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Location:
Melbourne
Registered:
November 2003
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Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:14
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Hi guys, just splitting this from the 4A F / G head debate to see if I can actualy get an answer on low compression head shapes.
As Bill Sherwood as said, a flatter top on a piston is better for performance engines. As such, a lower angle head is needed on pentroof designes to raise compression while avoiding doming the piston head.
My question is, does the actual angle of the top of the combusion chamber matter for flame propegation and flamefronts etc?
I ask this as I am wondering if a wide angle head would be any worse for a turbocharged engine, where you don't need to run high compression so can have a wide angle head and flat pistons.
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Location:
Brisbane / Gold Coast
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:21
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How does the engine 'know' if it's boosted or not?
They don't. 
The main problem is getting enough chamber volume without killing the squish areas.
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Location:
Melbourne
Registered:
November 2003
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Location:
Brisbane / Gold Coast
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:43
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Squish areas make the right sort of turbulence and promote complete combustion.
When I say the engine doesn't 'know' if it's boosted or not, I mean apart from compression ratios you do much the same stuff with a boosted engine as you do with an N/A one.
Mate of mine used to build Sierra turbo engine for the Group A series in Aus. When they went to restricted boost, he ended up building an engine with up around 12:1 compression, big cams, etc.
So what I'm saying is that the combustion chamber shape should try to be the same for both types of engines. It doesn't matter if there's lots of boost or not.
At the moment, the optimum shape seems to be a four-valve, shallow-angle, pentroof chamber with good squish areas.
Naturally depending on the engine type you have to make compromises on that.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:44
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it's all about the shape of the cavity at the time that ignition occurs.. take away piston and roof and just think about that remaining space...
aivui, in the squish area the gases get a high rate of compression just as the piston reaches TDC, ie if squish clearance is 1mm, then in the last 2mm of piston movement, the squish area gets compressed 3:1... this results in these gases being at _higher_ pressure than the rest of the combustion chamber, and as a result, sort of spray out into the rest of the chamber, causing much mixing.
This high rate of compression will also cause a large temperature rise in the gas, but since there is so little gas in contact with such a large surface area of piston and roof, this temp rise is dissipated quickly.
have to remember that ignition occurred a while ago relative to the squish (sayy 20 deg BTDC), the the flame front is already propogating away from the spark plug. the 'squished gas' is squirted toward the flame front, and causes tumbling and turbulence and mixing of burning and unburnt gas... resulting in a much smoother burn...
or have i got it arse-backwards? 
check out the magazine articles at the bottom of this page
http://www.theoldone.com/articles/
Bill, i'm interested to hear what you think of Larry's 'soft heads'.. i know he was talking 'static' compressions of 20:1, and dynamic will be a lot lower with the cams they run, but it's interesting nonetheless...
Cya, Stewart
edit: maybe i'll just let Bill answer next time
[Updated on: Wed, 27 October 2004 02:46]
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Location:
Brisbane / Gold Coast
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:49
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Wot Stuart said as well.
Just reading the Soft Head thingy now.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:50
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in a boosted motor, do you allow a larger squish clearance due to the higher cylinder pressures? or is it still the 'normal' amount (around 1mm)??...
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Location:
Brisbane / Gold Coast
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:56
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| oldcorollas wrote on Wed, 27 October 2004 12:50 |
in a boosted motor, do you allow a larger squish clearance due to the higher cylinder pressures? or is it still the 'normal' amount (around 1mm)??...
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Can't see why it'd be any different.
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:57
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Ahh, thanks guys. I think I understand a bit better now.
So for high end performance engines, a close angle head is just better for pentroof designs? What about if you were to leave your engine more mild for road use? Would a wide angle head effect things then?
How do you compress a turbocharged engine so high?! Don't you run into serious issues with detonation and things like that? Or was that 12:1 engine not running much boost?
I hear the SR20DET is prone to detonation, could this be beacuse it has a nice narrow angle head and good squish areas?
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 02:58
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neither do i since relative pressure rise will be similar, but absolute pressure will be higher.. but since it's in close proximity surfaces so the heat goes away, but but but.....
"now if i was a piece of air, what would i want to do" 
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Location:
Brisbane / Gold Coast
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 03:06
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| Quote: |
So for high end performance engines, a close angle head is just better for pentroof designs? What about if you were to leave your engine more mild for road use? Would a wide angle head effect things then?
