Best Exhaust Back-Pressure for 2.7L Engine?

[edevlin]

I have been playing with exhaust systems lately on my 2.7L 2000 Boxster. I have tried different combinations of Fabspeed Cat-Bypass Pipes, TTP headers, 100-Cell cats and both stock and Gemballa mufflers.

The car seems to like the stock header/Fabspeed Cat-Bypass/stock muffler combination pretty well (198 whp). But it runs and sounds best with TTP headers/100-cell cats/stock muffler (202 whp).

But what suprised me was that it did not like the TTP headers/100-cell cats/Gemballa muffler combination. I did not even run a dyno on the rig because it was not running well at all (also, that combination was too loud for my tastes).

The Gemballa muffler seemed like it had pretty free flow compared to the stock muffler, so it got me thinking about what the optimum back-pressure in the exhaust system is for the 2.7L engine.

Anyone have any ideas about what is the optimum back-pressure or exhaust is for this engine,

Cheers,

Ed <_<

2000 2.7L Boxster, 50K

[Topless]

Intake and exhaust just need to be matched. If you only change one, the motor won't really breath any better. You get very limited improvement.

[mee]

IMNSHO, engines never need back pressure. With that said, however, engines are optimized around a given set of parameters. The HP differences you're talking, though, are minimal and likely to be well within what the ECU can adapt. As Topless mentions, you can only remove bottlenecks in one place so far then a bottleneck somewhere (intake) takes over. I don't think that is the case here. I wouldn't be surprised if the Gemballa muffler just "sucks". There have been plenty of mufflers that sounded great but lost HP.

[insite]

IMNSHO, engines never need back pressure. With that said, however, engines are optimized around a given set of parameters. The HP differences you're talking, though, are minimal and likely to be well within what the ECU can adapt. As Topless mentions, you can only remove bottlenecks in one place so far then a bottleneck somewhere (intake) takes over. I don't think that is the case here. I wouldn't be surprised if the Gemballa muffler just "sucks". There have been plenty of mufflers that sounded great but lost HP.

true, back pressure, per se, is unnecessary. VELOCITY, however, is critical. generally what happens when those out of 'the know' try to reduce back pressure is that they enlarge tubes in the system. this can result in sub-optimal exhaust velocity, which will in turn inhibit the ability of the system to scavenge ('suck') gasses out of the system.

[mee]

Yeah! Someone who agrees with me. 'insite' has it right. Intake and exhaust air has mass. You want that air mass moving along at a smooth and swift speed. If you keep changing the pipe diameters in the "system" that mass has to keep slowing down and speeding up. If that mass has a high velocity, it has high intertia to use for filling combustion chambers on the way in as the valves close and for scavenging on the way out.

Of coure, even higher velocity could be achieved using pea shooter-sized exhaust pipes, of course ;) however that is where friction with the walls becomes too high and increases back pressure. Changing pipe diameters, though, causes impedance mismatches in the flow and increases back pressure. The same thing for too many bends/kinks, etc. What many people do with a "bigger is better" strategy is wind up with increased "back pressure" even though the component by itself has less back pressure.

[insite]

Yeah! Someone who agrees with me. 'insite' has it right. Intake and exhaust air has mass. You want that air mass moving along at a smooth and swift speed. If you keep changing the pipe diameters in the "system" that mass has to keep slowing down and speeding up. If that mass has a high velocity, it has high intertia to use for filling combustion chambers on the way in as the valves close and for scavenging on the way out.

Of coure, even higher velocity could be achieved using pea shooter-sized exhaust pipes, of course ;) however that is where friction with the walls becomes too high and increases back pressure. Changing pipe diameters, though, causes impedance mismatches in the flow and increases back pressure. The same thing for too many bends/kinks, etc. What many people do with a "bigger is better" strategy is wind up with increased "back pressure" even though the component by itself has less back pressure.

and to add to that, the only time comparatively large diameter tubing / ~ zero back pressure is desirable in a normally aspirated car is under racing conditions. scavenging won't occur at low revs with that type of set-up (no low range torque) but WILL occur at very high revs to maximize peak power. tractability is greatly reduced (not streetable). this is a difference between street and race headers (tubing size / length determines 'power band' within the rpm range).

