The formula for calculating the compression ratio is:
CR = (Vs + Vc) / Vc
- CR is the compression ratio.
- Vs is the volume of the stroke. (π/4)b²s
- π is Pi or 3.1416.
- b is the cylinder bore.
- s is the stroke.
- Vc is the clearance volume of the head.
We are checking the compression ratio of a Steve Maney short stroke 750cc engine.
This engine has standard Commando conrods and standard 850cc pistons giving us a bore of 7.7cm
and a stroke of 8.04cm.
The volume of the stroke (Vs) is calculate by using (π/4)b²s or (3.1416 / 4) x 7.7² x 8.04 giving us Vs = 374.394cc
To calculate the clearance volume of the head (Vc) you will have to cc the head to obtain the volume of the combustion chamber (Hcc).
To cc a head you will need the following :
- A light colored liquid. We are using Chevron ATF fluid.
- A burette or something that measures ML or cc. One ml is equal to 1 cc. The Commando combustion chamber measures around 50cc for one cylinder, so a 100ml burette is best. The head we are ccing measured 54.6cc (Hcc), so if we had a 50cc burette, we would have had to re-fill the burette and add the two readings together to get an accurate measurement of one combustion chamber.
- Some grease. White grease is recommended, but we used the grease we had available.
- A piece of Plexiglas that covers the compression chamber with a hole in it to fill the chamber.
We have a piece of Plexiglas that covers both chambers for convince.
Think about how you are going to mount the burette before drilling the hole in the Plexiglas.
If you are using a normal burette stand, you may need to drill the hole near the edge of the combustion
chamber or in our case, the center since we mounted the burette over the center of the combustion chamber.
Mounting the burette.
- You need to spread a light coat of grease around the cylinder to be measured, or in our case both cylinders, since we are covering both with one piece of Plexiglas. The grease is used to stick the Plexiglas to the head and make a seal. The idea is to fill the combustion chamber with fluid to the parting line of the head and cylinder and the Plexiglas allows you to do this.
- Press the Plexiglas on to the cylinder making sure you have a good seal.
- If you do not have a stand for the burette, improvise some way of holding it in place. You will get very tired holding the burette by hand, since the liquid flows out very slowly, a drop at a time.
- Fill the burette with your measuring liquid. Fill the burette a bit above the zero line and dispense the liquid into something other than the head until it is just at the zero line.
- When the chamber is almost full you will probably need to gently tip the head to make sure all air bubbles are removed.
- Read the volume directly from the burette.
The head ready to cc. Filling the combustion chamber with ATF.
Both combustion chambers should be the same volume, if not you may need to modify one chamber to make them equal. We are not going into how to modify the combustion chamber in this article.
We now have the volume of the combustion chamber and need two other measurements to calculate the clearance volume (Vc).
- We need to know the thickness of the cylinder head gasket. The flame ring (eyeleted) head gasket (06-5051) is normally .040" thick.
- The volume of the piston (Vp) that sticks into the compression chamber. For this article we are using pistons with a flat top and this is the case with all the 850cc Commando Pistons that we sell.
Using a depth gauge to measuring the height of the piston above the top of the cylinder, we get 0.1".
Subtracting the thickness of the head gasket 0.1 - .040, we get .06" of the piston sticking into the combustion chamber.
Converting the .06" to cm. .06 x 2.54 = .1524cm for the piston height (H) in the combustion chamber.
The volume of the piston (Vp), (π / 4) b² x H is (3.1416 / 4) x 7.7² x .1524 or 7.0967cc
Assuming no valve pockets in the piston we can now calculate the clearance volume of the head (Vc). If you do have valve pockets in the piston, you will have to measure that volume and subtract it from the volume of the piston (Vp).
Vc = Hcc - Vp or in our case 54.6 - 7.0967 = 47.5033.
The compression ratio (CR) is calculated by using the formula CR = (Vs + Vc) / Vc, we obtain a compression ratio of (374.394 + 47.5033) / 47.5033 = 8.88:1
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