How Motorcycle Engine Power Is Produced
TAKING AIM, by Chris Maida, Editor
Whenever you get around a group of riders, compression ratios, cubic inches, and cam lifts, as well as a multitude of other modifications, are discussed and compared, along with the resulting torque and horsepower numbers. But why does increasing an engine’s cubic inches (known as its displacement) result in more power? And why does installing the highest lift cam you can buy usually result in less power being produced?
All engine performance mods, from something as basic as bolting on a freer-flowing air cleaner to exotic systems like superchargers or nitrous oxide, are designed to do one thing: get more air/fuel mixture over the piston, so it can be ignited — hopefully when it will do the most good — and burned as efficiently as possible. Period. The heat energy released by burning gasoline and oxygen is what the piston and associated parts turn into the mechanical energy needed to turn the rear wheel. Think about it: it’s not the size of the cam’s lobes that allow an engine to develop more power, it’s how well a specific cam works with the engine’s other components to get as much air/fuel mixture into the cylinders as possible for each combustion event.
But getting the cylinders filled is not the only factor in getting the most from what you have. When the ignition system fires the spark plugs to burn the mixture and how efficiently that mixture burns also plays a big part in how much power will be produced. The spark plug should ignite the air/fuel mixture before the piston starts on its way back down the cylinder on its power stroke. This is done so the mixture has time to ignite and burn. The piston is driven down the cylinder by the pressure produced in the combustion chamber by the rapidly burning fuel and air. To get the most power, you want the piston just past the top of its compression stroke by the time the mixture has burned to the point where the now-expanding gases can push their hardest on the piston, driving it down the cylinder on its power stroke. As for having an efficient burn, the more completely you burn the air/fuel mixture, the more power you’ll get from it.
To get a very efficient burn, the fuel must be completely atomized and thoroughly mixed with air in the right proportions before it’s ignited by the spark plug. The fuel and air are initially mixed in the engine’s intake system. However, for a very efficient burn, the fuel and air should be mixed again in the combustion chamber just before it’s ignited. The combustion chamber and piston design on Evos and Twin Cams do this by turbulently mixing the air/fuel mixture as it’s compressed into the combustion chamber by the rising piston. Just for the record, you don’t want turbulence in the intake or exhaust tracts; laminar flow is the way to go in the ports.
See you on the road,
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