[Topless]

Yes! Yes!

That is exactly what they mean when performance builders talk about "tuned exhaust". It is "matched" to the intake airflow and maximizes cylinder scavenge for a given volume of exhaust gasses. This is why I am not a big fan of the "lets just bolt on a new set of pipes" method. Those boys in Stuttgart did their homework and barring any other changes the stock set is really pretty good.

Now if you are really serious about performance gains, get a high flow cold air intake, matched/tuned exhaust, burn 100 octane and have your DME performance mapped while on a dyno to really dial in your ign. timing and air/fuel ratios. Yes you will feel it. No it's not cheap.

[986Jim]

Tuned exhaust has nothing to do with the intake. What you do to the intake of the car is TOTALLY IRREVELANT to the exhaust. Yes, to get the best results from the whole setup on the dyno you want to do all the mods you can however if the exhaust gives you 15hp it will give you 15hp without the intake, you won't need an evo intake to get 15hp from the exhaust (though you will get more with the intake and exhaust together obviously) but one is not dependant on the other.

For some reason Porsche guys have this thing that there's no point to doing an intake if you didn't do the exhaust and vice versa. This simply is not true, I have no idea where this thought process came from. Obviously a lot of guys here have never been to a dyno or tuned a car before. I build and tune cars for a living so surprise surprise I have a different opinion. Not saying this is what you guys think, just sometimes seems to be a general consensus from many people...

Tuned exhaust means that the size of the collector and length and output side is designed for the engine. If you look at burns collectors (who made our race car collector) they want to know the displacement, bore, cam lift, hp, torque, where that power is made in the band etc etc so they can make you a collector that is TUNED to your car. It has nothing to do with the intake however.

From a racing standpoint when you say intake, you mean intake manifold and throttle body. An "intake" is not a pipe and filter generally, it's just a pipe and filter. So when you tune your collector to match the output of the engine you have already selected your size of intake manifold based on your desired results. Large Plenum small runner for high HP, vs. small plenum long runner for lots of torque type of deal. This is how it's Tuned, not with an evo intake. (I Have an evo btw, love it).

Your optimum exhaust system for racing is basically no exhaust because back pressure only robs HP on the high end, however that may not be optimum setup for the street. These are not high revving cars to start so it will create a peaky narrow power band which isn't really much fun, like an S2000 with lots of mods on it. So stick with a setup that keeps the pipe diameter consistent and not too big so you get a good balance of HP and mid range. If you make it too free flowing you will make the max HP at peak power, but not enough in the mid range. For racing this is what you want, but not really for the street unless you drive your car like that. You would be better off with something making the 198hp level which felt better than 202 with all peak power. Sacrificing 10hp in the midrange for 3hp at peak is fine for racing but your better off with the 10hp in the mid range for street driving.

http://www.etdracing.com/team/vibrant/finished4.jpg

This is our old exhaust header on our race car. As you can see right after the 4-2-1 collector it gets very narrow then goes quite large. This keeps velocity high to the collector then after the savenging effect where velocity is high, the pipe opens up to keep back pressure off the collector. This header was designed by burns stainelss (www.burns.com) on their computer model based on our cars design. Obviusly peak power was the only desire here. This is just to give you an idea...

I can elaborate on any of these points if you want more discussion on a particular area...

