbo back with NO problems


heh heh, i think 2" on a 1.3L NA is about perfect. for turbo, larger and larger is ok since the turbo itself is a very effective pulsation dampener, resulting in a smoother exhaust note that is less offensive.

as you go up in size (in ") the area increase quickly too..

2" = 20.2 cmsq
3" = 45.6 = 2.25 x 2"
4" = 81.1 = 4 x 2"
5" = 126.7 = 6.3 x 2"



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Farkurnell
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:45 Go to previous messageGo to next message
My car is a 1.8L with 3 inches from the turbo back and seems to go ok.
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draven
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:47 Go to previous messageGo to next message
it's really easy for an na exhaust to sound shit, veyr hard to get a good note (unless you get it done properly)

turbo engines all sound pretty much then same.. a bit flat and lifeless Razz
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 09:49 Go to previous messageGo to next message
all I can say is that which I have gained from experience...

my example is my old ke70 which had a 4age....

when I changed the motor from 4k to 4age I kept the 4k exhaust (1.75in I think) for about 10mths before it fell of one day...at which stage I hooked up a 2.25in with hotdog and free flow muffler....

with the 4k exhaust the car did feel more torquey down low and when I put the larger exhaust on it definately gained more top end power....

now the 4age had stock cams and it did seem to affected by the larger exhaust....

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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 10:12 Go to previous messageGo to next message
draven wrote on Wed, 09 February 2005 17:47


turbo engines all sound pretty much then same.. a bit flat and lifeless Razz

'Cept a 1G Razz
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rokusan
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 10:26 Go to previous messageGo to next message
so how long and what diameter would you recommend for the turbo manifold for a two litre for flat out trailer sailer and no noise restrictions?
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 22:28 Go to previous messageGo to next message
Just to throw another bit of info into the mix.
yamaha use a vlave (forget the name of it) in the exhaust on yzf1000's so that it is restricted down low and opens up fully up high...

this is so effective that we ran my mates one in the workshop without a muffler and ad idle and low revs it was actually quite.. but rev it up and it'd open up and was noisy as all fuck.

this is help create a nicer smooth curve with decent power down low and a screaming top end.
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willwal98
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 22:30 Go to previous messageGo to next message
Quote:


or continuously variable VVTi-L.

or use a motor that is tuned for a very specific rpm to turn sayyyy.... an electric generator, and then that power is transferred to eletric motors that drive the wheels and batteries to store power so you don't need to run the petrol motor as much....

and how about we call it a Prius

there have been continuously variable gearboxes made.. i remember seeing a Honda one at some stage (and someone else will remember what it actually was), btu the limiting factor will be the amount of power such a variable gearbox can take..

or you could use an auto gearbox, and use an electronically controlled torque convertor to keep the engine in a specific rpm range perhaps?


This is not a new idea. Those huge HUGE mining trucks use a large diesel (I think) motor to turn a generator and they have an electric motor for each wheel. This way they get the best fuel economy, and can you imagine making a drive train (geabox, diff and tailshaft) for one of those things.

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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 22:41 Go to previous messageGo to next message
Yamaha call it a EXUP
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Mr Revhead
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Re: Exhaust Myth... Fact or Fiction? Wed, 09 February 2005 23:55 Go to previous messageGo to next message
ppl get confused between back pressure and the pulse from the exhaust...

when ppl talk about backpressure they are talking about the actual gas itself... whereas extractors use the PULSE or shockwave from the exhaust port.... two different things.
having the gases build up pressure is very bad.
using the pules to increase performance is a good thing. but as stated above can only be tuned for a specific rev range.

as for variable exhausts several companys use those, ferrari, and toyota also use a crude system, although thats more of a muffler bypass under full throttle.
on ae101 20vs they have a flp in the exhaust that opens up the passenger side muffler under full throttle. under crusie or idle loads it stays shut.
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 00:58 Go to previous messageGo to next message
justcallmefrank wrote on Wed, 09 February 2005 20:42

draven wrote on Wed, 09 February 2005 17:47


turbo engines all sound pretty much then same.. a bit flat and lifeless Razz

'Cept a 1G Razz



my thoughts exactly frank, oh and a 2JZGTE with 60mm external gate and screamer, that also sounds pretty lively.

