David,
First let me say that I'm honored to be mentioned in such esteemed company.
I found the Chevy article you mentioned interesting in that it assumes a
(too) high cranking pressure (for pump gas) and then stated the long
duration bleed off reduces the potential to detonate. Put that back into a
reverse perspective of your high compression, stock cam (reduced bleed off)
motor and the situation would only get worst.
There is likely an "ideal" or maximum cranking pressure (of which value I
don't know). Yes, increasing compression in and of itself does help with
engine output (up to the unknown limit). However, typically a hotter cam
(longer duration and typically longer overlap) requires a higher compression
ratio to get an equal cranking pressure of a stock motor. Often cam changes
are related to the ability of the intake and exhaust systems to pass
fuel/air. It would be interesting to see the same engine's output with only
compression ratio changes. Likely with most things "engine" other factors
play in, but it would still be interesting to see. Because of the cost and
labor I doubt that many are up to the task.
I found this article below. It is for Motorcycle engines, but there is
probably a correlation to the auto engine.
>>>When modifying an engine, it is important to match the compression ratio
with the duration of the cam. While additional cam duration can produce more
useable power, to much duration can hurt the performance. The problem is
created when too much duration results in lower cylinder compression
pressures at low RPM which reduce the low RPM torque and power.
Below is a chart that provides guidelines for engine performance for a given
range of static compression pressures. These should provide a good means of
judging if the cam installed in the engine matches the other components.
Static Cranking Pressure Performance Implications
Less than 115 PSI Poor low speed performance, poor throttle response, hard
starting. Compression Ratio and Cam not matched or worn rings, valves.
120 PSI to 145 PSI Expected range for most stock or modified street motors.
This is a good range for a street motor.
145 PSI to 165 PSI Modified street motors. Static pressures in this range
should produce good results. This is a good range for a street motor.
165 PSI to 180 PSI Marginal for street motors. Possible hard starting,
detonation and overheating.
Over 180 PSI These are all out race engines. A street motor in this range
will be trouble. Stick to the track. <<<
Although not directly related to your original premise I offer this: I have
a friend who works for Shelby as well as has an engine building shop called
Mitech. He once commented to me that you can only get so much torque out of
any given bore/stroke combination. I would assume this includes a limit on
compression. He continued to say that the ONLY way to get more HP was to
increase the RPM (assuming the torque curve moved appropriately). I can't
recall if you were looking for the greatest efficiency or power, but I
thought I would throw this into the soup.
Tom
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