Many aspects determine the performance of the bike’s motor but one of the most important certainly is the compression ratio of the head/cylinder/piston assembly. The greater the compression rate is, the greater bang or force for every cylinder, up to a point.
A reciprocating piston motor produces horsepower by combusting an air-gasoline mix to drive the piston in the cylinder. This straight line push is referred to as the power stroke. The linear movement of the binding rod-piston set up is moved to the flywheel, which turns the straight line movement to circular motion. This circular movement is then transferred to the transmission, and also on to the rear wheel, producing forward motion. Seems simple, but in reality might be more complex.
The pistons inside a H-D V-twin are designed to produce a precise compression ratio from a specified application for the best operating efficiency. Past experiences and history have taught Harley-Davidson designers the most beneficial compression ratios for bikes driven on the highway.
To streamline things why don’t we work with a single cylinder motor as one example. An average four-stroke single cylinder motor (one half of a twin cylinder motor) works in a number of specific cycles. That is why it is called a 4-cycle motor (or engine). During the intake stroke (1), air/fuel is drawn in by vacuum throughout the opened up intake valve while the piston moves down. Since the piston starts back up (2) the intake valve shuts and then the air and gasoline mix is compressed. As the piston reaches up to the top cycle the air/fuel fusion is shot by the spark plug then the piston is forced downward strongly via the combustion of gasoline and air mixture (3), producing the previously mentioned straight line movement that is turned via the flywheel to circular power. When the piston comes back up (4), it forces the burnt mix released (exhaust gases). which is known as the exhaust cycle.
lf the compression ratio is minimal, the bike’s motor creates minimal power. lf the compression rate is pretty high the bike’s motor can make a great deal more power for each cubic inch of displacement than an identical motor using a lower compression rate. Many things impact the chance to be able to handle a motor with high compression, not the least of which is the use of high octane gas. With no availability of high octane gasoline, a high compression motor could be afflicted by pre ignition (pinging), due to the gasoline air mix firing beforehand. Pre ignition can be very dangerous for a motor.
Compression ratio is defined simply as the volume above the piston at bottom-dead-center (BDC), divided with the volume on top of the piston at top-dead-center (TDC). The greater compressed the gasoline and air blend is when burned the bigger the bang. A bigger hit translates to more power, and also extra force on all the physical pieces included. High compression motors need the usage of high-octane gasoline to stop pre-ignition and/or detonation, which can produce pricey problems on pistons, valves, and piston rings or maybe even worse, not winning the race.
For typical street riding most qualified technicians advise a compression ratio between 8.5:1 and 9.5:1. Any higher and a higher octane fuel qualification is needed. With compression rates lower than 7:1 a bike’s motor just can’t work effectively. Hopefully at this point you realize what compression ratio means. Then again, this is simply static compression ratio. Cam lift and valve overlap as well as other aspects decide the particular or functional compression rate. Also, remember that increased compression rates while increasing power may also increase wear and tear on the bike’s motor. High compression motors are not good cruising bikes, just as lower compression motors aren’t excellent racing bikes
Until Next Time, Ride Hard & Live Free!
Larry
