Have read many papers on various applications and spoken with top tuners and had products dynoed to show remarkable gains, but never tapped in to measure absolute back pressure at the turbo outlet.
Exhaust mass/density, volume, and temerature are all crucial in creating the ideal circumstances for optimized power / response - all of which are affected by key factors such as induction, fueling and boost.
This is an extract from a company that was faced with the dilemma of 3.00" or 2.50" choice of system...
Exhaust velocity is dependent on the temperature of the exhaust gas. As we know, the engine combustion and turbo restriction processes heat the spent exhaust gas. Gas expands when heated.
The expanding gas creates pressure within the confines of the exhaust tubing.
This pressure "seeks" out lower pressure, which is at the tailpipe. Basically, the pressure wants to escape, and this helps keep the gas moving away from the engine.
A properly designed performance exhaust "manages" this pressure properly. As the exhaust gas moves away from the turbo, it begins to cool down. As it cools down, its pressure inside the exhaust tubing reduces.
Too big of an exhaust tubing diameter for the given heat, and the gas will cool down prematurely, reducing velocity towards the tailpipe, which is not good for power.
Now, throw in the factor that when at lower rpms and idle, the entire exhaust system is cooler than at high rpm and full throttle. A 3.0" system has more standing air volume inside it than a 2.5" system, which means more air inside it that will absorb the heat coming from the exhaust gas. A drop in heat means a drop in pressure, which means a drop in velocity.
That is precisely why "special" programming is needed for a 3.0" exhaust.
What happens is that there is tremendous turbo/power lag with the 3.0" system because of this excessive air volume. That causes actual turbo boost to not match requested turbo boost from the software. And to make matters worse, when the turbo finally does spool up in an attempt to match requested boost, the factory N75 valve can't keep up and overboost happens. And then the N75 does catch up, but the ECU is already reacting to the overboost spike as read by the boost sensor, and the opposite happens (underboost). This dance/surge happens for a few hundred rpms as the ECU finally manages to get everything under control.
It is this surging down low that a special software flash is intended to fix.
However, this surging is simply a result of TOO LARGE EXHAUST TUBING. The software cannot change the physics of the mechanical parts it is working with and get the turbo to spool up faster down low. It can only help reduce the negative effects of over and under spool due to the 3.0" system. Therefore, significant power (torque) is left on the table simply because the exhaust is too large for the power and displacement it is working with.
Our full time 3.0" turbo back sport systems are designed to run in conjunction with a package of modifications including mapping, however, our unique Powervalve systems which operate utilising a 76mm boost actuated exhaust gas flow control fluctuate between an overall flow rate between 60 and 76mm which is optimised at ***psi of boost this is why the mid range torque balloon is far greater than any other system on the market as essentially it is 2 systems for the price of 1.
Cheers
Nige