Read: Tezuka's JZX81 chaser...
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 02:16 Go to previous messageGo to next message
I know this may sound silly, but back pressure IS required on most Normally aspirated cars.. not all. Less back pressure is required on multi valve engines than push rod V8's

Having formally worked in the exhaust trade, I have done a little study into it, see dyno charts and spoken to people that design headers for off the shelf purchase to V8 supercar.

The reason it is required is acually to assist in drawing more air from the next cylinder after it fires, if there is no back pressure, there is no vaccum behind each pulse of exhaust flow, therefore not sucking it through, slowing down the exhaust flow and decreasing the efficency of your valves. Why does it vary from pushrod to multivalve.. simple, more valves = more exit points for the gasses.

What I have seen, is that Not all n/a cars require it. A good example is the majority of Honda VTEC engines, for whatever reason, they work much better without back pressure.
Another example of the oppisate is the genIII LS1 motor, the ideal exhaust setup was actually the one that nobody purchased.. everyone wanted the big bore twin 2 1/2 inch.. rather than the system that worked best.. twin 2 1/4" into single 3" rear.

Hope I havent been too vauge.. But with exhaust.. there is no deadfast rule, it depends on head design, aspiration, mods, and about 100 other things.

As for turbo's.. well thats a different concept all together... and yes, you do want some back pressure.. but a whole 1 to 3 pound (which is next to nothing)and the end of the dump has been what most have found to ideal for a torqey street setup, so no not every turbo needs that big mutha 3 inch system. Usually a top dump pipe design will make more usable power than that big bore exhaust system you just spent a fortune on.

Before you deciede on what exhaust you want.. go and talk to a pro.. most exhaust places will have someone there who has some expierence with hi end exhausts.. they know much better than you or me!!!
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 02:32 Go to previous messageGo to next message
Quote:

for the dragster, it's a packaging thing..


another major reason top fuelers use this design is that it creates several hundred kg's of downforce when they fire out of the hole. with the pipes pointing up, a gigantic volume of (burning) gas being forced out at several thousand km/h forces the car into the track, a bit like a backwards rocket.

Quote:

there have been continuously variable gearboxes made.. i remember seeing a Honda one at some stage (and someone else will remember what it actually was)


dunno about honda, but i know Audi is currently producing CVT gearboxes. they're optional on the A4's and A6's i think.

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buck naked
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 03:47 Go to previous messageGo to next message
I had a similar result to chrisss when I did my exhaust.

I got defected by the police for noise, so I had to change it before I took it over the pits.

The system was a 2.25" with a straight through / cannon exhaust on a 1st gen 3SGE. First I put in an 18" resonator, but it was still too loud, so I added in a Lukey Turbo muffler.

I was really impressed at first, my tourqe was much better and the car became more comfortable to drive around the city. I took it out into the country and ran some tests (0-100 type tests) and my car's time had changed from 8.92 to 10.2 - consistantly.

I don't know what effect the resonator had on the results, as I never drove it before the muffler was on. Is it a contributing factor or is the extra back presssure created solely by the muffler instead of the straight through?
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 05:22 Go to previous messageGo to next message
OPPY wrote on Thu, 10 February 2005 13:16

I know this may sound silly, but back pressure IS required on most Normally aspirated cars.. not all.




please stop assisting the myth Wink
backpressure is NOT required IF the engine has been designed properly.