Edited October 3, 2006 by 986Jim

[insite]

Tuned exhaust has nothing to do with the intake. What you do to the intake of the car is TOTALLY IRREVELANT to the exhaust.

if by intake you mean strictly the filter / tube, tuning the exhaust indeed has nothing to do with the picture, although ensuring your intake is unrestrictive enough to take full advantage of a freer exhaust is important. for instance, in a car with significant cam overlap, the exhaust scavenging effect of a header will actually pull intake air into the cylinder. a handicap on the intake plumbing would reduce this effect.

on the other hand, if by intake you include the manifold, this should be matched to the exhaust for maximum benefit. the intake runner lengths / diameters should be tuned to the same desired RPM range as the header.

[Topless]

Tuned exhaust has nothing to do with the intake. What you do to the intake of the car is TOTALLY IRREVELANT to the exhaust.

986jim,

I guess you and I are just going to have to agree to disagree on this issue. Take your race car: You decide to enter it in three different classes this year. The first class is an intake restricted class choked down to a 2" diameter; the next class is a N/A unrestricted class; the third class is unlimited ( supercharged 30lb boost) class. Same basic engine, same displacement, very different intake air mass and very different exhaust gas volume. Would your exhaust system experts recommend the same exact set of pipes on all three setups or would they "tune" the exahust to match each different setup to maximize your performance in each class?? Ask em.

[insite]

The first class is an intake restricted class choked down to a 2" diameter; the next class is a N/A unrestricted class; the third class is unlimited ( supercharged 30lb boost) class. Same basic engine, same displacement, very different intake air mass and very different exhaust gas volume. Would your exhaust system experts recommend the same exact set of pipes on all three setups or would they "tune" the exahust to match each different setup to maximize your performance in each class?? Ask em.

excellent points!

[986Jim]

986jim,

I guess you and I are just going to have to agree to disagree on this issue. Take your race car: You decide to enter it in three different classes this year. The first class is an intake restricted class choked down to a 2" diameter; the next class is a N/A unrestricted class; the third class is unlimited ( supercharged 30lb boost) class. Same basic engine, same displacement, very different intake air mass and very different exhaust gas volume. Would your exhaust system experts recommend the same exact set of pipes on all three setups or would they "tune" the exahust to match each different setup to maximize your performance in each class?? Ask em.

You missed everything I was trying to say. Obviously that would not be the case. A smaller outlet on the collector on a NA motor is more beneficial where on a Turbo car you just simply make the exhaust as large as possible and a good collector doens't really help, there is no tuning..

What I'm refering to is our cars. I was using our race cars as an Example thats sort of related for our cars from an NA point of view. Our race car doesn't have an intake on it, it's has Individual throttle bodies with a custom made box and external injector setup. So what I'm talking about is completly a different league what what your refering to. (not to be rude, don't take it that way)

I hear what your saying. On a stock N/A car is what im talking about. It just doesn't make a difference and you don't really need to "Tune" the exhaust form that stand point. THe engine does make enough hp. 250-300 is diddly squat so the exhaust setup is not that huge a deal. If we were pulling lets say 140hp/L from our cars like our all motor car does (to the wheels to btw) then setting up the exhaust better is a much bigger deal. Take the GT3 for example. Thats 420hp from 3.8L, thats good HP/L and also good HP over all. It's gonna be hard to improve on that and will definately take some serious work. For racing, headers and straight pipes woudl work best as it would give all top end HP which is what you want for racing. On the street that would suck and stock setup would likely be the best. Thats all I was saying..

More or less the setup with 195hp that felt the best would be the best as all peak power is fine for somebody who really drives the crap out of their car, but generally we don't so it wont be the best. It all depends on what balance your looking for.

Our cars will make HP increases with only the exhaust and not doing anything to the intake. Intkae will make more power if you do that too, but one is not dependent on the other. Gains can be made with or without the intake or exhaust depending on the road you go.

