Quote:
Originally Posted by Cloudcapp
Not true. It doesn't matter whether it is FI or NA.
It serves two purposes: 1. Keeps backpressure at low end for performance and 2. Reduces noise.
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That flap is certainly there to reduce noise.
As far as it being there to control back pressure, you may be right. I am no expert, but I don't know if that's as important in a FI engine.
The way that I understand it, in a NA engine you want to control the sudden release of exhaust gases so that you don't slow it down enough to interfere with the next cycle. If the volume of space the gases have to fill is too large the gas will slow down causing back pressure and a loss of power. This is more likely to occur under low RPM.
However, in a FI engine, the turbo results in a steadier stream of gases from the engine, and since the turbo uses the differential in pressure to produce boost the lower the back pressure the more efficiently the turbo can operate. Thus for a FI engine freer flowing is almost always better.
There was an interesting post here awhile back about this mod and its effect on performance.
http://www.xbimmers.com/forums/showthread.php?t=341367