Quote:

The reason it is required is acually to assist in drawing more air from the next cylinder after it fires, if there is no back pressure, there is no vaccum behind each pulse of exhaust flow, therefore not sucking it through, slowing down the exhaust flow and decreasing the efficency of your valves. Why does it vary from pushrod to multivalve.. simple, more valves = more exit points for the gasses.


i don't follow you. as i tried to explain earlier.... when the exhaust valve opens, a POSITIVE pressure pulse travels down the primary at the speed of sound (higher in a hot gas), and when it reaches the primary/secondary junction, a negative pressure pulse is reflected. it is this reflected negative pressure pulse that assists in cylinder scavenging.

with back pressure, we are talking about a higher pressure in the REST of the exhaust, as a result of a restrictive system.

more valves = more exit points for the gas? how does this affect things when the ports merge in the head, and then you have a single exhaust primary per cylinder?


for anyone who seriously wants to know what is goin on, go and borrow the book "the scientific design of intake and exhaust systems" by Phillip Smith.. it is held in the UNSW library, and is also available elsewhere. that book explains the concepts and some mathematics behind the design of manifolds...

Quote:

What I have seen, is that Not all n/a cars require it. A good example is the majority of Honda VTEC engines, for whatever reason, they work much better without back pressure.
Another example of the oppisate is the genIII LS1 motor, the ideal exhaust setup was actually the one that nobody purchased.. everyone wanted the big bore twin 2 1/2 inch.. rather than the system that worked best.. twin 2...

Less back pressure is required on multi valve engines than push rod V8's


no shit....

the honda motor is designed properly with an efficient head, and SENSIBLE cam choice for the motors rpm capability.

with the V8... too big a cam for the rpm capability of the motor requires backpressure to keep the cylinder filled properly at lower (ie less than 6000) rpm.

i can't stress this highly enough. ANY TIME YOU INCREASE THE BACK PRESSURE WITHIN AN EXHAUST, YOU WILL DECREASE THE AMOUNT OF GAS ABLE TO FLOW = POWER.
YOU CANNOT MAKE MORE GAS FLOW THROUGH A MORE RESTRICTIVE SYSTEM BECAUSE YOU ARE DECREASING THE POTENTIAL DIFFERENCE BETWEEN THE MOTOR AND THE EXHAUST, RESULTING IN A LOWER DRIVING FORCE FOR THE EXPULSION OF EXHAUST GAS. THE ONLY REASON BACKPRESSURE WORKS IS BECAUSE THE CAMS ARE NOT SUITABLE.

the bike is a perfect example. cams are designed for high rpm. at low rpm, you restrict the exhaust to reduce the harmful effect of the large valve overlap at lower rpm. at higher rpm, when the overlap is working effectively, you remove the restriction.

read the book i have now mentioned twice, and tell me that is incorrect.

Cya, Stewart


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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 05:25 Go to previous messageGo to next message
oh, and as for cars that run better with more backpressure..
these wouldn't be cars running factory engine management and MAP sensors would they?
by reducing backpressure you alter the cylinder filling characteristics, and as such you should retune the fuelling to suit..

you know how you change the jetting of a bike carb when you put a better flowing pipe on? like that Razz
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Sam_Q
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 07:22 Go to previous messageGo to next message

I think this should be put in the FAQ section so much good info here and will help end this stupid myth.

as breifly explained earlier the yamaha valve in the exhuast is not for back pressure its to setup an earlier reflection pulse for the valve overlap period. This is effectivly creating a second resonance so the exhaust pulse is sent back lower in the rev range to stop all the air going straight through because of the high overlap. Not true? the try this is a car with normal overlap and see how far u get.

