Pros Of The 4 Stroke Engine

Well, since the YZ400F made its debut in 1997, when Doug Henry won the final supercross of the year in Las Vegas on one... things have changed dramatically. The fours have fast become the dominating force on any track or terrain. Why? Well it seems there are a number of debatable reasons. Their power to weight ratio is almost on par with the 2 strokes - and I simply mean in terms of how a 450cc four stroke is now a lot closer to the weight of a 250cc two stroke, but puts out a similar amount of speed and power. Back in the early 90's four strokes were heavy beasts and were considerably slower than the two's. With the new and improved 4-banger, many people find them easier to ride with their smooth, tractable power delivery. But on the other hand, a lot of riders don't actually want that. They want the thrill, noise and adrenalin of a powerband that only a two stroke can serve up.So why are the 2 strokes disappearing? Some people believe (and blame) the format of the AMA, and large companies such as Honda have a heavy influence over why we are witnessing the decline of the popular two's. Is it fair that a 450cc four stroke is able to compete against a 250cc two stroke? It has also been said that another contributing factor to the 2 strokes decline is the pressure felt by government groups and lobbyists to phase them out because of the environmental damage they cause. But how much more pollution do they really put out compared to the 4's? It seems that the technology already exists to create a two stroke engine that is just as, if not more environmentally sound as a four stroke.

http://www.dirt-bike-tips-and-pics.com/2-stroke-vs-4-stroke.html
http://www.btosports.com/p/PCTI4R4
http://blogs.internetautoguide.com/6564865/auto-news/2010-honda-crf450r-dirt-bike-available-now/index.html

A Honda CRF450 2009 at Washougal MX Park.

This is a Procircut pipe for a 4 stroke engine and bike.

Cons Of The 4 stroke Engine

Today's 4 stroke dirt bike engines are leap years better than they were 10 years ago. What has changed? Technological advancements have seen the weight decrease, the power increase and the all round performance of the 4 stroke engine become so much more efficient.The downside of this is today's high performance thumpers are far more costly to maintain than they used to be. Though they generally don't require as much regular maintenance as a two stroke - but when they do there are more parts to replace, therefore costing you more. The 4 stroke dirt bike engine consists of a piston and barrel, and the cylinder block which holds the valve train. The number of valves vary with different bikes - The '08 YZ450F has 5, while the '07 CRF450 has 4.

The four stroke engine has four unique strokes or movements that each perform a specific task.

 - The first movement is the intake stroke. As the piston moves from the top of the cylinder block to the bottom, the intake valve/s open to allow a mixture of fuel and air to be drawn in to the cylinder.

- The second movement is the compression stroke. All valves are closed as the piston rises, causing the fuel mix to be compressed.

- As the piston reaches the top of its stroke, the compressed fuel mix is ignited by the spark plug, causing an explosion which forces the piston back down. This is the combustion stroke.

- Now the piston begins to rise again. The exhaust valve opens up and releases the emissions from within the cylinder, out through the exhaust pipe. This is the exhaust stroke.

 - This whole process then repeats...

The 4 stroke dirt bike engine, unlike a 2 stroke does not have fuel inside the crank case. Therefore, you are able to fill the crankcase (where the crankshaft bearings and moving parts of the piston rod are) with a heavy duty oil to lubricate them. So, that is why you do not need to mix oil into the fuel with a four stroke engine. No burning oil - less damage to the environment.

 http://www.dirt-bike-tips-and-pics.com/4-stroke-dirt-bike.html

Cons Of The 2 Stroke Engine

There are some cons (bad things) about the 2 stroke though. Here are some of the things that make the 2 stroke bad or not really wanted. Two-stroke engines don't live as long as four-stroke engines, the lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
The two-stroke oil can be expensive. The mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.  Two-stroke engines produce more pollution from, the combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.

http://www.deepscience.com/articles/engines.html

http://www.btosports.com/p/PCPIPE2WKT

This is a picture of a Procircut Works Pipe. The other picture is a yYamaha yz125 Two stroke engine.

Pros Of The 2 Stroke Engines

The 2 stroke engine is one of the oldest engines still around the bad thing is they have stopped making them.The pros of this are a 2 stroke engine dose not have valves and contributes to it  lighter weight. The two stroke engine fires (produces power) once every revolution of the crankshaft. Where as a four stroke fires once every other revolution.The 2 stroke engines are easier to handle than 4-stroke engine dirt bikes. The 2-stroke dirt bikes have superior power-to-weight ratio than 4-stroke dirt bikes and kick starts easier then the 4 stroke.They have less parts and more power for half (close to) displacement. And the thrill, sound,smell, and the cheaper exhaust. So yes the 2 stroke is a really good engine and machine.

The 2 Stroke Dirt Bike Engine

http://www.motocrossreview.com/content/view/197/56/