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Registered:
August 2002
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N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 08:30
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I was just wondering if there is any benefit in using an exhaust system diameter that gradually gets smaller as it moves towards the tip. Theory being that the gas cools as it moves along and thus creates less velocity towards the end of the system than at the start when it was really warm, requiring a larger diameter to move the expanded gas.
I was thinking extractors-> 3inch -> 2.75inch -> 2.5inch
Just a thought, any ideas?
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Location:
Brisbane
Registered:
June 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 09:43
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Also factor in the effect of turbulence...
unless it could be down with one pipe instead of 3 chopped pieces then you could be slowing down the air flow hence defeating the purpose...
But in theory a decreasing pipe diametre would boost exhaust gas exit speed.. perhaps?
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 10:03
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| Quote: |
unless it could be down with one pipe instead of 3 chopped pieces
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wtf?!?!
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Location:
Brisbane
Registered:
June 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 10:06
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** Done with one pipe...
having over / under joins in the pipe work would slow / disrupt air flow 
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 10:08
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that's what a lot of performance cars are now using
Big hp cars (350+) that are for go, not show, are using a 4" dump pipe (or the biggest available), down to a 2.5" system after the cat (with the cat shifted back if flow is impeded).
on a small n/a there's not a huge point, as there isn't that much exhaust gas anyhoo
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 10:14
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well it wont be a small n/a application 270-300hp @ flywheel
270hp is the more realistic figure
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 10:18
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similar to the tuning pipes of 2 strokes?? they have a continually decreasing pipe (but i think that's for exhaust pulse reflection tho???)
another thing to consider is that as it cools down and becomes more dense, the friction on the smaller pipe wall may also increase.
dunno.... afaia there is not much point having an exhaust after the tuned section. everything else just increases restriction.
true that having a really big pip will mean the exhaust is just dawdling along, but how big do you have to go before this happens??
give it a go and see what happens????
Cya, Stewart
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Registered:
August 2002
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 10:35
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ahh K, i'm from a different school of thought...
i reckon that if you need backpressure, then you have too much exhaust scavenging, and the exhaust after the extractors is not the problem...
ideally you want a high degree of scavenging, but when the intake valve has just opened, and just before the fresh charge is about to come out the exhaust valve, you want a positive pressure pulse to hit the exhaust valve to sort of 'plug it up' and stop the fresh mix from escaping. ie pulse tuning, or reflection tuning...
rather than having an exhaust with backpressure to keep the mixture in the cylinder, you could cut down on the valve overlap... or put a resictor valve in the exhaust to create backpressure at lower rpms where the overlap is the biggest problem..... anyway i could babble on for hours 
Cya, Stewart
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 11:09
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Stewart - lay it on me, what is your best suggestion for the system i should use?
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 11:39
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I'd probably go
good extractors -> 3" -> high flow cat -> 3"-2.5" -> 2.5" -> muffler
extractors are probably the most important. and 3" might be overkill, I dont really know much about n/a exhausts
if that's too noisey, you can add another muffler (not a resonator), which makes it a 3 stage system.
expensive, but good
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 11:50
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Yeah, dont worry about the extractor design, leaving that up to the experts.
And yes, quietness (read: NO DRONE) would be nice.
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 12:28
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if you're pushing 300 horsies, drone won't be a large issue.
it'll scream
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 12:30
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Ive got nothing against scream 
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 14:21
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| BlackSupra wrote on Thu, 14 August 2003 21:09 |
Stewart - lay it on me, what is your best suggestion for the system i should use?
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ummm, i'll give it a go
what is your:
cam timing (and rpm range it's designed for)
exhaust port size or area (at manifold flange),
what rpm range you want max torque in,
what rpm you will be going up to,
cylinder size and number of cylinders,
what rpm will you be sitting at on the highway and around town? (ie 110-120 and 60-80)
do you have to have a cat? do you have much underfloor space for mufflers?
for my motor:
275/285, 65deg overlap (4000-7000)
24x27mm (6.5cm^2)
4000-7000
8000
330cc x 4
4500 on highway, 4000-4500 around town.
my extractors are 4-2-1, 37mm, 43mm, 50mm (OD's) lengths are 38cm, 38cm, 75cm, then into first resonator (4"x12"). this is designed for pulse tuning between 4500-6500, ie peak at 5500rpm.
Then about 1-1.5m of 50mm pipe to second resonator (taking that buzzy 4cyl noise out), then 50mm mandrel bent over diff housing to a 6x9x15" reverseflow muffler.
my torque peak is at 4600rpm (just above cruising speed) and tails off due to intake restriction. exhuast is much quieter than intake, and starts to resonate a lot at 5300rpm. resonance tails off about 7000rpm.
lemme know your details then i'll get back to you
Cya, Stewart
[Updated on: Thu, 14 August 2003 14:24]
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Thu, 14 August 2003 14:22
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(quote deleted)
oops..
it's not like i really know what i'm talking about  i just have my theories.....
