Rpm 315

What has the reliability of the 315 been like with the yamaha motors. I am thinking of using this approach on a new yamaha.

why dont you just get a new 450?

why dont you just get a new 450?

thats what i'm doing :excuseme:

I already own a CRF450. I want a smaller bike. I race hare scrambles and I don't need the power of a 450. I need a small bike with better handling and a decent amount of power. A few of the guys run big bore or bore and stroke 250f motors and have good success.

I run hare scrambles and am planning on the athena kit. Priced very well.

I already own a CRF450. I want a smaller bike. I race hare scrambles and I don't need the power of a 450. I need a small bike with better handling and a decent amount of power. A few of the guys run big bore or bore and stroke 250f motors and have good success.

maybe try the 290 kit? appreantly gives 6 HP :bonk: sounds like a good gain. run it with a pipe, cams and open up you intake, air box etc. and you'll have like 43-44 i beilve, for somethign that started life as a 250 thats REALLY good :excuseme:

stay with the 315. i heard the 290 kit signs off too early. reliability should be really good because you wont rev as high to make the same power as the 250. dirtbike, the mag, i think did a comparison of the rm 315 and yz315. they said it really closed the gap between a 450 and a 250. i would go with it.

The main reason for the large power gains in the lower rpm range but minimal gains (sign off) at higher rpm is because the breathing capabilities are designed to feed/exhaust 250cc. As you increase piston displacement the engine becomes more limited by breathing capacity as rpm increases, so don't expect higher rpm gains from an even greater piston displacement. Without increasing the breathing capability you should get more low end power from a 315, but don't expect it to perform much better on the high end than a 290 or even a 250!

The main reason for the large power gains in the lower rpm range but minimal gains (sign off) at higher rpm is because the breathing capabilities are designed to feed/exhaust 250cc. As you increase piston displacement the engine becomes more limited by breathing capacity as rpm increases, so don't expect higher rpm gains from an even greater piston displacement. Without increasing the breathing capability you should get more low end power from a 315, but don't expect it to perform much better on the high end than a 290 or even a 250!

EXACTLY!!!!! :bonk:

you'll need port/polish, more aggressive cam, free flowing exhaust and intake and that may not even be enough to give that 315 enough air :excuseme:

The 315 has a bore equal to a 276 and the rest of the displacement comes from stroking which means it still retains its top end.

The piston speed at a given rpm is obviously increased with the stroke increase, so I guess [/b]if the winner of the race is the one with the fastest piston, the big stroke might win. In reality, as piston speed increases, one moves closer to the designed margin of reliability. Personally I don't want to go there unless the power advantage makes it worth the risk.

The length of stroke has essentially nothing to do with how a four cycle engine breathes. Bigger bore/shorter stroke engines may have an advantage in design when all else is equal because of greater area for valves. On the existing engine that would not matter unless one could redesign the combustion chamber, IE, build a new head to utilize the area.

The requirement to fill the increased displacement remains, whether the increase comes from bore or stroke!

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now