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Kevin Cameron has been writing about bikes for practically 50 years, first for <em>Cycle journal</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

In the event you’re nerdy sufficient to note, you see that basically, compression ratios on air-cooled engines (particularly if they’ve giant bore) are typically decrease than on liquid-cooled engines, particularly these with reasonable to small bores.

The upper the compression ratio you possibly can safely run in an engine, the larger the torque it’ll produce. Why? As a result of as a rule of thumb, peak combustion pressure is 100 instances the compression ratio. Engine torque outcomes from that combustion stress urgent your pistons down to show the crankshaft.

What units the restrict right here is the diploma of heating the recent fuel-air cost suffers because it enters the cylinder and is heated by contact with it, then by compression, and eventually, as yet-unburned combination out close to the cylinder wall is compressed and heated by the increasing combustion fuel.

If that unburned combination out on the cylinder wall—which the engineers name the “end-gas”—is sufficiently heated earlier than the flame can attain it, bits of it could autoignite, burning at or above the pace of sound in a phenomenon referred to as “detonation” or engine knock. The knocking sound we hear throughout detonation is sonic shock waves, hitting the steel surfaces of the combustion chamber.

Compression ratio needs to be set low sufficient that even on the most well liked day, with the worst gasoline you’re prone to discover on the pump, your engine is not going to detonate. Greater compressions heats the fuel-air combination extra and produces greater peak stress, so the upper the compression ratio, the extra seemingly detonation turns into.

As a result of it’s more durable to chill effectively and persistently with air (summer time air? winter air?), head temperature of air-cooled engines tends to be greater than in a liquid-cooled engine, so fuel-air combination will get heated extra, probably resulting in detonation.

Regular detonation (versus the “occasional tinkle”) is harmful, inflicting overheating, piston warmth softening, and eventually, erosion of piston metal.

As a result of basically, the bigger the cylinder bore the longer it takes to finish combustion, detonation is extra seemingly in big-bore engines, which expose their combination to warmth longer. For this reason air-cooled engines and lots of engines with giant bore are given decrease compression ratios than liquid-cooled and/or smaller-bore engines.

There may be exceptions. I spent the summer time of 1963 in Denver, Colorado, the place I labored in a motorcycle store. One in every of our clients was a really severe evening warrior who had constructed an air-cooled Triumph 650 twin with a sky-high 12-to-1 compression ratio (that’s not sky excessive at present, when each sportbike engine has 13:1, however in 1963 it was loopy excessive).

Why didn’t his engine detonate itself into aluminum gravel? His mom had a again shed, and in it he saved a secret 55-gallon drum of purple aviation 115/145 gasoline. It was wealthy in extremely knock-resistant alkylates and contained 6 grams per gallon of the highly effective anti-knock compound tetraethyl lead. That was the gas that powered many supercharged and air-cooled World Battle II plane engines, all of which had enormous bores between 5-3/4 and 6-1/8 inches (146 to 155.6mm).

On this glorious however now unavailable gas his Triumph didn’t knock, however left the competitors for useless. Drag racers at present pay $60 a gallon for racing gasoline having related knock resistance.

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