basically if the car is mapped with bung in, then it needs to be driven off boost with bung out and visa versa.. I have seen a big drop in power with bung inserted and the back pressure makes a huge difference to exhaust gas removal from the cylinder and therefore how clean the charge is etc.. which influences how much ignition advance is possible to run and therefore the detonation level point and the boost is altered.
I have seen car mapped without bung totally safe with the bung fitted as it drops the boost at the same time due to the increase back pressure but I wouldnt like to assume that would be the same with every car. When I map a car if I know there isa bung for the exhaust I tend to map it in the configuration the owner drives the car in most of the time and double check it is safe in the other configuration.. generally they will make more power with the bung out.. with varying amount of alteration to the power depending on the power output of the car. ie. if it is a 280bhp car once mapped then the bung is going to lose very little power.. if it is 600bhp then expect a big drop in power with the bung.
Thanks
Simon
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