Lmao. I understand your frustration bro. I see it all the time. As you said, the turbo in itself creates resistence in the flow of the exhaust gasses = backpressure. Anyone wonder why race cars plumb exhaust pipes through the side of the front bumper. Disclaimer: I accept no responsibility for those who will disconnect their downpipe and roast everything under the hood...lol.

a 3" exhaust would be best for your goal of 400hp....and yes, even 2.5" could do it.

 

Can someone go to the dyno and test this back pressure theories and prove it?
you can just test with a silencer on and off then compare the bottom to mid range power.

PS: Or test with Gtech Pro.

 

Backpressure never did anything but REDUCE power for both normally aspirated AND turbo cars. In no instance is backpressure NEEDED or WANTED.

Those who speak of backpressure being beneficial do not have a clear understanding of how exhaust systems work.

And where exhaust systems for turbo cars are concerned, there is no such thing as too large where power is concerned. However, large beyond a certain point will not produce any additional power. In this case, I doubt a 3.5" system will provide significant benefits (if any at all).

 

I never said turbo cars NEED backpressure, and I didn't say big exhausts LOSE some power. I merely said that the power curve can be moved around with differently sized and shaped exhausts. I agree that the most peak power will be atained with an open DP/no exhaust, just like an NA with open headers. But I think a street car on pump gas with either a stock or moderate turbo upgrade could well see better midrange response with a properly sized exhaust. If you have proof of your claims I'd really like to see it.

 

On the contrary, there is no evidence whatsoever that a larger exhaust on a turbo engine reduces anything, anywhere.

Exhaust changes on a turbo engine do not work the same way they do on a normally aspirated engine.

 

go with a 3" youll be fine

 

IMO a 3.5'' would be a waste 3" is more than enough for 400whp.

 

For the benefit of those who are now confused . . .

A smartly designed normally aspirated exhaust has a mechanism to convert exhaust gas energy into negative pressure. This is a collector. If the collector and primary tubes are designed properly (and many are not), they will actually enable each exhaust gas pulse to gently 'suck' on the other primary tubes as it passes through the collector and on out into the exhaust pipe. The greater the 'suck' action, the greater the positive effect on VE and the reduction in pumping losses. If the exhaust is enlarged enough to reduce exhaust gas velocity through the collector, the exhaust pulses do not have enough energy to make the collector work, and our power isn't there. This is often misinterpreted as not having enough 'backpressure', which is incorrect. Of course, if the exhaust is too small, this increases pressure in the system, which begins to cut into our power as well. Anything more than ~2 psi in a normally aspirated exhaust system is going to reduce power, so sizing normally aspirated systems is like designing/sizing a musical instrument to sound a specific musical note when we blow into it.

A turbo exhaust has no such mechanism to create something positive from exhaust energy. Once that exhaust pulse spins its way out of the turbine, it can do nothing but get in the way. If pressure is created on the backside of the turbine, it just reduces turbine efficiency and VE as well.

With a turbo car, a larger exhaust will make the most difference at peak power, where the engine is consuming the greatest rate of mass air flow. IF we find that a larger exhaust somehow reduced power, it's because it increased VE and leaned our fuel mixture enough to make a difference. If we re-tune, we will get our power back, plus some.

 

Guys.....you need to learn a thing or too before posting garbage....seriously. There is only one person in this thread that i think has a clear understanding regarding exhausts and especially turbocharged engine exhaust.

I going to state FACT, not opinions.....so listen up, THERE IS NO DEBATE ON THIS! do some research and see.

With a turbochraged engine, the manifold and turbine housing is a very large restriction in the ehxaust stream. And the stock turbine section is even worse, because of it's small size. The best exhaust for a turboed engine is NO ehxaust at all! With this in mind, you want to replicate this as best as possible. You see, the reason why race cars dont have ehxuast is simple......you make more power without them...... Thats why they run open downpipes of 4in or more in some setups. Backpressure has a very negative effect on a engine..... It raises EGT's, cylinder temps/pressure. This in turn lowers the detonation threshold...which keeps you from running more ignition timing and more boost....which componds on itself. It's a lot more complicated than that, i'm just giving you the short version for the sake of time.

If you guys think that there is no gain from a 3in to a 3.5in or from 3.5in to 4in....you would be sadly mistaken. Because you want to have the LEAST amount of backpressure possible after the turbo.....and running a open downpipe is the best option for power......why do you think a 3in full exhaust isnt restrictive? trust me....there is some restriction in a 3in ehxaust. There is going to be restriction in any ehxuast thats not a open downpipe(there is even restriction in a downpipe...keep in mind that the manifold and turbine section is a big restricion...therefore anything after that is going to make backpressure worse by multiplying that restriction).

So, with that said, know this; 3.5in exhaust will make more power than a 3in ehxaust.....every time. A 4in ehxaust will make even more power. If you want to make the most power you can......go with a 3.5in exhaust! It's no louder than the loudest 3in. So don't let these people tell you that it's going to be a lot louder. They are louder by design, but i have heard some 3in ehxaust that were just as loud as my HPF3.5in.

So.....those are the facts, not opinions.....FACTS. Do what you wish with them.

One last thing. You guys confuse Naturally Aspirated engine science with turbocharged engine science, especially when it comes to exhaust! What holds true for the N/A engine does not nessicarily carry over to the turboed engine. We are really talking about two completly different engines...... sharing very few attributes. So, be careful how you compare the two and what you post regarding this kind of stuff.

CJ

 

Thanks for posting this....i simply didn't have time to get into scavenging and what it all means for them.

Also, once tuned for the increase in flow and increase in VE.....you will make more power....you are very correct.

CJ

 

Well, I figured it wouldn't be of much benefit unless I explained the 'why' after the 'what'. If it helped so much as one person understand how this works, it was probably worth it.

 

Then coating or wrapping exhaust components from the turbo back is a sound move and not a gimmick?

 

Yes, hot exhaust gases flow faster than cold exhaust gases. The faster you can expell those gases the better.

 

if you really are trying to squeeze power outwhy not go with a 3 inch with a cutout in the test pipe? still make power for the street but then will have that extra little oomph at the track? or is this just a moron idea?

 

my friend has a 3.5 inch tbe on his sti and it isnt that loud at all it just sounds badass