Compression Ratios I
TAKING AIM, by Chris Maida, Editor
Have you ever been in a pack of bikes and heard one that had an exhaust note that crackled with power when the rider blipped the throttle? That, my friend, is the sound of a high-compression engine! And though some try to imitate it with real short drag pipes or holes in their mufflers, nothing has the crisp bite of a high-compression engine, in sound or throttle response.
Boosting an engine’s compression ratio is a time-honored and effective method of increasing its performance. If done correctly, it’s a modification that will enable an engine to produce more power while also increasing its efficiency (fuel mileage). Increasing the compression ratio will also make your engine accelerate (build rpm) faster, which will make the engine more responsive when you crack the throttle. However, like everything in the world, every modification has its pluses and minuses. So before you head down to the service department with a fistful of dollars, you should understand what compression ratio is, why it produces more power, and what the possible drawbacks are.
As you’ve seen in countless tech stories and bike features, an engine’s compression ratio is usually ststed as, for example, 10:1 and read as 10 to 1, like any other ratio. What this means is that the volume of the area above the piston, which is mostly the combustion chamber, and the volume of the cylinder when the piston is at the lowest point of its stroke — called its Bottom Dead Center position (BDC) — will be reduced to one-tenth of that size when the piston is at the highest point of its stroke, or its Top Dead Center position (TDC). The notation 10:1 simply states that the air/fuel mixture will be compressed into the cylinder head’s combustion chamber until it occupies a space one-tenth as large as the volume of the cylinder and combustion chamber combined. A notation like 10.5:1 simply means the ratio is 10-1/2 to one.
As for why increasing the engine’s compression enables it to put out more power, remember when we discussed how power is produced? The piston is driven down in its cylinder by the pressure produced in the combustion chamber by the rapidly burning fuel and air mixture. How hard this pressure pushes down on the piston determines how much power the engine produces. And if the pressure that the air/fuel mixture’s under is increased before it’s ignited — meaning the engine’s compression ratio has been raised — the burning gases will exert even more pressure onto the piston, producing more power. It’s like a coiled spring in that if you compress it a little, it pushes back a little. But the more you compress it, the harder it pushes back.
We’ll cover possible drawbacks of raising an engine’s compression and other details in a future column.
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