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Old 01-19-2013, 01:15 PM
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You cannot just increase the cam timing or the intake valve closing point to adjust a high compression ratio engine to run on pump gas. it is not linear-progressive. The in cylinder pressure is high at running speed.
When the engine is actually running at speed the air does start to compress before the intake valve closes when its running at speed. Why ? because the air is still moving into the cylinder as the piston is rising.

Air is only bleed off at low engine speed. The exact point of compression and therefor the net resulting in cylinder pressure and temperature is very variable in the actual running engine based on many factors besides the intake valve closing point.
While its important to give an engine enough mechanical compression ratio, it is not a linear equation.
On a real street engine that has to operate in less than idea conditions, and for longer than a quick dyno test or 1/4 mile blast after complete cool down, once you get much beyond about 10.5:1cr the chance if it running correctly on pump gas dimishes REGUARDLESS OF CAM TIMING.

Most people won't be able to use 11:1 reliably on North American pump gas.
If you buy into this DCR calc consider it EXPERIMENTAL--very limited use. And you'd best have some method of detecting or anticipating detonation
and be able to adjust the ignition timing (retard it) on the fly.

And non computer controlled (no method of knock detection and on the fly-automatic spark timing adjustment) 11:1 cr engine needs about a 98 to 100 R+M/2 octane fuel for reliable use on the street. That has not changed in real terms in 100 years of 4 stroke internal combustion engine history.

Last edited by F-BIRD'88; 01-19-2013 at 01:33 PM.
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