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No different for a race engine or a road engine. The main difference there is the size of the cams, lengths of the inlet & exhaust, etc. A wider spread of torque for the road engine, etc.
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How do you compress a turbocharged engine so high?! Don't you run into serious issues with detonation and things like that? Or was that 12:1 engine not running much boost?
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With very careful intercooling and spark timing control. Also, the more revs you run (up to a point) the less likely you are to get detonation.
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I hear the SR20DET is prone to detonation, could this be beacuse it has a nice narrow angle head and good squish areas?
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Don't know!
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 03:39
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Don't road engines, at least road turbocharged engined usualy have much lower compression than race engines? Bill, you yourself said that to get more power than 200hp (I think it was) out of a 4A-GE the cheaper option is to lower the compression and attach a turbocharger.
Do you still have to match the angle of the piston to the angle of the head, even when running a lower compression?
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 03:49
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yes they do usually have lower compression, but thats a lot to do with 'bullet-proofing' a production motor against poor fuel, no servicing etc..
what Bill is saying is that you follow the same design principles for ANY motor, no matter what the induction is.
a good NA combustion chamber can be a good turbo combustion chamber, but depending on boost levels, you might want to reduce the compression a bit, BUT NOT the design of the chamber.
the static compression is affected by many things.. cams will reduce dynamic compression. turbo cars, as well as having boost, run smaller duration and so dynamic compression will be higher.
BUT, higher static compression in NA motors can result in high peak cylinder pressures (ie the pressure spikes), whereas in a lower static CR boosted motor, you may have a lower peak pressure, but higher average pressure = more torque.
or something like that?
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Location:
Brisbane / Gold Coast
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 04:00
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Wot Stuart said.
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 04:20
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Hrm ok. Sorry for all my (probably silly) questions, but I'm just trying to get this totally clear.
Okay, so the reason you can't run a wide angle head and a flat piston is because this will ruin your squish areas? Of course, assuming that the lowered compression from having a wide head and flat piston are not a problem (ie, to help guard your turbocharged engine against detonation).
This is sort of a continuation of my search for understanding why SR20s use rockers. I figured out that it's to have a close angle head, now I'm just trying to understand why they did that. I think I do now what with close angle heads being better.
Assuming I have this right in my head, this also clears up why SR20s are prone to detonation: 'cuz they are good.  Or as good as a Nissan Engine can be anyway. 
Thanks!
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Toymods Vice President
Location:
Sydney
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 04:28
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The SR20 uses rocker arms to be able to run a wider valve angle while still keeping the head small. It's all about packaging for the SR.
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 06:29
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| Joshstix wrote on Wed, 27 October 2004 14:28 |
The SR20 uses rocker arms to be able to run a wider valve angle while still keeping the head small. It's all about packaging for the SR.
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I think you'll find that it makes a closer valve angle. By the looks of things, they needed to do this so they could run nice big cam gears with that big 'ol chain they have.
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I supported Toymods
Location:
Melbourne Australia
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Wed, 27 October 2004 20:34
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this is kind on topic...
squish areas a great thing but how would you go about creating it? or is this a non existant aspect. for instance on your site Bill, you show the pictures of the 20V silvertop and blacktop heads, the silvertop having better squish areas. Is it possible to replicate similar squish areas in the black top, or any head that doesnt have/too little squish areas?
Thanks
Huy
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Location:
Brisbane
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Thu, 28 October 2004 01:49
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It's possible but it involves changing the shape of the combustion chamber by adding new material, ie not easy to do!
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I supported Toymods
Location:
Melbourne Australia
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Thu, 28 October 2004 11:24
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yeah thought so, plus there might be structural issues in trying to add new material.
twas just curious...
other thing, with pistons, dome, dished, flat and any other i missed, what would be the various usages of them. i can see that dished can be used to assist in the squishing/quenching aspect but that would be best if there are squish areas right?
what about dished topped pistons, would these be the choice in engines whereby there are little or no squish area? and what abt dimples?
oh and in FI cars where the CR has been lowered, will squish areas still play a major role in the combustion process as from what i have read, 2-1mm is ideal for good usage of squish areas.
Thanks in advance for your reply
Huy
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Location:
Canberra, ACT
Registered:
September 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Thu, 28 October 2004 11:40
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| Joshstix wrote on Wed, 27 October 2004 14:28 |
The SR20 uses rocker arms to be able to run a wider valve angle while still keeping the head small. It's all about packaging for the SR.
