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Old 10-12-2008, 07:51 AM
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Cylinder pressure

Is there any kind of chart that shows average cylinder pressure vs. compression ratio? I took some readings on my motor and they all were 210psi to 220psi.

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Old 10-12-2008, 10:34 AM
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atmospheric pressure x CR ought get you close, not considering any bleed off or etc. (e.g. 10:1 at 14.7 psi is the same as saying 10 atmospheres at 14.7 psi compressed into the space of 1 = 147 psi). There are many opinions to be found in this arena, some better than others. Fuel will become a consideration as well. Compression builds heat, and excessive heat from compression may lead to ignition of the A/F before spark if the fuel is not tolerant. This is going to get lengthy.
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Old 10-12-2008, 11:34 AM
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Two more things to consider; intake valve closing point and cranking RPM. Close
the valve sooner and pressure goes up; same goes for spinning the engine faster.
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Old 10-12-2008, 04:13 PM
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Quote:
Originally Posted by automotive breath
Two more things to consider; intake valve closing point and cranking RPM. Close
the valve sooner and pressure goes up; same goes for spinning the engine faster.
AGREE. Cam timing makes a big difference.
It is difficult to draw generalities about static compression vs. cranking pressures due to cam timing.

A little quad-four 95 Pontiac cranks at 225 and runs on cheap gas. (why?)
I've seen some late model Chevys that crank 160.

Saying that 175 is runable on 87 octane
and that 200 is about the limit for 93 octane
are both good generalities.
Special tweaks and exotic tricks can extend this somewhat.
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