Backpressure: The myth and why it's wrong

[runriot]

Can somebody explain me all of these ? hahaha

I´m not an engineer so, somebody can explain me this in human words?

Question: If i have a 2.4l 4cil turbocharged engine, can I install a 3" straight exhaust system?

yes you can

[FortKid]

Velocity (at least after the turbo) isn't nearly as crucial with turbo's as it is with NA engines, but just ask someone who specializes in your car, without doing lots of trial and error or horrendous amounts of math it is hard to tell what the optimal size is. Best thing is an expert, or look what other people are running. Also assuming your car has the same setup as the SRT4 it has a one piece turbofold (turbine housing and manifold and waste gate all in one unit) so there isn't much room for upgrades upstream of the turbo without replacing all 3. If you find a system that others have had success with on your model car, go for it.

[diego1990]

Unfortunetly theres no catback aftermarket for my car so it has to custom made

[FortKid]

there are lots of kits for the SRT4, just look (by look I mean google search) at one of those and tell the exhaust shop to use similar pieces and sized tubing in yours. If the engine is the same, you should be able to get a pretty good system that way.

[diego1990]

Yes, theres a lot for the SRT-4 but my car isn´t an SRT-4, they doesn´t fit

[FortKid]

I'm saying just copy their designs (but make it to fit your car), I'm sure they wont mind, it is their fault for not making a system for your car.

[diego1990]

Yes I know, but it´s more interesting make it by yourself !!

[falconnut]

Thanks, was a great read.

Glad to hear that bigger isnt always better, its how you use what you got.

Equated to Exhaust systems

[lildragon555]

Never heard of backpressure before...this was an interesting read...
So depending on a car, you need the perfect size exhaust to get optimal performance out of it?

[Lethal300]

Also, a friend bought a Skyline R34 Turbo Coupe, like mine. Same engine, same turbo, virtually the same everything, except his has a stock exhaust and mine a 3" catback. He swears mine is quicker (and it probably is) however his feels to have a lot more push / torque down lower.

Yeah this may be true but, you have to take in all the variables which can make a huge difference. Not just engine wear, clutch, mass, tyres etc etc but altitude, temp, road surface, A/F ratios and so on. Its just to much to calculate. Even if you did a before and after on the same car.

[jmacman12]

Now I know that the original author dates back a few forums and a few years ago but I like to see authors credited. So in the absence of a concrete example of proof (authors=credibility) I will mention that Fortkid is the real winner here as in basic principle what he has mentioned is correct

[Rake]

Yeah this may be true but, you have to take in all the variables which can make a huge difference. Not just engine wear, clutch, mass, tyres etc etc but altitude, temp, road surface, A/F ratios and so on. Its just to much to calculate. Even if you did a before and after on the same car.

I agree, ont forgetting the Driver him/herself as well, who's driving of the car could factor for several fractions of a second. But, doing a before-and-after on a single car is the only way to get some sort of result.

I still believe however that comparing two cars which are near identical in every way (inc condition) with the only major exclusion of the exhaust is still a fair test from which to draw comparison. Yes, everything you mention is true, but at the end of the day, we have driven both cars and the real-world proof is there. Perhaps not as hard paper fact, but I'm not claiming it to be. But the differences are real and are there to be felt, which we have. I would not describe differences such as engine wear, tyres and so on to be major in this case. Sure, if you were comparing a brand new R35 GT-R to a 1991 Hyuindai Excel, sure, that is a major difference. A good condition, stock R34 Skyline vs another good condition, stock R34 Skyline is a minor difference, if any at all (with the exception of the mods in question - aka exhaust).

At the end of the day, there may be physics involved no human is even aware of. I suggest it would be good if there was some algoritm to help determine "in theory" what the best size is, however at the end of the day it's all about the real-world effects, and if it's proven that a particular size has a particular effect on engine performance, then who's to argue.

[High_Octane_Love]

At the end of the day, there may be physics involved no human is even aware of. I suggest it would be good if there was some algoritm to help determine "in theory" what the best size is, however at the end of the day it's all about the real-world effects, and if it's proven that a particular size has a particular effect on engine performance, then who's to argue.

For a long time now I've followed a general rule of thumb... 2" pipe on NA 4 cylinders, 2.5" pipe on 3L V6's and 5.0 V8's and 3" pipe on any type of turbo engine. This has resulted in cars which perform well and sound nice without detriment to bottom end power. Also remember the rule on throttle body trumpets - Thin and tall for low RPM power and short and fat for high RPM work... This directly translates to exhaust as well

I find the use of a venturi particularly fascinating though as "conventional wisdom" suggests that any loss in pipe diameter adversely affects flow... But like the Dutchman says... It's on a racecar where every Kw counts so I'd suspect it works!

[FortKid]

Now I know that the original author dates back a few forums and a few years ago but I like to see authors credited. So in the absence of a concrete example of proof (authors=credibility) I will mention that Fortkid is the real winner here as in basic principle what he has mentioned is correct

I would like to credit Fox and McDonald for their fluid mechanics book, it is by far the most used engineering book I have had at school. (this might be due to the fact that all books for other classes changed editions so I had to buy new ones between quarters...but I did use it a fair bit) It was expensive though

[MY91MX]

Here is the formula I was thinking of. (it looks daunting but lots of things cancel out) you can use this to calculate the momentum of the flow. Also, luckily at speeds below .3 times the speed of sound, mathematically you can treat the fluid as incompressible. I think I just got waaaay of topic though.

Edit: Crap, that is not the one I wanted either, I'm bad at finding formulas on the internet, and I left my fluids book at home. I give up.

Edit: F**k this S**t I have a pen!

I'm all for treating gasses as incompressible but unfortunately there are not. If the exhaust stream was liquid then yes it can be assumed mathematically that it is incompressible but even the smallest amount of compressed exhaust would negate this equation.

Back on topic this tread is an awesome and informative read. Thumbs up for it