I see something missing, there's no mention of cylinder head upper chamber size - normally given in cc's. Since different heads may have larger or smaller upper chambers, this can increase or reduce the space that air and fuel are being compressed into. That in turn can cause radical changes in final C/R, with the result that absolute compression at the top of the stroke can vary widely between different heads, even if no other changes are made to the engine.

OOPS! Wait a minute - I just saw it. You were going with a standard head with a 78cc chamber.
But I don't use online calculators as much as I do plain old math so I thought I would pass on what I know...

Because of this - you need to know what size upper chamber you have before shopping for a set of nifty heads out of a performance catalog. If you go from an "Open Chamber" design, to a "Closed Chamber" design, such as a "QUENCH" head, and you were already riding close to having detonation issues: the reduced upper chamber volume might in fact be great enough that you won't even be able to get it to light off! (at least not in any useful way)

Alternatively - I have known of shops that purposely lowered compression for vehicles intended to run on low octane fuel such as PEMEX in Mexico

So the basics (for me) of Compression Ratio have always been total volume, including upper chamber shape and volume, plus any dishing, with the piston at bottom dead center as a first factor.

The second factor is total swept volume - which is the total amount of volume left after the piston has arrived at top dead center.

*Some people will disagree with me there, and define total swept volume as the amount of volume displaced by the piston as it moves from the bottom to the top of the cylinder, I may have used the wrong term. Perhaps "RESIDUAL" volume better describes what I am getting at.

*Also - depending on what rods were used, and the crank itself, the piston edges may not be level with the top of the cylinder, but slightly below. THAT upper cylinder area that is not used ALSO has to be accounted for.

A THIRD factor is volumetric efficiency, which even without a turbo or supercharger can vary from as low as 80% with an average stock set of heads and intake plumbing on up to 135% with a four valve per cylinder type of head with a good intake and tuned valve timing.

~Indeed, the attraction of the 4V setups is that they behave (in the RPM range they are tuned for) as if the WERE turbo or supercharged.

Once you have those three factors, you can divide the total volume by the volume left when the piston is at top dead center and arrive at what percent of the total volume is the end result. Depending on how you do the numbers you will have a fraction, such as 1/10, or you will end up with a percentage as a factor, and you can divide that into 100 to yield an end result.

To get a fraction (ratio) you would DIVIDE residual volume INTO total volume.
Example: TV/RV= ?/??
Example 2: If I have 10 CI as RV, and 100 CI TV, 100/10 = 10. If you cross factor you get 10/1
Example 3: 10 CI RV and 90 CI TV becomes 90/10 (90 divided by 10) = 9

Transposing the numbers - look at example 3. If I divide ten by 90 I get 0.111... which is actually eleven point one percent. Divide 11.1111 into 100 and I get 100 / 11.1111 = 9.000900

Once you have that, multiply by your estimated or measured VE, and it will give you the actual compression achieved

Have I lost you yet? Try this one:
Since upper chambers are measured in cc's, and most of us are familiar with cubic inches, you may have to translate the upper chamber volume into CI first.

Honestly though, the math might seem complicated at first glance, but it really isn't anything beyond the abilities of an average middle school student.

Just make sure you don't leave anything out, and make sure all of your measurements are in "the same language" IE: Don't cross metric with imperial in the same formula, cc's and CI's are totally different standards


What would really bomb your clyde is figuring total cylinder volume given a slightly enlarged bore and stroke, but that is a nightmare for another day. Factoring the area of a circle is the least of it...

I love cars and trucks, they are the most fascinating toys!
~ and I used to hate math in school