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# Compression Ratio Help

 06-14-2010, 07:08 PM #1 TECH Fanatic Thread Starter iTrader: (9)   Join Date: Jul 2005 Location: ASU, AZ Posts: 1,471 Compression Ratio Help I punched in my cam specs in the calculator above at all three locations Here is the ratios I got calculated for me: Static Compression Ratio 10.37:1 Dynamic Compresion Ratio 7.91:1 Rod to Stroke Ratio 1.68:1 Bore to Stroke Ratio 1.08:1 Can someone explain to me meaning of those ratios, the good and the bad? Also how can I make the better? The Car is a stock LS1 LTs ORX, Fast 78mm, Stock TB, 853 heads, El torro cam. Thanks, Dakota
 06-25-2010, 02:34 AM #2 TECH Resident   Join Date: Dec 2004 Location: K-W, Ontario Posts: 845 I'm not sure how much detail you need, but here's a start: The static compression is a fixed mechanical ratio between the volume of the cylinder when the piston is at Bottom dead vs. Top dead center. This value alone means very little until you compare it with dynamic compression. Dynamic compression indicates how much of the static ratio is actually used when the intake valve closes on the compression stroke. This is a function of the camshaft design and phasing with the crank. By looking at the static and dynamic numbers together you can see how aggressive your valve timing can be and estimate power curves. Based on the numbers you show, it's telling me your intake duration is fairly high (285 ish) and your torque will peak late in the band (let's say 3800 RPM). Sub 8's for dynamic is safe for gasoline; some will say 8.5 or thereabouts is pushing the envelope for pump grade fuels. You're not too far off the mark for a street / strip engine. In most cases, if the cam duration is high the static compression will increase to get more squeeze to produce enough heat, to help light off the mixture and create decent force during the initial degrees of the power stroke. Another sign to look for is the spread between the static and dynamic numbers. If the static is higher while the dynamic stays fixed, you can estimate that the engine will be efficient in higher RPM ranges. IE: 8.0:1 DCR with 10.0:1 SCR will tend to operate in a lower band than 8.0:1 with 12.0:1 SCR Bore and Rod stroke is another novel, and a gray area for most (me!). I've read different opinions on the use of these ratios. More bore is a plus for unshrouding valves and getting more air into the cylinder. More stroke allows for higher piston speeds at lower RPM (lower torque peak all else being equal) A combo of the two creates your desired displacement. Comparing similar cube engines: An engine with a longer stroke, smaller bore tends to peak sooner because the piston speeds are faster allowing more air to move at lower RPM. Downside is stress on the reciprocating parts (g's). An engine with short stroke, big bore will peak later because the same piston speed requires more RPM. RPM is rolling the dice with engine longevity...but the trade-off is chamber design (bigger valves, air entry, etc.) Then of course, most of the above is all dependent on the cam, heads, intake, exhaust setup so don't bet the house in all scenarios. Last edited by Adrenaline_Z; 06-25-2010 at 01:19 PM. Reason: spelling and errors. Remove 'cycle' from Power Stroke
 06-25-2010, 11:54 AM #3 TECH Fanatic Thread Starter iTrader: (9)   Join Date: Jul 2005 Location: ASU, AZ Posts: 1,471 Cool man, that was just what I was looking for. I really wasn't sure what those things meant until now. Thanks, Dakota

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