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fitting the J valve prior to mapping makes it a worthless mod!
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Exactly.
Same reason as quoted by PhilR above... he explained it perfectly.
If you fit the 'J' before mapping then it will be mapped with that valve to the same boost level as it would have been with the OE valve and 'lost in the system' so-to-speak.
Remember, the N75 just controls boost, it doesn't limit it. Your wastegate itself and the fuel/ignition/boost maps do that. It just happens to be slighlty higher rated than the OE one which means when fitted, it will increase boost levels (very slightly) across the range of the map that was written for the car.
It will yield improvements on any car it is fitted to, they will just not be as noticeable on a standard (non-chipped) car.
I fitted mine a while after having my car remapped and although I wouldn't say it made a massive difference, it did make a nice improvement across the rev-range with the car holding boost very slightly longer at the upper end of the rev-range. All but forgotten about now though since I got used to it /ubbthreads/images/graemlins/grin.gif
A very cheap and worthwhile mod in anyone's book though I think /ubbthreads/images/graemlins/smile.gif
Cheers
T