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Moto-cross 4 stroke VS 2 stroke

Posted: Wed Aug 22, 2007 2:26 am
by alex.barylski
I haven't followed racing in at least a decade, but I used to be right into moto-x, etc...

Back in the day, all the hottest bikes were two stroke....my very own beloved RM80 is most certainly a two stroke. The sound or should I say buzz of that little engine always pushed me ahead of my friends who otherwise had four strokes.

Today I have friend who bought himself a Kawasaki (125/250 or similar) and it's 4 stroke. I laughed at him and called him a wussy and he explained that 4 stroke had come such a long way they now equalled 2 strokes in almost everyway. He then proofed his claim by hilighting most bikes in his category were indeed four stroke.

He proved his point but I am at a loss as to how this can be, having a basic understanding of the principles behind the two different types of engines, a 4 stroke out-performing a 2-stroke seems to almost defy logic. I searched Google.

It sounds as though, 2 strokes are slowly being replaced by the heavier, more complicated, albeit more efficient 4 strokes, but not nessecarily due to equalled performance. More like, regulations, environmental concerns, etc.

I had a similar argument with another friend a while back about his crotch rocket - he had told me it was 4 stroke and indeed it is. I was shocked and still am shocked.

Anyone know why and when these bikes started turning to 4 stroke technology?

Posted: Wed Aug 22, 2007 4:29 am
by iknownothing
Fuel Efficiency, Emissions, Quieter and Engine Life, are basically it. Its technology which has caught up to the power of 2 strokes. A well designed 2 stroke will always have more torque, and far greater power-to-weight ratio.

Posted: Wed Aug 22, 2007 6:53 am
by feyd
I prefer 2-stroke.

Posted: Wed Aug 22, 2007 10:13 am
by ReverendDexter
For power to displacment, you're not going to beat a two-stroke. In a two stroke motor, you're getting a combustion on every downstroke of the piston. Contrast that with a 4 stroke, where you're only getting a combustion every other downstroke.

However, by the way they work, 2-strokes don't burn the entire air-fuel mixture, and they burn oil. Both of these lead to a lot higher hydrocarbon content of the exhaust, and lower fuel economy (also, you have to run premix).

Posted: Wed Aug 22, 2007 10:16 am
by RobertGonzalez
Man, I love our forums. Where else would you find a bunch of web nerds talking about combustion engines.

Posted: Wed Aug 22, 2007 12:59 pm
by ReverendDexter
I wanted to ad this earlier, but for all you car-driving types, if you're a fan of the 2-stroke, try a rotary engine. Again, where your normal 4-stroke engine gets 1 combustion for every 2 revolutions per piston, a rotary engine gets 3 combustions per every single rev! Which is why you can get 200+ hp from a naturally aspirated 1.3L engine. Of course, they suffer from all of the usual 2-stroke faults, and you have to learn to drive them correctly (i.e., keep the rpms up), or they become horrendously unreliable.

Posted: Wed Aug 22, 2007 1:29 pm
by RobertGonzalez
Rotary engines rip! I remember doing an engineering paper on Felix Wankle. It is a friggin' awesome design and concept.

Posted: Wed Aug 22, 2007 1:56 pm
by Christopher
ReverendDexter wrote:However, by the way they work, 2-strokes don't burn the entire air-fuel mixture, and they burn oil. Both of these lead to a lot higher hydrocarbon content of the exhaust, and lower fuel economy (also, you have to run premix).
It should be noted that this is where the four-strokes have closed the gap. Because two-strokes do not burn the air-fuel mixture as completely as four-strokes the gap in actual power produced is narrowed. Continued improvements in four-strokes have shrunk that gap. I recall reading that there are some thermal reasons why multi-cylinder two-strokes are less efficient than multi-cylinder four-strokes -- further closing the gap.

Posted: Wed Aug 22, 2007 6:38 pm
by alex.barylski
arborint wrote:
ReverendDexter wrote:However, by the way they work, 2-strokes don't burn the entire air-fuel mixture, and they burn oil. Both of these lead to a lot higher hydrocarbon content of the exhaust, and lower fuel economy (also, you have to run premix).
It should be noted that this is where the four-strokes have closed the gap. Because two-strokes do not burn the air-fuel mixture as completely as four-strokes the gap in actual power produced is narrowed. Continued improvements in four-strokes have shrunk that gap. I recall reading that there are some thermal reasons why multi-cylinder two-strokes are less efficient than multi-cylinder four-strokes -- further closing the gap.
Interesting.