I think people who say things is support of back-pressure are just people who don't understand what harmonic resonance is.
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 07:45 Go to previous messageGo to next message
Quote:

Handling the exhaust chores is a 4-into-1 system controlled by Yamaha's Exhaust Ultimate Powervalve (EXUP) that precisely matches the exhaust backpressure with engine RPM to create maximum power.


http://www.centurionyamaha.co.za/ssfz1.htm

Quote:

EXUP system
The engine is equipped with Yamaha’s patented EXUP system to further increase the torque. EXUP is a system that operates a valve in the exhaust collector in relation to the engine rpm-range. For optimal exhaust efficiency you need a wider exhaust diameter at higher rpm as compared to lower rpm and that is exactly what the EXUP system does: it reduces the diameter at lower rpm

Variable air intake system
A similar optimization as the EXUP is used on the intake side, where a valve system regulates the air intake diameter of the airbox just before the air filter. This keeps the airflow and air intake speed always at an optimum level, for the best throttle response




http://ymedc.introweb.nl/en/archive/street/mt01_04 sep.asp#b1
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Sam_Q
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 07:53 Go to previous messageGo to next message
corolla man, your second quote is right, vauge but still right your first one however is showing the common misconception. I can so this so easly because I have read a report from the head engeneer of yamaha when asked about this valve and its function. I think the guy who engeered this would be a more reliable source that the website of a motorbike shop/distributer
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 07:57 Go to previous messageGo to next message
http://ymedc.introweb.nl/en/archive/classics/FZR10 00_exup.shtml

http://ymedc.introweb.nl/en/archive/classics/images/FZR1000/89/exupdraw.gif
http://ymedc.introweb.nl/en/archive/classics/images/FZR1000/89/egdrawcol.jpg
Quote:

EXUP (Exhaust Ultimate Powervalve)
One of the most significant features on the new FZR1000 is the EXUP exhaust control system. Another Yamaha invention; in principle it is much like the YPVS system which improves 2-stroke engine performance by changing exhaust tuning in response to changes in engine mm.

As more horsepower is designed into production engines, the smooth powerband so desirable for the street is replaced by the "peaky", lumpy power curve of the racing engine. Especially pronounced with high-performance, 4-into- 1 exhausts, this results in a fiat spot at about two-thirds of peak-torque mm and a rough idle.

Technically speaking, when the exhaust valve opens, residual combustion pressure in the cylinder rushes in to the exhaust pipe, creating a primary "positive" pressure wave moving towards to collector (muffler). Upon reaching the collector, it expands, sending a primary "negative" wave back toward the cylinder. The header continues to reverberate, alternating positive and negative. primary, secondary and tertiary.

Header pipe length is set so that the primary "negative" wave reaches the cylinder at valve overlap (the brief instant when both intake and exhaust valves are slightly open). This negative or "suction" wave does two things. lt pulls residual exhaust gas out of the cylinder, and it starts the flow of fresh fuel/air mixture through the intake valve.

Unfortunately, because these positive and negative pressure waves move through the header pipes at uniform speed regardless of engine rpm, at lower rpm the primary "negative" wave arrives too soon (before overlap), and in its place a primary "positive" wave arrives at valve overlap. This positive wave forces exhaust gasses back into the cylinder, diluting the charge, and it blows back through the carburettor, delaying intake and causing double carburetion (carburetion in the wrong direction). This is what causes the dreaded race-engine flat spot.


Prior to EXUP, the only way to smooth out power delivery was to sacrifice performance (less overlap, use of less resonant exhaust pipes, etc.).
Think of EXUP as an exhaust throttle. By placing a rotary valve driven by a microcomputer-controlled servomotor between the header pipes and the collector, Yamaha engineers were able to control the pressure waves. The Computer senses engine speed from the ignition. By choosing this valve progressively as rpm decreases, the harmful positive pressure wave is prevented from reaching the cylinder at valve overlap. Double carburetion is eliminated, torque rises back to a normal level and driveability is restored.EXUP also reduces exhaust emissions at idle by producing back pressure that reduces boss of fresh charge through the exhaust. The idle is also smoother and steadier. And a new muffler has enlarged capacity to efficiently quiet the increased power.

