Racer's Guide To Motorcycle Engines.Most off-road motorcycle engines are delivered off the showroom floor in such a high state of tune that it takes expertise to get more usable power out of them. Due to their popularity this article will concentrate on today's four-stroke motorcycle engines, however many of these tricks also apply to two-stroke engine technology. If you decide to alter the powerband of your machine, after proper gear selection, the following are some recommended first steps:
Motorcycle Exhaust Pipes: The exhaust systems of today's four-stroke motorcycle engines are tuned to provide the optimum powerband. The major design elements of the pipe are head pipe length, head pipe diameter, taper of S-Bends, diameter of muffler core, and overall length of the muffler. Compared to the stock setup these features can often be modified by the successful aftermarket designer to both save weight, and increase horsepower and torque over the engine's workable range. Motorcycle Air Filter / Wind Screen Removal: Another cost-effective modification to increase the motorcycle engines performance is either to remove the stock wind screen with a pair of tin-snips, or purchase an aftermarket motorcycle air filter system which eliminates the restriction. While in truth this modification seldom results in improved numbers on the dyno, many riders comment on improved throttle response which is just as important. High-Compression Pistons: On four-stroke motorcycle engines the compression ratio is typically increased by using a piston with a higher dome. A high-compression piston generally delivers a harder hit and better low-to-midrange power. Increases in both horsepower and torque are often achieved. Stock compression on most off-road motorcycle engines is typically in the 12:1 range, however it can be increased as high as 15:1. If you should decide to go this route do yourself a favor and pay a little extra for forged piston design from a reputable company such as JE Pro Series Pistons. These forged pistons are designed for severe service found in motocross racing and are used by the majority of professional factory and satellite teams. These pistons are fabricated from a special aerospace grade 2618 aluminum. During the forging process (which occurs under pressure), these pistons have much higher tensile strength, yield strength, better elasticity, and longer fatigue life. These designs contain internal & external bracing, and are claimed to be 30% more durable than stock cast pistons (which become brittle under heat & compression). Forged pistons can also dissipate heat better, and withstand greater operating temperatures compared to a cast piston. The drawbacks of the forged piston design are slightly higher cost, and slightly higher engine noise upon startup. Don't be alarmed as the engine noise is simply due to the denser forged material requiring a longer period of time to reach operating temperature, thus requiring slightly larger clearance requirements. Cylinder Heads Porting: Unlike a two-stroke where porting is done in the cylinder wall, all porting on four-stroke motorcycle engines are performed in the cylinder heads. To port the head of a four-stroke, the intake and exhaust ports are smoothed and reshaped to improve flow. Popular with many expert racers, this modification is primarily performed to increase the engine's pull further into the top end of the powerband. High Performance Camshafts: Typically high performance camshafts perform their work in the way the power is delivered, although increases in both horsepower and torque can be achieved. Cam duration, lift, timing, and overlap are critical to achieve proper flow within four-stroke engines. These are parameters the successful cam designer will modify to achieve desired results. Do research however as there can be durability downsides depending upon the amount of development & testing performed, or if the cam is not installed & broken-in properly. Oxygenated Race Fuel: The quickest method to make engines produce more power is to feed it more oxygen. Many race fuel manufacturers such as VP makes race fuels with 6% oxygen content. The increase in octane, oxygenents & hydrocarbons (with greater energy potential) results in instant horsepower that can be poured into the tank in seconds. The downsides to this approach are cost, re-jetting carburetor is often required, and race fuel typically does not store for long periods of time. Another tip is many race teams pack their gas in ice prior to refueling. The lower fuel temperature increases the oxygen content, and thus make more horsepower. Big Bore Kits: The old saying "No Replacement For Displacement" continues to ring true. Whether it's a two-stroke or four-stroke engine, overboring a cylinder is probably the easiest, cheapest, and safest way of increasing an engine's power.
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