If you’re riding a Harley-Davidson motorcycle you must know it is powered by a twin cylinder, reciprocating piston, internal combustion motor. Reciprocating pistons means the pistons are going up and down, or back and forth, or from side to side depending upon motor configuration. Many factors determine the performance of the motor but one of the most important is the compression ratio of the cylinder/piston/head assembly. The higher the compression ratio, the more bang or power per cylinder, up to a point.
A reciprocating piston motor (motorcycles are powered by motors, that’s why they are not called “engine cycles”) generates horsepower by burning an air-gasoline mix to push the piston down in the cylinder. This linear push is referred to as the power stroke. The straight-line motion of the connecting rod-piston assembly is transferred to the flywheel(s), which converts the linear motion into circular motion. This circular motion is then transferred to the transmission, and on to the rear wheel, creating forward motion. Sounds simple, but in reality is a bit more complicated.
The pistons in a Harley-Davidson V-twin are designed to produce a specific compression ratio in a given application for best operating efficiency. Experience and history have taught Harley-Davidson engineers the best compression ratios for motorcycles being operated on the street.
To simplify things let’s use a single cylinder motor as an example. A typical four-stroke single cylinder motor (half of a twin cylinder motor) operates in four distinct phases or cycles. That is why it is called it a 4-cycle or 4-stroke motor (or engine). During the intake stroke (1), air/fuel mix is taken in by vacuum through the open intake valve as the piston moves down. As the piston starts back up (2) the intake valve closes and the air/fuel mix is compressed. As the piston reaches the top of the stroke the air/fuel mixture is fired by the spark plug and the piston is pushed down violently by the exploding air/fuel mixture (3), creating the aforementioned linear motion which in converted by the flywheel(s) into circular energy. As the piston comes back up (4), it pushes the burnt mix out (exhaust gases). This is called the exhaust stroke. One-Two-Three-Four….If the compression ratio is low, the motor makes minimal horsepower. If the compression ratio is relatively high the motor can produce a great deal more power per cubic inch of displacement than an identical motor with a lower compression ratio. Many things influence the ability to be able to operate a motor with high(er) compression, not the least of which is the availability of high octane gasoline. With out a supply of high octane gasoline, a high compression motor may be subjected to pre ignition (pinging), due to the fuel air mix firing prematurely. Pre ignition can be very destructive to a motor.
Compression ratio is defined quite simply as the volume above the piston at bottom-dead-center (BDC), divided by the volume above the piston at top-dead-center (TDC). The more compressed the fuel air mix is when ignited the bigger the bang. A bigger bang means more power, as well as additional stress on all the mechanical components involved. High compression motors require the use of high-octane gasoline to prevent pre-ignition and/or detonation, which can cause costly damage to pistons, valves, and piston rings or even worse, losing the race.
For average street riding most trained mechanics recommend a compression ratio somewhere between 8.5:1 and 9.5:1. Any higher and a higher octane fuel requirement is needed. With compression ratios lower than 7:1 a motor simply cannot operate efficiently. Hopefully you now understand what “compression ratio means.” However, this is just static compression ratio. Cam lift and valve overlap along with other factors determine the actual or functional compression ratio. Also, remember that higher compression ratios while increasing power also increase wear and tear on the motor. High compression motors are not good commuter motors, just as lower compression motors are not good race motors.
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