Pressure Ratio Made Simple

If you’re riding a Harley-Davidson motorcycle you must know it’s driven from the twin cylinder, reciprocating piston, internal fusion motor unit. Reciprocating pistons means the pistons go up and down, or back and forth, or laterally depending on motor unit configuration.

Many factors evaluate the functionality of the motor however, one of the most important is the compression rate of the head/cylinder/piston construction. The higher the compression rate is, the greater bang or power per cylinder, up to a point.

An alternating piston motor generates power by burning an air-gasoline mix to push the piston inside the cylinder. This linear drive is called the power stroke. The straight-line movement of the connecting rod-piston system is utilized in the flywheel, which in turn turns the linear movement into turning action. This circular action is going to be transferred to the transmission, and on to the rear wheel, producing forward motion. Seems straightforward, but actually may appear far more complicated.

The pistons in the H-D V-twin are meant to produce a precise compression ratio from a given use for best performance. Past experiences and history have educated H-D engineers the most effective compression rates for motorbikes driven on the street.

To streamline things let us work with a single cylinder motor as an example. The average four-cycle single cylinder motor (half of a twin cylinder motor) works in a number of distinctive phases or cycles. That’s why it is named a 4-stroke motor (or engine). Within the intake stroke (1), air or gasoline is drawn in by vacuum pressure throughout the open intake valve as the piston goes down. Since the piston starts back up (2) the intake valve closes and the gasoline and air mix is compacted. As the piston reaches up to the top cycle the gasoline and air mixture is fired by the spark plug then the piston is forced straight down strongly via the expanding fuel and air mixture (3), creating the aforementioned straight line motion which is transformed through the flywheel into circular power. When the piston returns upward (4), it drives the burned up mixture out (exhaust gases). which is called the exhaust cycle.

lf the compression rate is decreased, the motor makes minimal hp. lf the compression ratio is comparatively high the motor can produce a whole lot more power for each cubic inch of displacement than an identical motor using a reduced compression setting ratio. A lot of things influence the cabability to operate a motor with higher compression, not to mention the availability of high octane fuel. With no supply of high octane gasoline, a high compression motor unit may be afflicted by pre ignition (pinging), due to the fuel air mixture firing ahead of time. Pre ignition is often very harmful for a motor.

Compression ratio is described basically as being the mass on top of the piston at bottom-dead-center (BDC), separated by the mass above the piston at top-dead-center (TDC). The more pressurised the gas and air mixture is when burned the stronger the punch. An increased force would mean more power, as well as additional stress on all the physical components involved. Higher compression motors need the use of high-octane fuel to prevent pre-ignition and/or detonation, which can cause pricey problems on piston rings, pistons and valves or even a whole lot worse, not winning the race.

For average cruising a lot of trained mechanics suggest a compression rate approximately 8.5:1 and 9.5:1. Any higher and a higher octane fuel requirement is needed. With compression ratios less than 7:1 a bike’s motor cannot work efficiently. Hopefully at this point you understand what compression ratio means. However, this is just static compression rate. Cam lift and valve overlap along with other components determine the actual or functional compression ratio. Also, remember that higher compression ratios while boosting power also increase deterioration of the bike’s motor. High compression motors are not good cruising bikes, just like lower compression motors aren’t good race motors.

Remember, it is extremely important to wear a strong motorcycle helmet.

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