Engine Tech: Compression Ratios and Cam Specifications

Ben Alameda - Compression Ratios and Cam Specifications


1) AVOID USING A RACE TYPE CAM WITH STOCK COMPRESSION.
The resultant loss in cylinder cranking pressure will result in decrease engine response and sluggish acceleration.(ex. stock cranking pressure-160psi. after cam installed, cranking pressure is now 135psi. pressure drop because of the valve overlap and duration of the cam.) Use a street type instead. Less duration and overlap. If you must insist, try advancing the intake centerline or advancing the cam to improve low and mid-range throttle response.

2)AVOID USING HIGH COMPRESSION WITH A STOCK CAMSHAFT.
The higher compression pistons will yield excessive cranking pressures that will detonate the engine as too much DYNAMIC COMPRESSION is achieve. If your compression is too high for your “mild” street cam, try retarding it to reduce dynamic compression pressure present. this will also make it rev a little higher…

3) DYNAMIC COMPRESSION: this is the actual compression ratio achieve after the valve closing events are taken into the consideration. This is the working compression the engine sees!

4) Competition Compression Ratios of 15.5 to 1 or higher is achievable by manipulating very high ratios with cam duration! (Intake valve closing in the compression cycle).
utilizing “High” dynamic compression ratios with cam timing the sharp racing engine builder can achieve 1.9-2.4 horsepower per cubic inch!!!
ex. 2.5 liter(150cid) 150×2.4=360 horsepower NORMALLY ASPIRATED
NOTE: Mechanical compression ratio=10.1. duration and overlap increases in valve timing has produced an intake valve closing way past BDC resulting in delayed valve closing and producing a dynamic compression or working ratio of 8.8-1 ! this additional cam timing effectively drop the dynamic pressures in the cylinder and thereby had a measurable effect on performance….

5) BE VERY CAREFUL!!! when you have achieve very high compression ratios as to cylinder chamber volumes. ex. 16.1 comp. with 40cc heads. You flow tested the heads and in the search for more airflow you have increased your head CCs by 10cc. from 40cc -50ccs. YOU HAVE VIRTUALLY MADE YOUR ENGINE NOT COMPETITIVE ! BECAUSE NOW YOUR COMPRESSION STANDS AT 13.1 ! I DON’T KNOW ANYONE THAT CAN EFFECTIVELY COMPETE AT THIS LEVEL.A LOSS OF 3 COMPRESSION POINTS IS DEVASTATING…EX. 10.5-1 ENGINE AND HEADWORK FOR FLOW ALSO INCREASED 10 CC’s ( BECAUSE OF CHAMBER MODIFICATIONS FOR FLOW) WILL ONLY LOSE TO 9.25-1! A LOSS OF A LITTLE OVER A POINT OF COMPRESSION….What you gained in headwork you lost in compression. probably now your worse off.

6) Competition engines are very sensitive in combustion chamber volumes. The sharp engine builder knows a cut to clear for a valve pockets might have disastrous consequences to power. the added CC from this will drop compression!

7) To summarize this section. The higher the compression ratio, the less tolerant it is to added volume. at my level of compression ratio and chamber volumes.
EXAMPLE: BEN ALAMEDA RACING V-8 496cid 1200 horsepower
Normally Aspirated
2×1250 cfm Holley Dominators
Cam Lift approx. 900+ @ valve
Duration Adv. approx 330+ degrees
Lobe Centers ? no comment
Valve Spring Press. 450lbs at valve closed
1025lbs @ max lift.
Compression Ratio ? very very high…
Shift RPMs 9400-9600.

IF THIS ENGINES HEAD GASKET THICKNESS-VOLUME IS INCREASED BY JUST 5cc’s. I WILL LOSE 2 COMPRESSION POINTS!!!

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