Calculate Your Compression Ratio

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• published: 26 May 2014
• views: 73190

This is everything you need to do to calculate your compression ratio. No foolin'. Every equation and process demonstrated. Find all your variables. Know your exact compression ratio in every cylinder. This is how you do it. Just because your service manual says your car is 7.8:1 or 8.5:1 compression doesn't mean that it is. Whenever there are casting irregularities, variations in piston height, parts that have been machined, non-OE parts, or changes to your head gasket selection, your compression ratio WILL change. It's highly probable that you're only CLOSE to spec if you've never touched your engine at all since it was "born", and that it doesn't MATCH spec. Even if it did, how would you know? This. 5 variables. V1 Swept Volume V2 Deck Volume V3 Piston-to-deck clearance V4 Piston dish cc's V5 Head combustion chamber cc's The ratio math: V1+V2+V3+V4+V5 = volume of combustion chamber at BDC V2+V3+V4+V5 = volume of combustion chamber at TDC The ratio is... (V1+V2+V3+V4+V5) ÷ (V2+V3+V4+V5) : (V2+V3+V4+V5) ÷ (V2+V3+V4+V5) or BDC ÷ TDC : TDC ÷ TDC First you fill in the variables, then you calculate volumes, then you add the volumes, then you reduce the ratio (fraction). It's that easy. Here are your magic numbers: 0.7854 = Pi quartered to the ten thousandth 16.387 = number of cc's in a cubic inch. If you divide any number in cc's by 16.387 it gives you inches. If you multiply any number in cubic inches by 16.387 it gives you cc's. Quartering pi lets you use the calculation: BORE x BORE x STROKE x .7854 = volume of a cylinder instead of... π x (BORE ÷ 2) x (BORE ÷ 2) x STROKE = volume of a cylinder Either way is right. You get the same result if you calculate pi to the ten thousandth. While I apologize for all the math, no I don't. I'm really not sorry. You actually clicked here for it whether you realize it or not. This is ALL the math, the tests, and the whole process to calculate your cylinder volumes and compression individually even if you don't know any of your variables yet. All of my numbers are present for those who want to calculate out the last 3 cylinders out of curiosity just to see how it affects cylinder volumes and compression ratios from one cylinder to the next. Why would I do that for you? Why would I deprive you of that practice? Just assume that all 4 of my combustion chambers are 41.75 ml if you do this. Clicking like share and subscribe helps a channel grow. It also motivates me. Don't sweat the camera. It's enough to know that so many of you care about what I'm doing here. From the bottom of my atmospheric dump, I thank you all! This gift horse's teeth are all over the place, but he sometimes poops gold nuggets. PS: Use ATF for your piston dish volume tests, not alcohol. Of course it's better just to use the spec sheet included with your pistons... but not everyone gets that luxury. Water is just fine for head combustion chamber tests. Dry and re-oil all parts that water touches.