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really? I thought they did it just to piss off people building SR20DE race motors
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Location:
western queensland
Registered:
September 2004
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Re: Head flow and flame propegation issues in turbocharged cars
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Thu, 28 October 2004 12:04
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what about 3sgte's, don't they have wide heads and flat pistons?
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Fri, 29 October 2004 03:23
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| indigoid wrote on Thu, 28 October 2004 21:40 |
| Joshstix wrote on Wed, 27 October 2004 14:28 |
The SR20 uses rocker arms to be able to run a wider valve angle while still keeping the head small. It's all about packaging for the SR.
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really? I thought they did it just to piss off people building SR20DE race motors
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Nope, they do it to have a closer angle head. Have a look at an SR with the cam cover off. The rockers are on the inside of the cams and so are the rocker followers (or whatever they are called). Cams couldn't be any closer together otherwise the cam gears would be in contact with eachother.
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Location:
Adelaide
Registered:
May 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Fri, 29 October 2004 12:41
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Lash adjusters they are called.
Stupid Nissan nomenclature.
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Location:
Canberra, ACT
Registered:
September 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Fri, 29 October 2004 12:44
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eh, don't mitsubishi and numerous other manufacturers use the same terminology?
4G63 has them, for example, but unlike the SR20 it has one per valve
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Location:
South Australia
Registered:
July 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Fri, 29 October 2004 14:25
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| Quote: |
in the squish area the gases get a high rate of compression just as the piston reaches TDC ... this results in these gases being at _higher_ pressure than the rest of the combustion chamber, and as a result, sort of spray out into the rest of the chamber...
...This high rate of compression will also cause a large temperature rise in the gas...
...have to remember that ignition occurred a while ago relative to the squish (sayy 20 deg BTDC), the the flame front is already propogating away from the spark plug. the 'squished gas' is squirted toward the flame front, and causes tumbling and turbulence and mixing of burning and unburnt gas... resulting in a much smoother burn...
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Does this mean that squish areas near the spark plug (for example) will cause higher levels of NOx emissions - due to the increased peak temperature at the flame front (this increased flame front temperature provides energy of activation, or merely changes the equillibrium? I haven't done any chemistry since year 12!!!). What would happen to the overall temperature of process - I'm guessing it would be more complete combustion, meaning a higher level of chemical energy would be changed into heat/kinetic energy? ie hotter (although more efficient)?
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Location:
Rocky Mountains, Canada
Registered:
May 2002
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 Re: Head flow and flame propegation issues in turbocharged cars
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Fri, 29 October 2004 16:44
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From what ive read, squish areas were introduced as a method of reducing emissions, not increasing power levels (although they *may* do this as well).
But you would think that if more of the fuel was burnt, you would end up with more power... anyone have proof of this?
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Location:
South Australia
Registered:
July 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Sat, 30 October 2004 03:59
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It probably increases efficiency, thus reducing general emission levels. Maybe due to better mixing it reduces carbon monoxide and carbon emissions...
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 03:15
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Then why do modern N/A engines tend to have about the same power of older N/A engines, even though the new ones run variable timing and higher compression? Shouldn't more power follow from more efficiency?
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Location:
South Australia
Registered:
July 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 03:54
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Then why do modern N/A engines tend to have about the same power of older N/A engines, even though the new ones run variable timing and higher compression? Shouldn't more power follow from more efficiency?
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would say that, in general, modern engines have more power per displacement than old engines. Alot of it is to do with the way modern engines are tuned, it's generally geared towards good emissions and good fuel economy. Also you have to take into account that modern engines have WAY better torque curves than older engines...to get good power just requires alot of air flow (think big ports, cams etc) alot of older engines were totally gutless outside of their good torque range, 90% of torque was seen over only 1000rpm or so rather than modern engines where 90% of torque is usually available over a 4000rpm range (at least).
That's just the half of it though.
The S2000 for example puts out 170+kW from a 2.0 litre engine and can still produce a modern level of emissions, that is one HELL of a feat.
As for it's combustion chamber shape - good question
afaik modern engines are designed to be efficient over a big rpm range, it's part of the whole 'economy' thing.
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But you would think that if more of the fuel was burnt, you would end up with more power... anyone have proof of this?
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Generally yes, if the combustion is more complete then this would usually be the case...I think...