Equipped with EXUP, the engine produces about 10% more top-end power than an engine without EXUP. Most importantly, driveability and throttle control are greatly improved in that critical upper portion of the power band. There is an astonishing 30 to 40% increase in bow- and mid-range torque and smoother acceleration. The idle is much smoother: 30 to 50% less fluctuation at idle mm. The exhaust note at idle is quieter. And, hydrocarbon emissions are reduced.

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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 07:59 Go to previous messageGo to next message
Sam_Q wrote on Thu, 10 February 2005 18:53

corolla man, your second quote is right, vauge but still right your first one however is showing the common misconception. I can so this so easly because I have read a report from the head engeneer of yamaha when asked about this valve and its function. I think the guy who engeered this would be a more reliable source that the website of a motorbike shop/distributer


yeah yeah i know Wink i just grabbed the first two quotes i read... but it does effectively do that in a way... or can be thought of as increasing the exhaust restriction in order to improve torque..
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YelloRolla
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 08:06 Go to previous messageGo to next message
Wow Stew you have got too much time on your hands to go typing all of that.

There are a couple of points that have not been accurately conveyed IMO.

1) Top fuelers - short primaries because they are supercharged - the supercharger blows a fresh charge across the the top of the piston on overlap ensuring that there is no torque-robbing exhaust gas to contaminate the intake charge (length is as Stewart has pointed out - a top fueler is at 7500 to 8000rpm the entire time that the throttle is open).

2) Back pressure robs power - especially when manifold pressure is low - exhaust gas often flows backwards thru the open intake valve and into the intake manifold. The effect that this has on the BMEP of the engine is considerable.

3) Short pipes lead to the "atmospheric pressure" (love it Razz ) reaching the exhuast valve very quickly. Having vacuum in the intake and atmospheric at the exhaust while both valves are open = reversion = no good for power. Keeping the exhaust pulse intact and maximising the benefit of the negative pulse is what this is all about.

4) DON'T belive anything that you read here - Do what Old Corollas has suggested - get the book and read it.

Stewart - I think that I have caught your typing bug arrgghhh.

DON'T EVER BELIEVE THAT BACKPRESSURE IS HELPING TORQUE, or you are missing the point entirely.

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Mr Revhead
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 09:54 Go to previous messageGo to next message
Rolling Eyes

ppl get confused between back pressure and the pulse from the exhaust...

when ppl talk about backpressure they are talking about the actual gas itself... whereas extractors use the PULSE or shockwave from the exhaust port.... two different things.
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Big T
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 10:57 Go to previous messageGo to next message
me and a mate had this exact same conversation a couple weeks back. he asked me why is it necessary for some NA cars to have backpressure? my understanding is that ALL backpressure is restrictive of power and if at all possible, it's eliminated. its simply a by-product in lazy engineering, not a necessity for an engine
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 11:57 Go to previous messageGo to next message
So would the consensus of opinion be that the equal length 2" primaries -> 4-1 merge collector -> 2"1/2 straight thru muffler be better suited then the standard 20V extractors to our 560kg Clubbies running 20V ST on standard ECU's?
Aftermarket ECU's and big cams are planned later. We haven't been particularly concerned with torque problems due to the low weight of the cars and short gearing (200ish kph at 8000RPM in 5th).

Cheers,
Julian.

http://img68.exs.cx/img68/2039/FMS02-3.th.jpg

Thanks to ImageShack for Free Image Hosting
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Hunty
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 February 2005 12:09 Go to previous messageGo to next message
oldcorollas has totally owned everyone one that posted their opinion in this thread lol
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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Fri, 11 February 2005 05:24 Go to previous messageGo to next message
2" may be a little large but will be good for top end... how logn are the primaries??

i calculated mine from the graph on Bill Sherwoods page... and to my surprise, it came out almost exact in real life (once i got intake sorted Wink )


heh heh, i don't think so.. YelloRolla has gotten the detail much better Wink i was trying to keep in in more laymans terms, but if ppl want to learn more about intake and exhaust harmonics.. read that book.. the graph on bills page seems to be quite good as a prediction tool, as long as you choose sensible pipe diameters (i have 37-43-50mm, but i think maybe 34 or 35mm primary would have been better for my 1300)

