Postby SgtRauksauff » Fri Dec 16, 2016 12:59 pm
my gut says it's cam profile and timing. when I built my 16v, with stock cams, there was the typical dropoff after about 6700 rpm or so. when i put on the Toda cams with longer duration, the whole curve moved up, but the drop in torque was less pronounced. I agree that with a 12% increase in cylinder volume, duration and overlap of the cams can have a more significant effect.
I don't really know a lot about the VVT system and exactly what it does, but it looks like it just advances or retards the intake cam, so you're changing your overlap between exhaust and intake cycles, and starting the intake event a little sooner or later. duration and lift of the valve opening will remain the same, so it could be reaching the limit of air that it can move into the system.
The VVT solenoid works based on RPM, and allows oil flow to the VVT cam gear, correct? Do you maintain the same oil pressure at the higher rpms, or does it drop, maybe causing the overlap to increase, reducing the volume of air/fuel mix in the combustion cycle?