Compression ratio is limited by fresh charge heating.

If you’re nerdy enough to notice, you see that in general, compression ratios on air-cooled engines (especially if they have large bore) tend to be lower than on liquid-cooled engines, especially those with moderate to small bores.

The higher the compression ratio you can safely run in an engine, the greater the torque it will produce. Why? Because as a rule of thumb, peak combustion pressure is one hundred times the compression ratio. Engine torque results from that combustion pressure pressing your pistons down to turn the crankshaft.

What sets the limit here is the degree of heating the fresh fuel-air charge suffers as it enters the cylinder and is heated by contact with it, then by compression, and finally, as yet-unburned mixture out near the cylinder wall is compressed and heated by the expanding combustion gas.

If that unburned mixture out at the cylinder wall—which the engineers call the “end-gas”—is sufficiently heated before the flame can reach it, bits of it may autoignite, burning at or above the speed of sound in a phenomenon called “detonation” or engine knock. The knocking sound we hear during detonation is sonic shock waves, hitting the metal surfaces of the combustion chamber.

Compression ratio has to be set low enough that even on the hottest day, with the worst gasoline you are likely to find at the pump, your engine will not detonate. Higher compressions heats the fuel-air mixture more and produces higher peak pressure, so the higher the compression ratio, the more likely detonation becomes.

Because it’s harder to cool well and consistently with air (summer air? winter air?), head temperature of air-cooled engines tends to be higher than in a liquid-cooled engine, so fuel-air mixture gets heated more, possibly leading to detonation.

Steady detonation (as opposed to the “occasional tinkle”) is destructive, causing overheating, piston heat softening, and finally, erosion of piston metal.

Because in general, the larger the cylinder bore the longer it takes to complete combustion, detonation is more likely in big-bore engines, which expose their mixture to heat longer. This is why air-cooled engines and many engines with large bore are given lower compression ratios than liquid-cooled and/or smaller-bore engines.

There can be exceptions. I spent the summer of 1963 in Denver, Colorado, where I worked in a bike shop. One of our customers was a very serious night warrior who had built an air-cooled Triumph 650 twin with a sky-high 12-to-1 compression ratio (that’s not sky high today, when every sportbike engine has 13:1, but in 1963 it was crazy high).

Why didn’t his engine detonate itself into aluminum gravel? His mother had a back shed, and in it he kept a secret 55-gallon drum of purple aviation 115/145 gasoline. It was rich in highly knock-resistant alkylates and contained 6 grams per gallon of the powerful anti-knock compound tetraethyl lead. That was the fuel that powered many supercharged and air-cooled World War II aircraft engines, all of which had huge bores between 5-3/4 and 6-1/8 inches (146 to 155.6mm).

On this wonderful but now unavailable fuel his Triumph did not knock, but left the competition for dead. Drag racers today pay $60 a gallon for racing gasoline having similar knock resistance.