Cya, Stewart

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Byatch
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Re: Exhaust Myth... Fact or Fiction? Sat, 12 February 2005 05:57 Go to previous messageGo to next message
I'm supprised no one has mentioned the clear parallels between exhaust and airoplane wings (or maybe someone has, and i missed it).
I always thought the principle was the same. an increase in gas speed => a decrease in pressure. (This is how planes fly, soccer balls curve in air, those cool balls stay in the jet stream when you put the vacuum in reverse).
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3S-GE_Man
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Re: Exhaust Myth... Fact or Fiction? Sat, 12 February 2005 06:52 Go to previous messageGo to next message
ok i didn't go and read all of the thread but in the 21st century performance book...he goes... once past the TUNED LENGTH of the the exhuast (header/manifold/extractors) the there should be no back presure what so ever for maximum performance and fuel econimany! and i will believe that no matter what cause u see it every where in performance appilcations...but not top fuel dragsters cause they just wnat the air in and the exhaust out as fast as scientificy posible, lol!

Nezza Cool
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EMD-AE86
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Re: Exhaust Myth... Fact or Fiction? Sat, 12 February 2005 08:25 Go to previous messageGo to next message
my exhaust i got from ET (exhaust technology) it is 2" mandrel bent. My engine in my celica is a 3SFE and it sounds awesome, really punchy, fairly loud but i think it is legal, it has a cannon too from autobarn. It has phat burbles too, not loud backfire just when i slow down there are crackles goin' off.
It wasnt too bad in price either for the cat back without the cannon was $380 and the cannon was $185.
Soo nice, sounds punchy like a little rex.
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RWDboy
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Re: Exhaust Myth... Fact or Fiction? Sat, 12 February 2005 13:25 Go to previous messageGo to next message
The only reason you want 'backpressure' in the exhaust (ie some level of restriction of flow) is for emissions purposes on low-tech old engines with bad combustion chamber shape and port design.

I think someone already mentioned this, but dragster primaries are also tuned to a certain length as even an open pipe will 'reflect' pressure waves (think woodwind and brass instruments) although the length of them suggests that they are tuned for a very small RPM range.
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Dunno
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Re: Exhaust Myth... Fact or Fiction? Sun, 13 February 2005 13:08 Go to previous messageGo to next message
ha newb reading the forum (ie me lol)

so if i want to get an exhaust for my 7afe motor what would be a good size to get??
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jkvsnn
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Re: Exhaust Myth... Fact or Fiction? Wed, 16 February 2005 22:58 Go to previous messageGo to next message
Oldcorollas, Length of the primaries is 763mm +/- 1mm across the four of them. This is measured from the head to the entry into the merge collector. We used the 3D solid modelling software that we use for structural drafting at work to design the headers, it was a real struggle when working with pipes so large to get them all the same length and also have them all neatly aligned when they come out the side of the chassis.
My friend did a really cool animated gif that overlays a photo of the finished headers and an image from the model and you can see how close we got to the design. I'd post it up but its 5mb Sad

Cheers,
Julian.

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Hunty
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Re: Exhaust Myth... Fact or Fiction? Thu, 17 February 2005 02:01 Go to previous messageGo to next message
sounds cool, if you want i cant host it for you

just email it to me calum AT teegee.com.au


also re exhaust for 7afe, dude read this thread again or just go for a 2" or 2.25" system from the exhaust primaries back, dont worry about the cat, unless you wanna gut it and have a striaght pipe put on instead, a hi flow one wont make much difference on a 1.8litre N/A 'FE motor

and get a cannon muffler and hang it out to the side dorifto spec....fully sik
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jkvsnn
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Re: Exhaust Myth... Fact or Fiction? Thu, 17 February 2005 02:13 Go to previous messageGo to next message
Thanks Hunty, I'll email it to you tomorrow

