englertracing wrote: ↑Tue Aug 11, 2020 10:51 am
we don't want pictures, we want video
Not only will I deprive you of video but downgrade the thread to text for this reply...
As many people hot-rodding 1987-1995 Porsche 928 S4/GT/GTS engines already know at this point, the stock S4/GT/GTS active resonance intake manifold doesn’t really want to make power above 6000 rpm. It's great in the 2000-6000 rpm range though.
I just recently saw some data where someone had dynoed otherwise sensibly tuned normally aspirated 928 GT 5.0l engine (11.5 compression, E85, Colt 2 cams, headers, exhaust) and picked up literally 100+ hp in the 6000-7000 rpm range by swapping the S4 intake to 48mm AT Power individual throttle bodies with trumpets and no other changes (except ECU calibration, of course).
So even with valve events timed appropriately for higher rpms with bigger cams, the S4 intake manifold reduces the volumetric efficiency of the engine pretty dramatically above 6000 rpm. Furthermore, the penalty gets bigger the higher the rpms.
Now, since we’ve got turbos, we can literally force feed the engine whatever medicine we think it needs thru the reluctant intake manifold. The question is how much do we need to ramp up the boost at high rpms, starting from 6000, to keep the torque curve about flat?
The engine produces about 19.7 lbf-ft per psia at 6000 rpm, 17.3 lbf-ft per psia at 7000 rpm, and estimated/interpolated 14.3 lbf-ft per psia at 8000 rpm. From drivability and traction perspective, we want flat torque curves and will employ low torque curves at low gears and high torque curves at high gears. This is how much I think we need to ramp up the boost:
1) To produce a flat torque curve at 500 lbf-ft, the system needs to generate 10.7 psi boost at 6000 rpm and 20.3 psi boost at 8000 rpm.
2) To produce a flat torque curve at 600 lbf-ft, the system needs to generate 15.7 psi boost at 6000 rpm and 27.3 psi boost at 8000 rpm.
3) To produce a flat torque curve at 700 lbf-ft, the system needs to generate 20.8 psi boost at 6000 rpm and 34.3 psi boost at 8000 rpm.
The "bad" news is that with the stock intake manifold, the existing data says that the boost ramp up after 6000 rpm needs to be very steep. Much steeper than I expected before running the numbers, we're talking about numbers like 5 psi extra at 7000 and 10 psi extra at 8000. Which instinctively sound like high numbers, I guess substracting the 14.7 psi when going from psia to psig has a large psychological effect!
The good news is that torque/absolute pressure is almost perfectly linear in rpm up there so a linear correction will work really well and that’s much easier to calibrate than a full rpm-specific boost profile.