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Originally Posted by ROB-80E
Firstly, a 3" mandrel exhaust is perfect for the BPT engine in standard form. Being a turbo efi engine, it doesn't really care how big it is, and I've proved that there is a difference from just going from a 2.5" muffler up to a 3" which is the same size as the rest of my system. The difference was boost coming on alot quicker and also increasing power in top end. I'd never go back to a 2.5" system.
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This is due to backpressure generated by the 2.5" system. What Mal is trying to say I believe is that if you want a std conversion which is smooth in power delivery (ie no cuts) then go a 2.5" system. If you can handle the boost cut syndrome, go 3" and enjoy the things you outlined.
If you are planning to go big with your setup, definetely a 3" system is probably best.
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Originally Posted by ROB-80E
Boost cut on the GTX ECU is 14.7psi. With running a 3" system with a pod filter you will find that you'll run boost around 10-11psi @ 6000rpm. In the lower rev range when you plant it you boost will be the standard 7-8psi and then go up with rpm. Better known as boost creep or overboost. Porting the wastegate didn't solve the problem for me either...although I don't call that amount of creep a problem because it never goes over it. I've only know one other person who has had problems with running a 3" exhaust...he fixed his problem by changing the position of the wastegate pressure feed to before the intercooler instead of after it like the GTX setup is.
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The dual stage behaviour isnt boost creep. The factory system has a wastgeate solenoid plumbed in which will open the wastegate at 7.1psi upto 5300rpms and then close again for a bit of extra punch after you reach 5300rpms. If you dont have that solenoid plumbed in then you wont have that behaviour.
You cant really put a figure on the exact point the ECU cuts as it will differ based on environment. Based on mals setup we have observed a boost cut of around ~0.82bar and this has varied based on what pump fuel is used. Obviously the GTX doesnt havent a MAP sensor so I dare say that there is no "static" boost cut. It will depend on engine load, air flow, 02 reading, air temp etc etc etc.
When Mal had a leak in his system, we experienced cuts at different RPMs which demonstrates the stock ECUs ability to adjust based on different conditions and hence saving your motor.