[Editor’s Note: This article originally ran in the October 2008 issue of Grassroots Motorsports, but we felt the method for measuring compression ratios was worth mentioning again.]
Once you’ve finished building an engine, the bombardment of questions soon follows: What camshaft did you use? How much power do you think it makes? Why is it leaking oil already? Unfortunately, you might not …
We started the compression ratio measurement process by setting the head upside down on our work bench. While we could have made some sort of fancy stand to hold the head, we’ve found that two blocks of wood work just as well. In this case, the valves were fully installed with springs and keepers. If they weren’t, we could have applied a small amount of grease to their edges to seal them in. We also installed a spark plug so that liquid would not leak out of the combustion chamber.
Then we made sure the head was fairly level.
In order to seal off the combustion chamber and provide an accurate volume reading, we applied a small amount of grease to the outer edge of the combustion chamber, making sure not to get any grease inside. This grease will keep the plastic plate sealed down, preventing liquid from escaping. Before sealing the plate to the head, we needed to figure out where to put the filler hole. Since we’d be using water to displace the air in the chamber, it was best to give the air a natural escape route. Our level told us which edge of the chamber was higher than the others; we placed the hole near the high edge.
Time to bring in our 100cc burette—but first we needed to zero it. We filled it with water well past the top mark and then drained it until the water reached the zero line. Because water has significant surface tension, we could see it creep up the sides of the burette. We had to make a decision: Read the top edge or the concave bottom as zero? Either way works, but you have to be consistent. Once zeroed, we set the burette in position above the head.
We placed the tip of the burette inside the plastic plate’s hole and began slowly pouring water into the combustion chamber. As the chamber filled, we occasionally needed to slightly tap and reposition the cylinder head to shift some small air pockets. Once all of the air bubbles were gone, we shut off the burette. We could now read the burette to determine the volume of our combustion chamber.
This process is fairly accurate and easily replicated. We’ll usually repeat it to confirm our numbers. Also, we’ll often check each combustion chamber to make sure the findings are consistent.
Do the Math
After measuring the cylinder head’s volume, we used the same process to measure the volume of the head gasket and the volume of the top of the cylinder. Now we could measure our engine’s compression ratio using the following data:
stroke: 8.9 cm
head volume: 38.6cc
gasket volume: 4.5cc
piston in block volume: 10.6cc
swept volume (one cylinder):
π x (.5 bore)2 x stroke
π x 4.052 x 8.9 = 458.4cc
clearance volume (one cylinder):
head volume + gasket volume + piston in block volume
38.6 + 4.5 + 10.6 = 53.7cc
(swept volume + clearance volume)/clearance volume
(458.6 + 53.7)/53.7 = 9.54:1
Extra Credit: Dealing With Pop-Up Pistons
If your pistons are domed or rise above the deck of the block, you need to do some extra math before computing the engine’s compression ratio. First, calculate how much volume those pop-up pistons displace. Then, subtract that figure from your combustion chamber and head gasket volume amounts.
For flat top pistons that rise above deck height, the calculation is quite simple. Just use the formula for calculating the volume of a cylinder:
π x (.5 bore)2 x height
Height, in this case, is how much the piston rises above deck height.
For irregularly shaped pistons that rise up above the deck, you can either consult the piston manufacturer or measure it yourself using the trusty burette. Here’s the trick: Mount the piston in a piston ring compressor so that the highest point of the piston is flush with the top of the compressor. Use a burette to determine how much volume is not filled by the piston. Subtract that value from the above formula. (and in this case, height is measured to the tallest point of the piston above deck height). This final figure is the volume displaced by the piston’s dome.