With your setup, I'd calculate you are between 9.48:1 and 9.64:1 static compression ratio. a leak down won't tell you how much cc you have, only if you have a bad cylinder. You are using an out of date browser. Compression and Pressure Ratios The compression ratio is defined as the ratio between the volume of the cylinder with the piston in the bottom position, Vbottom (largest volume), and in the top position, Vtop (smallest volume). What are the common signs of low engine compression? You should start to worry if one of your cylinders is getting close to 100 psi. For a better experience, please enable JavaScript in your browser before proceeding. I also think your generic statements do nothing to help this post and are only taking space. the piston reaches the bottom and stays closed the whole way, and If you're doing a manual calculation, that usually means using cubic centimeters (cc). Please support our sponsors and let them know you heard about their products on Cruisers Forums. How do you convert compression ratio and psi? The definition of compression is the action or state of being squished down or made smaller or more pressed together. I have no way to know if its zero decked. No doubt, I am very familiar with that combo though. And hence all the above is why compression testing does not tell you a lot. If you have 155 cranking psi then you don't have "too much pressure". Let's take a look at how all these volumes interact by detailing the simple math that's used to determine compression ratio. As a rule, most engines should have 140 to 160 lbs. Two-Stroke engines should have a compression reading of around 110-130 PSI. Throttle position affects the accuracy, also. Low compression in one cylinder usually indicates a bad exhaust valve. You take the head ccs, then add the gasket ccs and then allow about ten thousandths for the deck. Like DCR calculations, cranking compression in psi gives us a number that represents the mechanical static compression ratio less the intake closing point of the cam. Heres the air pressure at varying altitudes: When you buy through links on our site, we may earn an affiliate commission. It looks like the pressures are too low. An engine's static compression ratio is only a rough indicator of its ability to tolerate a particular gasoline octane ratio. Heres one answer Ive found while searching: Since air pressure at sea level is 14.7psi then just multiply 14.7 times the compression ratio. I got the info from him but it was his first modular engine build so I am hoping he was misinformed about the "normal" comp ratio. you have what you need for static pressure vs compression ratio. I am swapping these heads for some Edelbrock Performer RPM heads. REAL way of telling an engines compression ratio without getting The CCP will drive the need for compression releases. A forum community dedicated to Nissan and Datsun Z car owners and enthusiasts. I calculated you compression ratio using the following: 4.030" bore 3.75" stroke 62cc chamber-6cc(estimated) for the .100 dome pistons.039" gasket thickness pistons .020" below the deck This calculates to 12.47:1! The specs for engines lists the static compression ratio. Is there a relationship between compression ratio and compression PSI checked at the plug hole? static compression can be figured out by math. Or bored over or the exact timing of the cam to ensure its advanced 4*. All of these elements must be measured with the same units. Thanks. Dynamic gas flows in the intake and exhaust alter the amount of air entering the cylinder.