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Old 03-16-2006, 06:23 PM
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Quote:
Originally Posted by fearsomefairmont
You have to remember cam choice is not really a function of engine displacement as it is compression ratio.

Pontiac engines, being that the vast majority of them were made after '71, are severely detuned smog motors and later in the '70's had an abysmal compression somewhere around 8:1 or 7.5:1. So, cam choice has to be conservative in contrast. The heads flow similar to the average Chevy iron head.

I would say the average mildly built chevy has around 9.5:1 compression, so adjust the duration figures accordingly. Compression ratio and cam are fundementally related, as cylinder pressure is what actual makes power.

Andy

Even though this is an old post, I think its pretty much completely false. All V8 engines were underpowered and had low compression in the late 70's not just the pontiac motors. Most chevy V8's from 305 to 454 had a compression ratio of 8:1 and in some rare cases, even less.

First thing that's misleading about the above cam specs is that they really tell you very little about a cam. Just because a chevy cam and a pontiac cam have 214/224 duration @.050 really tells you very little. This tells you nothing about the ramp speed or overlap. That said, however, they probably are relatively similar cams, and the two motors will respond differently. Here's why:

A cam will make its most power at a certain RPM because of the timing of the intake pulses coming into the chamber. A bigger cam runs choppy at a lower idle because the intake pulses are not moving fast enough during the overlap part of the cam timing (when both the intake and exhaust valve are open) for the rushing exhaust flow to actually evacuate itself and create a low pressure area behind the exhaust valve to actually suck more itake charge in. Because the velocity is too slow, the engine is reburning a charge that already has exhaust gasses in it, and thus gives you the choppy idle of the "big cam."

THe reason a chevy motor and a pontiac will respond to a similar cam differently is because each motor's intake pulses are different at different RPM's. This has to do with the bore to stroke ratio and the head design, but also the length of the total runner. Each pulse constitutes a "runner's full" of intake charge. Given that the distance from the carb to the actual combustion chamber is different in either motor (most notably because a pontiac has a much wider intake manifold) the pulses are actually different amounts of air and fuel and are gotten at different RPM levels.

K
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