Engine Compression Ratios:  What They Are, How They Work

One of the most commonly asked questions we hear at Delkron is: “If I use these parts what will my engine’s compression be?” This seems to be one of the most misunderstood areas of building a performance engine. In this short series of articles we will discuss both types of compression ratios—Mechanical and Corrected—and how to calculate them.

First let’s understand just what compression ratio means and how it affects the internal combustion engine. Compression ratio is simply the volume of the cylinder and the volume of the combustion chamber of the cylinder head when the piston is at Bottom Dead Center (BDC) and the volume of the cylinder head combustion chamber when the piston is at Top Dead Center (TDC). Let’s use a hypothetical engine to make things a little simpler. If we have an engine at BDC that has a swept volume of 900cc in the cylinder and a combustion chamber volume of 100cc, then this volume is reduced to the 100cc of the combustion chamber at TDC. This would be a compression ratio of 1000:100, or reducing it fractionally, a compression ratio of 10:1.

Compression ratios can be a double-edged sword in many ways. First, the higher the compression the more power the engine will make. This is due to being able to extract greater mechanical energy from a given amount of air/fuel mixture that is created by its higher thermal efficiency. Higher compression ratios place the molecular structure of the fuel and air into a smaller area, along with the adiabatic heat of the compression, which causes a greater evaporation and mixing of the fuel droplets in the combustion chamber.

High compression engines make great power, but it needs to be understood that engines with higher compression require fuel of higher octane and grade. Low octane, low grade fuels can cause severe and irreparable damage to an engine due to detonation. Detonation is caused when the fuel self-ignites under compression—not during the firing phase of the ignition system. Detonation can be the cause of connecting rod failures, piston failures, and more.

Just as high compression wants better fuel, the other side of the coin is running higher octane fuels in low compression engines. Running high octane fuel in a low compression engine is, well, throwing good money down a black hole. You are not going to make any more power than you would using the correct, lower octane fuel due to the fact that the lower compression engine just simply does not have enough compression to support the higher octane fuel. Running high octane fuels in a low compression engine many times is the reason riders bitch and complain about tuning issues of carburetors, ignition systems, etc.  In many cases the only reason that the operator feels more power is due to the fact the he has spent more money so it must be working!

A simple rule to remember is that the lower the octane the faster the burn, and the higher the octane the slower the burn. This is why high compression engines like higher octane fuels because they burn slower and are not as prone to self-ignition, or detonation. The same rule applies to low compression engines liking lower octane fuels; lower compression engines do not have to work as hard to light the fuel mixture due to the lower octane fuel burning faster with out a lot of compression.

Bedford, Ohio

Story and Photos by Steve “Posie” Pfaff, Delkron