Strokin’

How many times have you got yourself involved in a discussion or debate on the whys and why nots of two strokes and four strokes. Everyone is entitled to an opinion. But in this debate, how informed is your opinion, other than what you know from the feel of riding one or the other. So, for the uninitiated, or those that are just interested in an update, here’s the skinny on the differences between the two.

The basics – a two stroke motor is so named because there are only two strokes in a cycle – a compression stroke and a combustion stroke. A four stroke motor has intake, compression, combustion, and exhaust. In a two stroke engine, start with the point where the spark plug sparks. Fuel and air in the cylinder have been compressed, and when the spark plug fires, the mixture ignites. The resulting explosion forces the piston downward. Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder. As the piston finally bottoms out, the intake port is uncovered. The piston’s movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel. Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor. Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle. A four stroke engine works like this: The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and petrol. This is the intake stroke. Only the tiniest drop of petrol needs to be mixed into the air for this to work. Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the petrol. The petrol charge in the cylinder explodes, driving the piston down. Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the exhaust pipe. Probably the most confusing novice question on the issue is why do you mix two-stroke oil with petrol in a two stroke engine? To most rookies, this is the definitive difference between a 2-stroker and a 4-banger. Now that the way the motors work has been explained and the oil isn’t really why they are different, let’s check the oil issue out. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston’s connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can’t hold thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, the engine isn’t going to last very long! Why is the exhaust header (the piece that comes directly out of the motor) so thick on a two stroke compared to the thin exhaust pipes on a four stroke? This little bit of engineering always intrigued me. You ever notice on a two stroke the oily discharge out the back of the pipe and the four stroke always looks so clean? Well, since the exhaust is pushed out of the cylinder in the same stroke that fuel/air mixture is sucked in from being compressed in the crankcase, and even though the intake port and the exhaust port are at different heights in the cylinder so that the piston can block one of them while the other is open, some of the fuel/air mix will still find its way into the exhaust. Bearing in mind, the high pressure that this “escaping fuel” will be under, imagine that when the exhaust is expelled some of the expelled air/fuel mix will be leaving the cylinder at a really high speed. All of this leaves into the thick header pipe which tapers off into the thin exhaust pipe. The shape of the header and the tapering off causes some of the escaped air/fuel mix to “bounce” back into the cylinder to be used in the next combustion stroke. Obviously some of it doesn’t make it back and that’s the discharge you see on a two-stroke pipe. Some of this explanation may be making some lights come on in terms of why they race 250 4-strokes in the MX2 class and 400-525 4-strokes in the MX1 class. Well, the classes used to be called 125 and 250. This was based on the engine capacity that was allowed to compete in the class. With four stroke motors making such great strides in engineering allowing them to be competitive in, what was once, a two-stroke dominated sport, the engine capacity classification had to be revisited. Why, you may ask? Well, considering that a 125cc four stroke has double the piston strokes that a 125 two stroke has, it is obvious that the two stroke 125 is going to have double the power of the four stroke 125. This is because power is delivered to the back wheel in half the time – 2 strokes instead of 4. Therefore, the average horsepower of a four stroke is the same as that of a two stroke half it’s capacity. So, MX2 now has 125 two strokes competing with 250 four strokes and 250 two strokes competing with 400+ four strokes. Theoretically, the power in the classes is consistent for competition. So now you’re waiting for the million-dollar comment – which is better? Well I’m not going to tell you! Each has it’s pros and cons. But which is best depends only on you. They are different rides, the provide different rushes, and for every champion of a two-stroke, you’ll find a champion of a four-stroke. What bike do I ride? Well……….I ride two-strokes. In fact, I am a two-stroke nut. But that’s just me! The bad news however, is that two strokes have a shelf life right now. Environmentalists have shouted them down and in some states in the USA they are illegal. The gut-feel on the street is that two-strokes will only be available for the next 5 – 6 years. If you are new to the game, try get a ride on both types before you go out and buy a bike. You really need to “feel” the difference to really make your personal preference. Keep the shiny side up and ride it like you stole it. Steve “Tombstone” Lauter