![]() Static compression is theoretical, in that its based on the piston stroke compressing the cylinder volume trapped above the piston from bottom dead center to top dead center being compressed into the combustion chamber and head gasket volume and you also need the piston deck to block deck heights and piston dome or dish volume, you need to know that static compression volume to start your calculations, its theoretical because all modern cams use timing figures that don,t close both the valves at BDC, allowing the full cylinder volume to be compressed. I generally look to see about 145-160 psi on a street gas compatible engine compression test, but even if the reading are down near 120-130 psi I look more at cylinder consistency that the number ![]() Ive found the KB calculator to give good results On a N/A engine you'll rarely if ever exceed the static compression, with the effective dynamic compression, but you can markedly reduce it with cam timing changes, that delay the valves closing as the piston compresses the cylinder volume. Uncontrolled detonation will rather quickly damage pistons and ringsĬylinder pressure is greatly influenced by the cylinders,displacement, and valve timing as well as the static compression ratio Your fuels OCTANE or tolerance, to heat before you tend to get into potential detonation, issues limits the effective compression. Usable torque the engine produces tends to go up as cylinder pressure increases Ive built enough engines that I think I can post a bit of info regarding the effect of the need to consider dynamic compression, in the engine planing If you had 8:1 compression, and boosted compression to 12:1 you could theoretically expect a 11.5% boost in torque as a result What the chart below shows is the theoretical gain in torque with an increase in compresion Measure the piston deck height after the machined block has the rotating assembly test fitted You just cc the heads, combustion chamber volume, place the piston 1" down the bore and seal the rings gap around the pistons,Ībove the rings with moly grease or Vaseline and measure that volume and calculate what a bore diameter cylinder 1" tall wouldĬontain, minus the volume you,ve measured, the piston dome took up.then calculate the true compression. ![]() , if you have any engine and want too find the true compression you need to deal in verified fact, not assume what the manufacturers suggest is always correct. Keep in mind a huge percentage of the details you need to understand is contained in the sub links so don,t just skim thru, take the effort to read the links also as youll save hundreds of dollars and weeks or nu-necessary work doing so in avoiding potential problems Your engines tendency to get into detonation is the result of higher effective compression that the fuel octane your using can permit, but your piston is only compressing the fuel air mix, trapped between the rising piston and combustion chamber, AFTER both valves seat.and that normally happeneds well after the piston passes BDC and starts back up the bore.ĭetonation is a condition that occurs as the result of both cylinder temperature compressions effect on your fuel/air ratio and several other factors that cause fuel to self ignite under some conditions, lowering the EFFECTIVE compression, lowering the heat in the combustion chamber, or designing the engines quench,squish,turbulence and other factors to greatly reduce the engines tendency to get into detonation will obviously help prevent it.īut you need to understand it before selecting a combos components , Its obvious some of the guys on this site need to understand the difference between static and dynamic compression ratios, and that's understandable as its a difficult concept to grasp at first.
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