Quote:
Originally Posted by slowmover
An RV cam emphasizes low rpm lift at the expense of duration (when done correctly).
Bruce Crower (to go back to the OP) was the one with high compression experiments in the late 60's/early 70's. What Yunick was doing was a bit different.
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During one of the gasoline shortages, in about 1978, I installed an RV Isky Mile-A-More cam in a 327 Chevy engine. Slightly less lift, but very noticeably (even by eye) duration. Side by side, the RV cam and the mld performance grind that I removed, looked like a Geo Xfi cam next to the stock Geo cam.
With less duration, the cranking compression noticably went up with the RV cam, low end torque went up, as well, making it very easy to drive slow and easy. My goal, bet with my dad, was to get 20+ mpg.
The problem with high compression and longer durations cams is exacty as has already been pointed out, you loose low rpm torque. I think there are several drivers, but reversion back up the intake tract is a huge one. During dyno testing of race engines, I've experienced this 'fog' above the velocity stacks, at lower rpms. Late closing exhaust vale event is the culprit. However, at higher rpms, this effect is less pronounced, the 'fuel stand off' visually goes away, cylinder over-filling goes up, and torque increases. VVT (which I've never played with) seems to fix this.