Readers often ask us for guidance on getting more performance from their motorcycle. Most are looking to improve top-gear acceleration from moderate speeds, like passing a car or riding safely on rural roads. Riders in the North Thailand need the same sort of power to comfortably climb mountains. Most motorcycle owners refer to this mid-range or mid-rpm power as torque. Almost-everybody with a stock motorcycle wants more torque.
Torque is relative easy to improve, and it need not be expensive to do so. And while it is possible to increase power across a engine's rpm range, that isn't what usually happens, at least not without massive engine rebuilding and modifications. Almost all bore-up and performance kits and services are directed at the top third, which is, depending on the engine displacement size, around 5,000 to 7,000 rpm for most motorcycles. In many cases, the resulting engine is sluggish in the middle third, where we do most of our street riding. Adding power in the 2,000 to 7,000rpm range is the key. Below 2,000rpm is for parking lots where good balance is more important than acceleration, and above 8,000rpm is where few of us ride our motorcycle in city traffic anyway.
Most people say, Size Equals Power. And yes, this equation is a bit simplified, yet it's largely accurate; bigger engine make more power - especially in the mid-range. An engine's power output is determined by how much air/fuel mixture gets burned in a given time. A small-displacement engine at high rpm can match the power output of an engine twice its size turning half as fast.
The simplest and most effective way to get more power out of your motorcycle at a given rpm is to make it bigger. If you can find a way to stuff 10 percent more in, you'll get 10 percent more out, everything else being equal. And with 'stuff' we mean here air/fuel mixture or bigger bore.
You might bore your motorcycle cylinder a few millimeters to get a a extra 25cc. If you do nothing else, the mid-rpm power increase will be proportional. That would bring your stock engine a few extra horses, depending on the engine displacement this can be 8 to 30-plus percent more power. Those numbers may not seem all that impressive, but you'll get them in a useful rpm range.
Engine Efficiency Equals more power. If you can increase pressure in a combustion chamber at the time of ignition, pressure during combustion will also be higher. Higher pressure pushes the piston harder and more work gets don“ your motorcycle accelerates quicker or climbs the hill more easily. Higher-compression pistons, thinner head gaskets, or anything else that make the combustion chambers smaller can achieve this result.
There are many limitations to both of these approaches. For instance, enlarging the engine quickly becomes expensive if one goes beyond simply boring the cylinders; and too much compression can make an engine overly sensitive to gasoline quality, which can lead to detonation and serious engine damage.