Quote:
Originally Posted by Ecky
Aside from reducing RPM and increasing engine load (easily the best way to improve this), I would think another way would be to just reduce the volume on the other side of the throttle plate(s). One could:
1) Reduce plenum volume
2) Reduce runner length/volume
3) Advance the intake cam considerably, so the valves close early, meaning more of the vacuum production would be inside the cylinder and less in the intake manifold (smaller volume)
4) Individual throttle bodies (ITBs)
 |
1,2,4 are all basically the same thing. Only 4 would have a noticeable effect, and it would still be small. Once you have a plenum, there's a big enough path to other cylinders that the pressure becomes fairly stable. ITBs only trim a small % of the pumping losses (e.g. at runner volume = 1/2 cylinder displacement I'm estimating around 7% pumping loss reduction at typical idle, but that number gets better if you can drop the runner volume more). They also don't reduce pumping loss as much if you have LIVC.
3 has the obvious problem of residual exhaust. Every previous gen DI engine used internal EGR and were limited by the ~20% charge dilution limit. If you don't care about high end power, you can regrind the cam for very little duration to get EIVC, but it's better to go the opposite way and use LIVC like every OEM does, since that preserves top end power, and reduces pumping loss.