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Toymods Vice President
Location:
Sydney
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 06:04
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The rockers used in the S2000 head that are used primarly to allow Vtec to work actually widen the angle of the valves.
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 06:15
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| Joshstix wrote on Mon, 01 November 2004 17:04 |
The rockers used in the S2000 head that are used primarly to allow Vtec to work actually widen the angle of the valves.
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I assume they use a single cam gear setup?
Interesting that they would widen the head. Eh, I still don't totaly understand all this.
I love the S2000, but it seems to be the exception to the rule. But perhaps your right and they just have more power down low. It's harder to see that though on paper, especialy with VTEC or VVTL-i and CVT (or whatever nissan calls their version of VTEC) as the max torque jumps up the RPM range so you can't just say "That has nice low down torque". 
Ohwell.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 06:18
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remember that the S2000 IS gutless down low.. so it's still a trade off between very high rpm, and low rpm torque...
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 06:19
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| oldcorollas wrote on Mon, 01 November 2004 14:18 |
remember that the S2000 IS gutless down low.. so it's still a trade off between very high rpm, and low rpm torque...
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Actually, for a 2l engine its probably not gutless at all, it's just in comparison to the top end its gutless Same thing gets thrown at the 2ZZGE, but it is about lineball with a 1ZZFE right up until the lift changes.
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Toymods Vice President
Location:
Sydney
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 06:21
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Yes the S2000 uses scissor gears.
CVT stands for constantly variable transmission or some such description from the new gen auto's.
The fact that the only specs you get to see for most of these newer engines are maximums lead people to believe that Vtec VVTLI etc engines don't make any low down torque, the fact is though they tend to make a reasonable amount of low down torque for their capacity it's just that they make more torque higher in the rev range.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 06:48
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| Joshstix wrote on Mon, 01 November 2004 17:21 |
The fact that the only specs you get to see for most of these newer engines are maximums lead people to believe that Vtec VVTLI etc engines don't make any low down torque, the fact is though they tend to make a reasonable amount of low down torque for their capacity it's just that they make more torque higher in the rev range.
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exactly...
whereas a lower revving, modern 2L motor might make 190-195Nm for a fair amount of it's midrange, the Beams and S200 motors make around 200Nm high in the rev range...
that they 'only' have 160-170Nm or so in the mid-range rpm, means that at those rpm, they _underperform_ compared to the lower revving motors, in that rev range...
160Nm is not exactly gutless, but it feels that way when they are driven .. and 160Nm from a 2L is still better than most older motors...
Cya, Stewart
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 07:34
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I was under the assumption they were pretty much lineball for torque production with a normal 2l lower-revving engine before they overtook it at the changeover point, not that they were underperformed before this.
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Location:
Brisbane
Registered:
October 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 08:28
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wow, this thread rocks,
what about the old hemi 265's, they'd have no squish area at all then??, then why did they haul ass so hard, did they have huge CR's or massive cams??????
so long as there was sufficient piston-head room would it be possible to "add" a squish area to say an 18RG head
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 09:04
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| justcallmefrank wrote on Mon, 01 November 2004 18:34 |
I was under the assumption they were pretty much lineball for torque production with a normal 2l lower-revving engine before they overtook it at the changeover point, not that they were underperformed before this.
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aiui, a 'very efficient' 2L motor will make close to 200Nm of torque. whether it makes it low or high in the rev range depends on cams and port velocity etc etc...
it's not like the S2000 motor makes 200Nm in mid range then magically gets 230Nm up high... same with the 2ZZ-GE motor... the great thing about these motors is that they are making peak torque around 6500-7000rpm, so you can take advatage of gearing etc... but by making their torque high in the rev range, they lose it down low.. even with the fancy cam bits, they will still have poorer port velocity down low than a motor designed for that lower rpm...
then again, maybe i have it ass about
anyone feel like finding an S2000 or 2ZZGE dyno graph where they are making 100Nm/L in the mid range?
the Hemis have similar CC to the 2TG? possibly it was just better port flow, and 'pre-ignition' minimisation..
as far as CC's go... from bad to good..
1. pre-igniting due to shit deisng with sharp edges and heat retention.
2. no pre-ignition points, but no squish either.