Attached are some more pics just for fun... That is our new header which we totaly re-designed to move the power around in the band a little better. We have changed the intake 3 times but never changed the exhaust, again one is not dependent on the other. This car makes 280whp from 2.0L or about 315 at the crank which is like 157hp/l at the crank. The newer style engines are getting 315 whp from 2.2l which is crazy... (they are also running 9's we go 10.6 @ 129 currently)

[Topless]

Jim,

Well I guess we have pretty much beat this horse to death haven't we? Nice pictures by the way. You sound like a bright guy with a passion for cars. I hear what you are saying... I just don't agree and that is ok. Spirited debates are part of what this forum is all about. I am no exhaust system expert, but I have been around the block and built an engine or two. Maybe someone who is smarter than both of us will come along and tie this whole thing together and we will both learn something. Until then I will stand by my principles if you will stand by yours.

Peace :)

Edited October 4, 2006 by Topless

[986Jim]

Jim,

Well I guess we have pretty much beat this horse to death haven't we? Nice pictures by the way. You sound like a bright guy with a passion for cars. I hear what you are saying... I just don't agree and that is ok. Spirited debates are part of what this forum is all about. I am no exhaust system expert, but I have been around the block and built an engine or two. Maybe someone who is smarter than both of us will come along and tie this whole thing together and we will both learn something. Until then I will stand by my principles if you will stand by yours.

Peace :)

Yeah thats cool I guess. I take what I say from my knowledge building tuning and racing some of the countries fastest all motor import race cars. So what I say is just based on my own mistakes and later changes to our own car in an effort to get the maximum amount of HP out of it. Most if not all what what we learn on that car is directly applied to setups on street cars.

Typically we build custom headers and exhaust systems for cars before using manufacturerd parts as we can more easily customize them to our setup/needs vs buying a part. In a way, we are tuning that exhaust and intake to match our setup. I guess its all in what you call "tuning" ... Tuning to me is a lot different than smart design to maximize hp. I still however respectifly disagree what the intake pipe and filter has any baring on exhaust.

Any car with enough overlap in a cam to make a difference is already running a non-restrictive intake becuase if you pop in a set of performance cams, your not about to do your intake next lol.. So generally speaking when you have enough cam you already have the intake and exhaust system inplace. If you were to tightly restrict a car with a lot of cam and exahust then yeah its gonna hurt it. Which is guess is what your saying.. The stock intake on these cars with the stock cams is not restrictive enough to alter your exahust choice however. The stock intake is pretty good as seen with the mild to non existant gains made by aftermarket exhaust systems, so there really isn't much consideration there when it comes to exhaust systems..

Anyway, enough I think we have beaten this to death really.. Maybe some day somebody will come back and read this post later on and it will give them some extra knowledge that we have decided to share...

On another note...

I think what we 2.5L guys would relaly like to know is will the 2.7L intake manifold with the the second cross over chamber work on our cars? Apparently a few HP was picked up from that second tube across the top of the intake manifold. It would create a larger reservce and also serve to balance the two cylinders out better. I would love to swap one over on my car and port my throttle body if anybody knew is the 2.7L intake manifold works?

Edited October 4, 2006 by 986Jim

[Topless]

Jim,

I took a closer look at your race car pictures and I think you guys may have still left 15-20 hp on the table...but you are going to laugh!!

[986Jim]

Jim,

I took a closer look at your race car pictures and I think you guys may have still left 15-20 hp on the table...but you are going to laugh!!

I'll bite... What is it?

[Q-Ship986]

Jim,

I took a closer look at your race car pictures and I think you guys may have still left 15-20 hp on the table...but you are going to laugh!!

I'll bite... What is it?

Heat shield? ;) --Brian

[Topless]

Jim,

Manifold your intake. See I knew you would laugh.

Maybe I am missing something from your pictures but it looks like your current intake has tons of air volume but there are also some flat sections at 90 degrees to the direction of airflow. These can cause air to stack up and at high velocity cause standing waves which interfere with cyl. intake velocity.

