Calculate Your Compression Ratio

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.

[marklowe]

I have contacted youtube and demanded they allow multiple thumbs up just for your channel. I've learned a lot here. Very practical and detailed.

[TheCarPassionChannel]

Thank you for this, I knew the math with the volumes but it really helps to see all the measuring techniques executed!

[DENicholsAutoBravado]

When measuring your chamber size, accounting for the space below your valves, "...use gravity from whatever planet your from." Thanks for the humor!

Even if humor was missing, I'd still watch, but these little goodies help.

[jasonedge]

Caboose definitely deserves some wet food. Thanks for ur time and energy Jafro. Glad to see you're back at it. Best production automotive videos on YouTube as far as I'm concerned.

[poojadharankar]

I haven't seen any perfect videos like this on YouTube ..Thanks for your level of deep explanation..Thanks a lot..

[coc]

im not a mechanic and i love this kind of KNOWLEDGE sharing videos like what he did. but i wanna know what is the reason why some DISLIKE this video?are there something wrong with the calculation?can someone please give me a reason.

[brianwilson]

Fantastic work Jafro, I'm not an engine builder by any means but i did stay at a holiday inn express last night!

[allryzeme]

Wow man that was some impressive mind boggling information. I had to pause and rewind this video so many to actually concentrate and understand what was happening. Great job man... Keep it up.

[TGsrao]

if it were possible to give more than one like for a video, this would be it! Extremely brilliant explanation! Textbook perfect!

[lpngcf]

Your work ethic is out of this world! Brilliant!!

[pgrens]

Glad to see you're back, Jafro! Also happy to hear that the repair came in under what you anticipated. Hoisting a cold one in your direction!

[izumimarcelo]

I just wake up with my android YouTube notification and was forced to watch this. Really good video. Nice math bro. Keep the good work.

[gergorian]

YAY! So happy to see you back in action Jafro. Cant wait for the next video already.

[hadriansmith]

I got a Honda Super blackbird and all the things you have done I will try on the bike's engine. I spoke to an old man who used to tune exhausts for formula one cars in the old days. He said that they would make the headers out of copper pipe because it was easy to cut. Fit a long length to each cylinder. Start the engine and shine a strobe light down the pipe. Where the light shone on the pulse from the cylinder, they would know where to cut the pipe. They would measure the distance using a long piece of wire which they would slide inside the pipe to the pulse.

[markphilpot]

Say hello to the new math whiz! Enjoyed the video because I am a math major and I like these type things. Do I or will I remember it? No. That’s why you take notes. That way you have examples and even descriptions of things. Maybe your memory is infallible. Good for you! Mine isn’t so I take notes! You do what you have to to get where you want!👍🏻😊

[madunwerkstatt]

Just came across your video about calculating compression ratio.

I usually did direct measurements to total cylinder volume plus sweep volume using burette filled with low viscosity oil such as ATF or 0w-20 oil.
Oil poured through spark plug hole on both TDC and BDC.
Yes it's messy, but more convenient (at least for me).

[IanOberhotlzer]

Thanks for being so informative, comprehensive and easy to follow. Very helpful! Thanks for passing it on!

[uprise]

Thank for all your hard work. Your videos have kept me thinking positive while being deployed and glad you can continue making videos

[anthonywendt]

I always revisit this video for reference. Thanks Jafro!

[johnh]

GOOD DAY ALL : I'm a technician in the aerospace industry from Toronto Ontario Canada , I've seen the video . If you'd also like to calculate you're compression pressure is goes like this . To-days atmospheric pressure (ie 14.6 psi times you're compression ratio ie 9.5 :1 = 138.7 psi . Each day there will be a slight difference in compression pressure . If you live at sea level you'l have the highest compression pressure, if you live in "Denver Colorado" you might want some nice 11.5 :1 pistons to off set the atmospheric problem . Bye Bye all and have a nice day ! ! !