Cheers,
Julian
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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 March 2005 08:25 Go to previous messageGo to next message
jkvsnn wrote on Thu, 17 February 2005 09:58

Oldcorollas, Length of the primaries is 763mm +/- 1mm across the four of them. This is measured from the head to the entry into the merge collector. We used the 3D solid modelling software that we use for structural drafting at work to design the headers, it was a real struggle when working with pipes so large to get them all the same length and also have them all neatly aligned when they come out the side of the chassis.
My friend did a really cool animated gif that overlays a photo of the finished headers and an image from the model and you can see how close we got to the design. I'd post it up but its 5mb Sad

Cheers,
Julian.




bugger, missed this post...

those 4-1's look and sound like they have been designed very well Smile not sure if they are better than the stock extractors, but the length of the pipes suggest they will be good across a reasonable rev range, not just up high (esp compared to the 'normal' 4-1's which are only 1 or 2 feet long) (edit: also funny to see 4-1 "tuned" extractors that don't even follow firing order Wink )

have you had a chance to see how they go? would be very interested to hear what you think of the resonance and where the sound 'smooths out' etc etc..

i'd be very interested to see the gif too.. i can host at uni for a while also.
Cya, Stewart

[Updated on: Thu, 10 March 2005 08:26]

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indigoid
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 March 2005 09:22 Go to previous messageGo to next message
open headers are definitely the way to go.

i drove my ke26 (mildly warmed up 3K bigport motor) around for 2 weeks with basically open exhaust (it broke clean through just after the factory 2-1 merge) before getting it fixed. not only was it loud as poo, but it went like stink. I'm still not sure how I avoided Mr Plod's attention Very Happy

would like to make my own headers someday!
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Re: Exhaust Myth... Fact or Fiction? Thu, 10 March 2005 23:07 Go to previous messageGo to next message
oldcorollas wrote on Thu, 10 March 2005 19:25


would be very interested to hear what you think of the resonance and where the sound 'smooths out' etc etc..




what is happening when the sound flattens out? this is very audible on my motor at about 3000rpm, which is where the "powerband" seems to start. i assumed it was more to do with the overlap being most effective past that point. there are 3 or more audible changes in exhaust note timbre as far as i can tell.
i have 4-2-1 pacemakers.

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oldcorollas
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Re: Exhaust Myth... Fact or Fiction? Fri, 11 March 2005 00:34 Go to previous messageGo to next message
greenta22 wrote on Fri, 11 March 2005 10:07

oldcorollas wrote on Thu, 10 March 2005 19:25


would be very interested to hear what you think of the resonance and where the sound 'smooths out' etc etc..




what is happening when the sound flattens out? this is very audible on my motor at about 3000rpm, which is where the "powerband" seems to start. i assumed it was more to do with the overlap being most effective past that point. there are 3 or more audible changes in exhaust note timbre as far as i can tell.
i have 4-2-1 pacemakers.




this is what happens with mine at 5500rpm when the powerband starts (also 4-2-1 extractors.. home made, tuned length, made for 5500rpm)
what i think is happening is that the nodes of resonance are moving around, and when it goes 'smoother' that is where the standing waves are at the right point to help the exhaust out of the motor.

you WILL get multiple points where this happens, since oyu will get primary, secondary and tertiary resonant frequencies for given lengths.

it is also a bit to do with overlap, but my cam is advertised as 3000-7000, but it's more like 4500-8000. before i put the EFI on i was not getting the smoothing outof sound, or the high rpm powerband, so you are definitely right, but it is also to do with intake design.... well, it's a complex system Wink.

i have done various changes of heads, exhaust anmd intakes and each step gives different audible results, and you can sort of tell what does and doesn't work.. ie too much reversion, resonance in wrong spot etc..

