SimonH

With all the chat going on about P1's blowing up etc etc, could someone explain to me how or if a decatted exhaust can effect the fuelling of a car - making it run lean etc?

TA

john banks

Depends

Not really a one dimensional answer.

By fitting a decat the volumetric efficiency of the engine increases. Part of the load input to the map is from the throttle position, part from the airflow sensor - only the latter will adjust to (therefore partially) compensate the AFR. So you may hit different areas of the map, especially when the car overboosts a bit more with the better breathing. So it can be richer or leaner in different areas of the rev/load range. In addition the ignition timing will also be altered similarly.

Moles Dad

Are you the poor bast4rd with the 'blown up' P1?

If so, were you not only doing 20mph?

Would fuelling really affect a car at such a low road speed...presume it wasnt at full chat with the loud pedal!!!

ozzy

Simon's not had his long so I hope not.

SimonH

Thanks John.
What I find a bit confusing is that the exhaust is downstream of the turbo. I could see that decatting might affect the back pressure in the system that the turbo works against (dodgy ground here ) but how does this affect things upstream of the turbo?
Is fuelling not driven by throttle position and MAF sensor inputs (genuine question btw not rhetorical!)
I can see that it might confuse the fuelling slightly with the turbo coming on song earlier down the rev range but cannot the ECU compensate? Or is the danger in reaching the limit of ECU correction?

MD - fortunately twas not I with the dead P1 as ozzy says mine is still wrapped in cotton wool

I posted a link on the P1 thread in General back to another post in Drivetrain where Mike Wood was saying that all the P1s he'd seen with decats were suffering from bad det. Pete Croney was arguing the complete opposite. I mentioned it in Powerstation this morning and he couldn't see how a decat could cause det either.

john banks

What I am saying is that the MAF/TPS load system doesn't compensate entirely, and the different airflow at different engine speeds makes the ECU hit some rather suspect areas of the map. This along with overboost and you have a recipe for detonation (leaner, more advance and more boost = all three substantially adversely affect the knock point).