3. lots of well placed squish areas.
fwiw, the K motors have at least a third of their combustion chamber as squish area, opposite the spark plug, but that doesn't make them a good head
Cya,S tewart
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I supported Toymods
Location:
Perth
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May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 09:07
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The whole point is they have cams optimised for low-down torque below the changeover point, the switch to the big cams made for high rpm work. For sure there will be some compromises in not having the entire engine optimised for low-rpm work, but no way near in the magnitude you're describing.
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 09:26
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| Mr DOHC wrote on Mon, 01 November 2004 19:28 |
wow, this thread rocks,
what about the old hemi 265's, they'd have no squish area at all then??, then why did they haul ass so hard, did they have huge CR's or massive cams??????
so long as there was sufficient piston-head room would it be possible to "add" a squish area to say an 18RG head
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Hemi 265s are big. Isn't that how they make power? displacement?
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 09:27
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| justcallmefrank wrote on Mon, 01 November 2004 20:07 |
The whole point is they have cams optimised for low-down torque below the changeover point, the switch to the big cams made for high rpm work. For sure there will be some compromises in not having the entire engine optimised for low-rpm work, but no way near in the magnitude you're describing.
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Perhaps they just feel gutless 'cuz as the RPM builds, their balls drop.
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I supported Toymods
Location:
Perth
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May 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 09:32
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| Shraka wrote on Mon, 01 November 2004 17:27 |
| justcallmefrank wrote on Mon, 01 November 2004 20:07 |
The whole point is they have cams optimised for low-down torque below the changeover point, the switch to the big cams made for high rpm work. For sure there will be some compromises in not having the entire engine optimised for low-rpm work, but no way near in the magnitude you're describing.
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Perhaps they just feel gutless 'cuz as the RPM builds, their balls drop.
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Hehe, they go from a Corolla engine to a real engine over 500rpm
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Location:
South Australia
Registered:
July 2002
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 13:08
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So what has all this got to do with squish areas making better power? 
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Location:
Melbourne
Registered:
November 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 13:13
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| FWDboy wrote on Tue, 02 November 2004 00:08 |
So what has all this got to do with squish areas making better power?
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Everything. Can't you see?
Do squish areas change depending on what RPM the car is designed to make power in? I imagine the S2000 engine would be set up for screaming, so low down it wont make great torque, but a probably still the same amount of shopping trollies.
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Location:
Perth
Registered:
May 2003
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Re: Head flow and flame propegation issues in turbocharged cars
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Mon, 01 November 2004 14:38
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I might be wrong, but after reading this thread there might be some confusion of squish area with some.
Squish is created by having a small gap between the piston top and head surface(for all those who haven't read Bill's Page). In closed chamber heads (Holden 6, 3K Toyota etc) there tends to be a large squish area created due to combustion chamber design. In open chamber heads the squish area tends to be smaller. (I will leave the 4AG heads to those that know about them).
For the hemispherical combustion chamber of the 18RG the squish area is the area around the very outside of the piston that surrounds the outside of the combustion chamber in the head. (I.e. the bore is slightly larger than the diameter of the combustion chamber, creating a ring of squish area around the combustion chamber).
An even squish area around the outside of the combustion chamber coupled with a centrally mounted spark plug and flat top piston should theoretically provide the best combustion process.
I know the 2TG 88260 head uses a slight modification on the perfectly circular combustion chamber. The head surface has more area on the inlet side in order to create more squish, obviously to create more turbulence on that side of the head in order to 1) mix the gases better and 2) to force the mixture across the piston top (as the air/fuel mix would tend to be slightly more concentrated on the inlet side of the head).
As stated by many previously, the squish effect causes the in rush (in towards the centre of the cylinder) of gases as the piston reaches TDC. Too large a squish gap and the squish effect is next to nothing (no benefits), too small a squish gap and the velocity of gases can become too high and there is also the possibility at high RPM the piston will kiss the head.
A point worth mentioning, one problem with why decreasing an engines compression (for turbo/supercharging) by using a head spacer is classed as a bad idea is because it kills all chance of making use of the squish effect.
Squish gap can be dictated a little by rod choice (especially in drag engines) where using aluminium rods a larger gap is needed due to expansion of the rods due to heat and high rpm stresses (this is Stewarts field) cause the rod to lengthen substantially compared to forged steel rods.
Stewart, that article was an interesting read.
Here is some more:
http://www.speedomotive.com/Building%20Tips.htm
Well thats my 2c
[Updated on: Mon, 01 November 2004 14:41]
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