Can anyone explain how back pressure in the exhaust system, can modify where the torque curve starts in the rev range, and how fast it rises.

The reason for asking is that I fitted a PPP ECU to a MY99 boost is approximately 1.1 bar, and the torque started to rise from about 2400 rpm, this was on a standard exhaust system not PPP so had two cats in place.

Then replaced the exhaust system for decat centre with resonator, and a free flowing back box.

The torque now starts to rise at about 2800 but comes in hard and feels stronger and boost is approximately peaking 1.2 bar.
Just keen to learn

 

Looks like this is a difficult one

 

Have a look here :
http://www.auto-ware.com/combust_bytes/p_goal.htm

May help you to understand

Regards
Dave

 

My own interpretation is:

when there is less gas flow (lower rpm) like at 2400rpm the smaller exhaust diameter will aid flow and the gas will travel faster through the thinner pipe (more restrictive pipe) like a water through a straw but when the engine revs get higher the more restrictive exhaust becomes a restriction to flow.. and the exhaust gases back up and hinder the combustion process and the turbo is trying to push out fumes rather than spinning freely and sucking in more air (hence the less boost on the standard set up).

It is all compromising.. less down low and more uptop etc.. it is playing to get what you want from it.

 

Thanks for the explanation guys