Ok, I think the max rpm of my scrammy is about 7k and on my never ending search for another quad I'm now thinking about the yfz450 (I work at a yam, kawi. shop now) and it turns something like 10500 rpm, I thought that two strokes spun quicker than 4's. or is the 400 just a slow turner and the 450 just a spinner?

 

you have to think about the difference between a 2-stroke and a 4-stroke when making that comparison.

your motor has a power stroke on every revolution. the 450 however needs 2 revolutions to for every power stroke.

 

I understand "2" vs. "4" strokes and what that means I've just allways thought a two stroke spun faster for some reason. none the less isn't 7000 rpms pretty slow for a 2 stroke? and 10500 damn quick for a four?

 

Pennyless's "Bubba" bike was running over 10000 rpms when it cracked the cylinder. They can do it

 

It has alot to do with age of these things. The new 4 strokes are alot more hightech than things used to be. Turning a stock 4 stroke 10,500 rpms a few years back was un-heard of. They just keep making these things better and the newer ones keep revving higher and higher.

 

you should look at it another way,if you known 2 stroke motor a little
you'll find that a scrammy 400 is consider a big bore 2 smoke.
a big bore as any single 2 stroke can't have those very short stroke in order to spin the engine that fast cauze of the very quick load of HEAT
generated by the piston and cylinder,for example the zilla 500 have a long stroke of 86mm wich just can't get any higher than 6800 rpm top,the massive rotating mass of the crank and connter balance slow the engine down but it's a must to help keep the motor assembled,,now the yamaha are pretty **** about those short stroke to
rev their engine through,that because of that,and 4 stroke will
have more high rpm capabilities cauze of the valves that speed up the engines speed.

 

It all has to do with piston speed. There is a threshold that ANY piston or rod can handle. How fast a piston moves in a given amount of time is measured in feet per second. The longer the stroke of the motor the more piston speed is has. As a basis, you generally see smaller motors buzzing higher than larger ones. Not only is it because of piston speed limits, but also a larger motor does not have the need to spin at a very high RPM to achieve good power and torque. Now when you can build a big motor and spin it fast and make power, then you make big power, but at the cost of reliability and it takes more money to put good parts in it. The NHRA Pro Stock cars run a 500 cubic inch motor, but they spin them to 9500 RPMs. That sounds unbelievable, but when you look at the motor specs, it falls into place. They only have a 3.5 inch stroke and a huge bore at around 4.5 inches. That is about the same stroke as a 350 cubic inch Chevy. AS A GENERAL RULE, more stroke equals less RPM, but more torque. Shorter stroke means you have to generate more RPMs to get the torque and HP. With the case of Polaris 2-strokes the motor must be matched to the pipe, but more so important is that the motor has to be able to make good useable power everywhere else in the RPM range. There are 2-strokes running lots of RPMs, such as drag Banshees turning over 10K, But they are drag motors. The builders of the new 4-strokes are finally making use of technology and better motor materials.Hope this makes some sense, it's early this morning[img]i/expressions/face-icon-small-happy.gif[/img]