Real World Compression Ratios

Compression ratios can be understood in two different ways: static compression and functional compression. Static compression is based ONLY on cylinder volume versus crankshaft movement. Static compression is fairly easy to determine. Functional compression accounts for cylinder volume, crankshaft movement, AND valve opening. This is how you figure “real world” compression numbers. Because air flows differently into a cylinder at different RPM’s, it is very difficult to know your functional compression ratio at every point in the RPM range.

When figuring your static compression ratio, you need to know the following:

Using the measurement for bore and stroke you can figure the volume of the cylinder. Using the same formula you can also calculate the volume of the compressed head gasket. Now you take your stoke and the compression height of your piston and add them together. For most motors, you will have a combined length that is less than the deck height of the block. You must then find the volume of the remaining room in the cylinder. Should the piston rise above the deck of the block, you’ll need to factor the volume of the head gasket into into your work. You should now have the volume of the cylinder and the excess volume. The excess will factor into your combustion chamber size.

Finding the volume of your piston’s dome or dish can be as easy as getting the specification from the supplier or as reliable as using using Archimedes’ principle of density. For those that don’t remember, that’s where you put an odd shaped object into a liquid and see how much the level of liquid changes. In this case, you’ll need to put a piston into a cylinder and seal around it so the water won’t run past. Heavy grease such as used with wheel bearings tends to work well. Measure the distance from the deck of the block to the “top” of the piston. Use this height in the volume = πr²h equation to find out how much liquid should fit into the cylinder. Using a graduated beaker, fill the cylinder with water or a liquid of your choosing. Be sure to measure how much you pour in. The difference between what you put in and what you found for total volume is your dome or dish volume.

Using the same method as you used to check the dome and dish volume, you can check your cylinder head combustion chamber volume.

Next you take the volume of your combustion chamber + the volume of your head gasket + the volume of the cylinder above the compression height of the piston and get your total combustion chamber size. You’ll then add more if you have dished pistons or subtract from it if you have domed pistons.

Using the volume of the cylinder you got from your bore and stroke, you divide by the volume of your combustion chamber to find your static compression ratio. So a cylinder volume of 100cc and a total combustion chamber volume of 10cc results in a compression ratio of 10:1.

Something to keep in mind, and you can factor it in as well. The piston isn’t tight in the cylinder. The gap around the piston from it’s top to the top of the compression ring is truly part of the total volume of the combustion chamber. This area is largely ignored as it doesn’t burn well and the air trapped there tends to circulate very little. This does lower the compression ratio slightly.

When you attempt to calculate your functional compression ratio, you need the following:

This is something very few individuals are able to fully calculate. There have been many tools made to measure air flow through a motor in an effort to calculate functional compression ratios. In recent years, some manufactures have developed advanced computer software to make these calculations. The most commonly done, is to factor in the leak down percentage and degrees of valve overlap.

You remove the percent of leak down from the total cylinder volume and an additional 0.5% for every 3.6 degrees the exhaust valve is open after the crank shaft passes top dead center. Yes, the piston will be traveling down with the exhaust valve open, but the exhaust gas is already flowing out of the motor and is drawing some intake air flow along with it. This is a very controversial point in factoring functional compression ratios.

Based off this, a cylinder of 100cc with a static compression ratio of 10:1 with 5% leak down and 10 degrees of valve overlap after top dead center, would have a theoretical functional compression ratio of 9.36:1.

To recap, what we found using the 100cc cylinder with 10cc combustion area was a static compression ratio of 10:1. With a functional compression ratio of 9.36:1 on the same cylinder. This drop in compression is very typical. When checking your motor, be sure to use the same set of measurements at all times. Remember that even NASA lost a Mars rover when the metric to SAE conversion was wrong.

If you have looked at our high compression performance pistons for Fiat 124 and wondered if they would be too much for your car, be certain that we have done extensive compression calculations and actual WET testing. We now have numerous successful performance engines on the street using pump gas and our pistons, and not only are they safe and reliable (with a properly tuned fuel system), they are freaking rockets!

Happy crunching,

Andy