the extractors are the most important part as they tune the motor. the rest of the exhaust is there to appease neighbours and police. generally, i'd suggest to run the same diameter as the last collector, with 1 or 2 resonators (good noise abatement vs flow if they are designed right) and then the largest chamber muffler you can fit. a reverse flow in a size or two larger than the pipe will give quieter and smoother sound than a straight thru muffler, but will sacrifice a little flow. if your pipe is 2.5" and you have a reverse flow muffler or turbo muffler with 3" guts, then that will still flow well compared to the 2.5" pipe, and be much quieter than a straight thru.
an exhaust is only as good as it's greatest restriction, and really the biggest restriction will be the cat. have ya seen inside one? it's an array of 0.8-1mm suare holes about 10-12cm long, and must be horrible for fast flowing gases to go thru....
if you have to run one, then the larger the cross section, and the smoother the in/out pipes are, the better...
you don't have to spend a fortune on an exhaust system to get it to be quiet and sound ok, but you might want to try a few different ones till you are happy...
Cya, Stewart
[Updated on: Thu, 14 August 2003 14:32]
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Fri, 15 August 2003 01:51
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Haha, thats alot of theory i haven't thought about at this stage.
Basically it is going to be a staged approach. Getting the engine in and running stock with the exhaust, then modifying it later on when i have some more $$$. But i just wanted to do the exhaust right the first time.
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Location:
Brisbane
Registered:
November 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Fri, 15 August 2003 13:21
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Basically if you set up your engine correctly (cam timing, valve overlaps, etc) and the right extractors, the trick after that is to have the least restrictive exhaust as possible (ideally no exhaust, but that's impractical for the street).
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Sun, 17 August 2003 14:21
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| BlackSupra wrote on Fri, 15 August 2003 11:51 |
Haha, thats alot of theory i haven't thought about at this stage.
Basically it is going to be a staged approach. Getting the engine in and running stock with the exhaust, then modifying it later on when i have some more $$$. But i just wanted to do the exhaust right the first time.
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oki cool.. easiest way to do it
what sort of CFM will your motor be ingesting?? (since exhaust will be proportional to ingoing air) you can compare that with other motors and their recommended exhaust sizes..
i think that about 2" on a 1.3L is a good size....
hmm, 1.3L -> 7000 -> 2"
gives about 3L -> 5000rpm -> 2.56" (assuming they have about the same volumetric efficiency, and simple math between sizes/rpms)
if you were to go to 7000rpm, then 3" might be good...
i'd be guessing that somewhere's between 2.5 and 3" would be ideal... so what are the specs of the grunter motor?? ie size, rpms etc??
Cya, Stewart
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Mon, 18 August 2003 02:47
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3L, longer duration cams (280ish), ported head paying special attention to the flow through the valve seat and exhaust port with general smoothing out, adjustable cam gears, at least a fuel computer and a custom intake. Probably want to take it out to 6000-6500rpm dpeending on cam setup.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Mon, 18 August 2003 03:38
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| BlackSupra wrote on Mon, 18 August 2003 12:47 |
3L, longer duration cams (280ish), ported head paying special attention to the flow through the valve seat and exhaust port with general smoothing out, adjustable cam gears, at least a fuel computer and a custom intake. Probably want to take it out to 6000-6500rpm dpeending on cam setup.
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sounds like a nice plan
i would hazard a guess that you'd want to go closer to 3" or at least 2.75" diameter, all the way from extractors back. you may need to experiment with different muffler/resonator setups to find one that sounds to your liking..... give a few exhaust shops a call to see what they'd recommend, and don't let anyone tell you that you need backpressure to work properly
as was said before, it's the extractors that tune the motor, and the exhaust is there to quieten things down and kep the gases away from the passengers.....
if you ar ein ydney (or can call from work ) give Australias Best Muffler Service a call (canterbury road, sydney) a call, as they seem to give prety good advice.
Cya, Stewart
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Location:
Lost in the K hole
Registered:
May 2002
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Location:
Fl
Registered:
February 2003
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Tue, 19 August 2003 02:06
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not exactly.... the collector diameter and length is usually sized for the reflection of the exhaust pulse (or sound wave) rather than flow, and will often seem a little large. i would wager that a 3" inlet/outlet cat would be more restrictive than a 2.5" pipe... i can take a pic of the cat material tomorrow if ya like awful stuff...
Ed, would you like to share your formulas for those calculations?? i'm very interested to see what would be recommended for my motor.. see how far off i am
Cya, Stewart
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Registered:
August 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Tue, 19 August 2003 08:34
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Ed - does this not say that your collecter is 2.85inch an not 2.5?
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Location:
Lost in the K hole
Registered:
May 2002
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Re: N/A exhaust diameter and decreasing diameters vs flow.
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Tue, 19 August 2003 11:13
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yes, it does say that the collector should be 2.85"
but the collector is not the final pipe, its the 'plenum' so to speak of the exhaust system, and as you know, your plenum is far bigger than any piece of your intake pipe...
so, a 2.85" collector, should happily feed into a 2.5" pipe to the back of the car...
stewart - i used a nice little website to calcuate that graph. lots of formulas. lots of java, it was great - pity i cant find it...
ill keep looking
cheers
ed
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