what rpm do you notice these changes? it is possible that you could use those rpm, and the lengths of your primaries and secondaries, to work out what the exhaust gas 'speed of sound' is at different rpm, and from this you can work out how to make them better!
Cya, Stewart

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79Rona
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Re: Exhaust Myth... Fact or Fiction? Fri, 11 March 2005 00:57 Go to previous messageGo to next message
wow, what a interesting read....

saying that, i run a 2" all the way with a muffler at the rear...
too quiet for my liking, but ohwell,
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mick
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Re: Exhaust Myth... Fact or Fiction? Fri, 11 March 2005 10:10 Go to previous messageGo to next message
I'd think I'd only go 2inch with my car
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towe_001
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Re: Exhaust Myth... Fact or Fiction? Fri, 11 March 2005 12:23 Go to previous messageGo to next message
oldcorollas wrote on Wed, 09 February 2005 19:47


<cut and move>read "the scientific design of intake and exhaust systems" by P.Smith.

Cya, Stewart
<.....>
by choosing the correct configuration (ie 4-2-1, 4-1, 6-2-1 6-1 etc) AND the correct pipe lenghts from primaries, secondaries and collectors, you can increase the VE significantly (around 10%?) over an untuned exhaust.. the other aspect is that as well as 'good' rpom regions, there will also be 'bad' rpm regions where it performs worse (but not much worse over untuned).
you can also tune for 1st reflection, or 2nd reflection etc...




I see you've been reading the book.
Its a bit old but its still a good read even if it does get a little too technical for my simple brain Smile
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Toobs
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Re: Exhaust Myth... Fact or Fiction? Sat, 12 March 2005 14:54 Go to previous messageGo to next message
This page is an interesting read for those wanting to know more about the low pressure pulse created by a properly tuned exhaust and or manifold:
http://sunflower.singnet.com.sg/~adriant/rc-exhaus t.html
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river
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Re: Exhaust Myth... Fact or Fiction? Sat, 12 March 2005 23:33 Go to previous messageGo to next message
Hi,

Good topic and interesting info.

In reference to aircraft, specifically the WW2 fighters... these engines were supercharged and they wanted the exhaust out asap and with the air being forced in (via the supercharger) the smallest possible exhaust pipes were the most effective. Also in these aircraft and especially being wartime, there was little concern for noise and, as in all aircraft design, weight was an issue and large exhaust pipes just added weight.

However, for night flying operations there were large baffles fitted to the exhausts to dampen the flames and also to hide the red-glow from the exhaust stacks. Nothing like a big red-glowing and flaming exhaust on your plane to show the enemy where you were.

In the post-war period where the big radial piston engines reigned supreme, the exhausts were altered to provide considerable additional thrust to enhance the planes performance.

seeyuzz
river
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towe_001
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Re: Exhaust Myth... Fact or Fiction? Mon, 14 March 2005 01:36 Go to previous messageGo to next message
river wrote on Sun, 13 March 2005 10:33

Hi,
In reference to aircraft, specifically the WW2 fighters... these engines were supercharged and they wanted the exhaust out asap and with the air being forced in (via the supercharger)

Not all. The P-38 Lightening was turbo.
Quote:

In the post-war period where the big radial piston engines reigned supreme, the exhausts were altered to provide considerable additional thrust to enhance the planes performance.

seeyuzz
river

Even dueing WW2 many of the american pilots that saw ground attacks preferred driving the Thunderbolt over a Mustang.
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notorius
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Re: Exhaust Myth... Fact or Fiction? Mon, 14 March 2005 04:41 Go to previous messageGo to previous message
ok, i seriously cbf reading over the whole thread, so this may have already been said

some ppl are impling the engine would work beter if u took off the exhuast, i jacked my car up, with exhaust on, put my foot flat to the floor, my N/A astron 2 went off the clock, so wheels were spinning fast enough for the car to say over 180 kph, took the compleat exhaust off and it wouldnt go past 140 kph, thats my 2 cents, cya
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