[$evy]

We had to write papers in school trying to convince people why we think something is better but stating it all in facts....so I decided why not write about 2 strokes? So most of the report I wrote off the top of my head.

Two Stroke Engines
Almost 20 years ago, just south of the United State’s border, in Baja, Mexico, the two stroke that is said to have kept sport ATV’s alive through the late 80’s and early 90’s made its first appearance, and a very good one at it too. The Yamaha Banshee was first introduced to the public in 1986 at the Baja 1000 race…one of the most grueling races known covering 1000 miles of Mexico. The Banshee dominated the competition and won the race. The Banshee, powered by its 350cc twin cylinder two stroke race bike inspired motor, eliminated the competition and awoke a new sport. Since then companies started producing more two strokes and advancing some of the technology. Yamaha also came out with the Blaster, which was a 200cc single cylinder air cooled two strokes, that to this day is still going strong. Also, both Honda and Suzuki came out with liquid cooled 250cc single cylindered ATVs that were race inspired. Suzuki also came out with the LT 500R that dominated its competition with over 40 horsepower stock from the factory. For the past 20 years two strokes dominated their classes and nothing could come close. With the less moving parts the two strokes are a lot easier to maintain. Also they have a higher power to weight ratio because of the less moving parts that it required to operate them. And when compared engine size cc’s to cc’s, (cubic centimeters) two strokes will always produce more power. This explains why a 450cc four stroke dirt bikes can compete in the 250cc two stroke classes.
Two strokes have less moving parts than four strokes and are much easier to maintain. Two strokes are powered by the air/fuel mixture being taken into the cylinder by a port in the side of the cylinder wall and another port in the piston. When the ports are uncovered the mixture flows into the combustion chamber, is ignited, and the piston moves down and uncovers an exhaust valve where the combusted gases are released. Four strokes have valves on top the cylinder head that are powered by a cam and a timing chain to open the valves. The valve covers, springs, rods, timing chains, and the cam are all extra weight that must be added in order for the engine to work. Two strokes are also able to be oil-injected. The lubricating oil is mixed in with the gas either in the gas tank or in the carburetor and then injected into the cylinder. Four strokes have a separate oil container that is usually made of metal, and is run through the cylinder to keep it lubricated so it requires more weight in order for the engine to operate. On two strokes, there are no valves to be adjusted or replaced, because the valve is the piston itself.
Two strokes have a better power to weight ratio. The power to weight ratio is the weight of the ATV compared to the amount of horsepower it produces. Because two strokes have less moving parts, and require fewer fluids to operate them, they weigh less and therefore have a better power to weight ratio. Of all the ATV’s made, the Suzuki LT500R has the best power to weight ratio. It’s a 500cc single-cylinder two stroke. When compared cc’s (cubic centimeters) to cc’s, two strokes have more power, because it requires less weight to build them up to such a large engine size. With four strokes, the larger the engine size the more fluids it takes to run it, and also the parts made for the ATV must be of a heavier, stronger strength so it can operate. The less weight your ATV weighs, and the more horsepower you can produce, the better the p-w ratio will be. And in a motocross race, saving weight is everything.
When comparing the same engine size, two strokes are always going to make more power than four strokes. It is a known fact that in the 125cc AMA class, 250cc four strokes can compete. . Also in the 250cc AMA class, 400-450cc four strokes can compete. It is also a known fact that when comparing two strokes and four strokes, in order for the four strokes to be equal it must be double the displacement of the two strokes. When looking at the ATV industry, the Yamaha Banshee (350cc two stroke) dominates drag races, hill climbs, and even won the Tour De Voux six years in a row. The Tour De Voux is a grueling race in France that is a test of how well your ATV can perform under immense stress, and how long the rider can endure the beating the race gives. Speeds on the track range anywhere from 15mph to over 90mph. The Yamaha Banshee has won the race six years in a row. The amazing thing about the race is that it’s an open class race, so quads as big as 700cc’s are competing in it. Another amazing fact is that the Duncan Racing team has won the race on a stock bore and stroke ATV. It is a known fact in the small engine industry that two strokes make more power when compared to a four stroke of the same engine size. Also the world’s most powerful engine is a two stroke diesel, making over 108,000 hp and 5.6 million ft.-lbs. of torque.
Although four stroke engines do have a smoother powerband and at times, can be more reliable, they still do not compare to the power and the speed that two stroke engines have. But two strokes come ahead in just about every class. Two strokes weigh less than four strokes and therefore are able to create a higher power-to-weight ratio. The world’s most powerful motor is a two stroke diesel engine, and even though it stands about three stories high, it still produces over 5 million ft.-lbs. of torque. No other motor comes close. Two strokes also have less moving parts and a lot less maintenance to do. With four strokes you constantly have to change oil filters and setting valves. Two strokes are also very easy to work on. Anybody with an owner’s manual and the right tools can rebuild the motor within a few hours. Whereas four strokes are very complicated and require lots of skill and time to rebuild. Two strokes will always be the most powerful engines when compared cc to cc. The Yamaha Banshee has been in production for almost 20 years, and the motor inside it was from an old two stroke race bike. That says a lot when a quad stays alive for 20 years and is still the top competitor.


I'd like to note that I'm sure some of this is wrong but my teacher doesnt really know but I'm sure you guys can point it all out and prove me wrong in a few places...but enjoy