Quote:
Originally Posted by mlai
BTW, for turbo cars, back pressure does not help performance at all. In fact, the actual back pressure is not advantages to NA engines either. Let me explain a bit from researching on the net.....
Let's start from NA (yes I know. We are all turbos now. But bear with me for a little....) Back pressure actually prevents exhaust gas flow and and impedes proper scavenging on the engine cylinder cycles.
What the NA engine needs maximum exhaust gas VELOCITY, so that it creates temporary vacuum which sucks out residual exhaust gas still in the cylinder. The easiest way is to squeeze the gas in a tube that is at the correct diameters. I did say correct, there is a sweet spot to tune the exhaust speed. If the pipe is too wide, the gas slows down and scavenging becomes less effective as the vacuum is not as long. If the pipe is too narrow, well, you have excess back pressure which makes it more difficult to suck residual gas out from the engine.
This is also one of the reason why, when one simply replace exhaust systems on a NA car with bigger pipes, people will generally feel lost of low-end torque because the exhaust at low rpm is not moving fast enough for scavenging, hence engine is less efficient. Improper tuning and exhaust matching can also cause exhaust gas moving too fast, which sucks not only residual burnt gas, but also sucks out some of the air that just entered the chamber and unburnt fuel. So, in NA, simply replacing exhaust without adjusting engine tune almost always results in less performance....... 
Now for Turbo cars like ours, scavenging the exhaust gas out of the chamber is less of a problem. Why? Because we have blowers force feeding the chamber with compressed air!!!! And contrary to what people may say about back pressure "creating loads on the turbine to shorten spooling", that is not at all correct. The turbo units essentially is a turbine ran by differences between intake pressure and outflow pressure. The intake is from the exhaust manifolds from the engine and the outflow is everything back from the turbine. So if you create back pressure behind the turbine, it will make the turbine harder to turn, thus in turn makes turbo lag worse.......
So the bigger the exhaust pipe from turbo-back, the lower the pressure (limiting up to atmospheric pressure in theory) at the turbine outflow port. In theory, the turbine fan should be easier to turn, and thus lowering spool time of the turbo?
Is my understanding somewhat correct on this? |
I can agree to this reasoning. I upgraded exhausts and headers on my 2 previous NA cars, and I notice poor low end torque on both cars. That isn't the case for this car. First, I was thinking that it might have to do with twin-scroll turbos which allow engine to be responsive at low rpm/less exhaust gas. But, it seems like exhaust systems on turbo cars are really just for noise suppressing, filtering backfiring noise and droning.
Now, I wanna get X-Pipe for this thing, but something I can swap in and out myself without visiting the shop.
