twin cylinder vs single cylinder design questions

[68chevyc10355]

I have been browsing over some other forums in addition to this one. There seems to be a common theme that nearly all owners of fourwheelers compare fourwheelers to a banshees in the way of horsepower, torque, and drag racing. Multiple times I have seen “imagine what ours would run like if it had 2 cylinders.” To me this seems like they are overlooking many variables because they make it seem like they are going to double their horsepower, or see a huge gain. Another common theme when comparing to a banshee, the four strokes always bring up that fact that they have so much more torque, which is very true. So, its seems to me that if a twin cylinder motor with a certain amount of cc’s is compared to another motor of the same cc’s, that is only a single cylinder, that there wouldn’t be much to gain in the way of drag racing over a short distance. Or another way of thinking about it would be like comparing a 350 shee to a 350r, but I am sticking with the four strokes because that’s what the subject gets brought up about.

 

1st: I would think that the twin cylinder would have less in the way of low rpm torque, which so happens to be the number one thing that the big bore four stroke brag about over the shee.

 

2nd: I would think that the motor would be able to achieve a higher rpm due to the shorter stroke, and could possible achieve a higher hp number. But once again this would be moving in the direction of a street bike motor, which is moving in the opposite direction of the low end torque that gets bragged about by them.

 

3rd: Wouldn’t traction be a little harder to get with the twin because instead of a power stroke every 720 deg of crank rotation, there would be a power stroke every 360 deg of rotation for a four stroke. Also, to leave out of the hole as hard as they do now they would have to be higher into the rpms. So the “power pulses” would be closer together and wouldn’t give the tires as good of a chance to re-grip, because once a tire starts spinning its hard for it to get good traction again.

 

I was hoping that some of the drag racing gurus and guys who know a lot about motors would be able to enlighten me on this subject, and whether or not my thinking is in the right direction. It seems to me that these guys in other forums are basically just trying to make a four stroke version of the shee, but it would be simpilar to just get one because of how much time has been put into these motors.

[Larry's Shee]

Intesting topic. In Gorden Jennings' book at one time AMA allowed 350cc single cyl Kawaskis to compete against 250 two cyl. (both 2 stroke) He showed mathmatically that the 350 couldn't produce as much hp mostly due to the piston/rod weight prevented it from spinning as fast. I would have to think that would hold true with 4 poke.,a 2 cyl would make more power less torque , with relative bore/stroke ratio than single. Plus adding another cyl would probably add at least $1000-1500 to the cost. How many RPM does a YFZ 450 turn? Think of what a 2 stroke could do if you could turn the revs of a Hybusa !! "Course them little slipper pistons will start cocking and rocking with a lot less wear.

[burhead]

I'm by no means a guru but I have been running a similar question around in my head. I have polaris 2 strokes as well as a banshee. My 400 sport runs damn near dead even to my banshee in a 300' race. Both bikes have the same mods (pipe, air filter mod and shaved head. the sport has a lightened flywheel and a bigger carb) but the sport is a little heavier and has a PVT trans in it. This is really an apples to oranges comparison. These 2 bikes are totally different to ride trails but in a drag race they run so close. It is fun to watch though because the sport gets beat off the line everytime due to the clutch but it launches pretty consistantly and ends up running the banshee down. I guess to relate it to your obsevations: There is no doubt that the bike with the single larger piston has a broader power band where as the banshee has a very narrow power band. This could be due to alot of different factors though. All I know is that they are both a blast to ride.