Manifold your intake with all curved surfaces and individual runners for each cyl. Tune runner length and diameter to match your desired intake air pulse rpm. This should give you great top end performance and improved low end torque as well. These changes may be tough to measure on the dyno because you are standing still... At 100mph you will notice. This is what the fastest cars in the world are doing. Maybe a good winter project. :)

[986Jim]

Jim,

Manifold your intake. See I knew you would laugh.

Maybe I am missing something from your pictures but it looks like your current intake has tons of air volume but there are also some flat sections at 90 degrees to the direction of airflow. These can cause air to stack up and at high velocity cause standing waves which interfere with cyl. intake velocity.

Manifold your intake with all curved surfaces and individual runners for each cyl. Tune runner length and diameter to match your desired intake air pulse rpm. This should give you great top end performance and improved low end torque as well. These changes may be tough to measure on the dyno because you are standing still... At 100mph you will notice. This is what the fastest cars in the world are doing. Maybe a good winter project. :)

Our intake has a manifold around it, thats what the big silver box it. The hood of the car goes over it which makes it a cowl induction. The size of the box around the ITB's is specifically designed to be exactly that size per the displacement of the engine. The ITB's were cut and had a 45* pitch added to them between the head and the throttle plates for a few reason (Velocity and entry angle into the head). The injectors are also external at that particular length for air cooling the intake charge with our cooled fuel.

Look at the picture that shows the overall engine. The box has a slight say 25* angle in it. Then the itb's go straight into that box then have about a 45" pitch on them which enter the head straight. Re-making the intake like this vs' the old setup made huge power gains...

Here is how it used to be http://www.etdracing.com/team/vibrant/finished7.jpg

See the ITB's go straigth back but still external injectors (F1) style.. I dont have another side shot like that, but we now have the itb's pointing upwards towards the windshield and that box around it still with the external injectors..

Edited October 8, 2006 by 986Jim

[Topless]

I see now. Didn't get a good look at your runners from the previous pic. I knew it was a custom sheet metal intake box and figured reverse cowl induction. I still don't like the flat surfaces at the bottom. Possible standing waves in the box. You are creating a ventouri effect now which is pretty good. As you already know, a little intake turbulence is good because it improves air/fuel mixture. Standing waves off the bottom of the box work against you.

[seafeye]

I see now. Didn't get a good look at your runners from the previous pic. I knew it was a custom sheet metal intake box and figured reverse cowl induction. I still don't like the flat surfaces at the bottom. Possible standing waves in the box. You are creating a ventouri effect now which is pretty good. As you already know, a little intake turbulence is good because it improves air/fuel mixture. Standing waves off the bottom of the box work against you.

If i could add my .02c, In regards to the exhaust i belive that porsche has left much room for improvement with the stock system.

Being the worlds most profitable car company they have to make money somewhere. And they do it with the simple exhaust headers

on the boxster. The flat six does make exhaust performance limited because of how difficult it is to match up the oposite cylinder

for proper air flow out the tubes, but they could have made the stock system better by just making all the tubes the same length.

Of course this would be more time consuming and eat the profits. I also believe the system is designed to reduce horsepower. Look

at every one of Porsche's race car headers. The tubes are all the exact length. This is just too difficult to do on a production car.

Or too expensive. Your choice.

[986Jim]

After having now been under the car on the hoist from putting on my muffler I got a chance to look at the headers clearly and photograph them. See the attached picture.

The problem with the stock headers on the 97-99 particularly is the clear lack of a collector which works to scavenge the cylinders after each one fires, secondly the runners are not equal length. These are designed only to fit the space and budget of the car. It's no wonder the aftermarket headers make 10hp and 15ft/lbs of torque from simply changing them out.

The 2.7 I believe got a proper collector even though there is a cat attached to it it's much better than the earlier design on the 2.5l. So not as much power was picked up on the 2.7L cars with a header (and 3.2 as they are the same) but there was still gains.

I have ordered some 2.5l headers to replace what you see in the pic and I will report the difference / install shortly.