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93 Octane can utilize a pretty high comression ratio (12-13:1) *only* if the camshaft has been ground (custom or bought from a parts supplier) to compensate for the higher static compression ratio.
The static compression ratio is the cylinder volume + the combustion chamber volume divided by the combustion chamber volume. These are the numbers you regularly see posted.
But the compression ratio that counts as far as fuel octane is concerned is the dynamic compression ratio or for purposes of simplification the 'effective' compression ratio. This is the compression ratio measured at the point at which the intake valve closes. The volume of the cylinder is smaller because the piston has already started moving up on the compression stroke.
The reason that this ratio counts is because there is little to no pressure (compression) in the cylinder until the intake valve is closed and the combustion chamber is completely sealed. For an engine running 93 octane (combustion chamber design, size of the intake and the exhaust ports, piston crown coatings and the general design of the engine alters this greatly) you can usually get away with an effective compression ratio of around 7.5 - 8.0:1 (some cars even higher, Honda's are built quite well).
What this boils down to is the intake cam duration (and therefore the point at which the intake valve closes) is for the most part dictated by the static compression ratio. Calculating this ratio requires some trigonometry, unfortunately.
High compression ratios benefit the power of the engine at higher RPM's so it is best to have a 'built' shortblock and the rev limiter set to a new, higher maximum RPM (redline). This allows you to spend more time in lower gears, thus taking advantage of the higher torque multiplication.
Thanks,
-Scott Tucker
The static compression ratio is the cylinder volume + the combustion chamber volume divided by the combustion chamber volume. These are the numbers you regularly see posted.
But the compression ratio that counts as far as fuel octane is concerned is the dynamic compression ratio or for purposes of simplification the 'effective' compression ratio. This is the compression ratio measured at the point at which the intake valve closes. The volume of the cylinder is smaller because the piston has already started moving up on the compression stroke.
The reason that this ratio counts is because there is little to no pressure (compression) in the cylinder until the intake valve is closed and the combustion chamber is completely sealed. For an engine running 93 octane (combustion chamber design, size of the intake and the exhaust ports, piston crown coatings and the general design of the engine alters this greatly) you can usually get away with an effective compression ratio of around 7.5 - 8.0:1 (some cars even higher, Honda's are built quite well).
What this boils down to is the intake cam duration (and therefore the point at which the intake valve closes) is for the most part dictated by the static compression ratio. Calculating this ratio requires some trigonometry, unfortunately.
High compression ratios benefit the power of the engine at higher RPM's so it is best to have a 'built' shortblock and the rev limiter set to a new, higher maximum RPM (redline). This allows you to spend more time in lower gears, thus taking advantage of the higher torque multiplication.
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