question on the new yz250f for you engine guys

In the August 2000 issue of dirt bike it tells how the 250f was developed from the yz125 frame, swingarm, rear rim, clutch and even the 125 crank. However the engine is obviously downsized from the 400, they still used the 5 valve engine. The bore had to be large enough on the 250 to accomidate room for 5 valves to be used. So Yamaha created the yz250f with a very large bore and very short stroke and 13,500 rpm limit.

My questions is in the past what type of engine power is most closely related to a large bore and a short stroke with high rpm's. Is this going to give more torque or HP than ussual. What other characteristics do you see coming from this engine?

The fact that it's a 5 valve engine doesn't mean it's short stroke, although that's likely. The R6 has 5 valves and only 150cc cylinders. The YZ250F could be a single R1 cylinder. The shorter stroke/higher RPM motors are usually high HP with a narrower power band and less low end torque, but they are going to be racing 125s which have NO low end!

PeteN is right on the money on all his comments.


'99 WR400

'92 GSXR 7/11

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There was a good article in one of the mags on this engine. It seems that with the 5 valve head, you cant have a bore smaller than 77 mm. Smaller and the design is inefficient. So the bore on the 250 isn't much smaller than the 400. The stroke is apparently as short , if not shorter than the current 125 2 strokes. That is why the rpm limit is so high(less piston speed). I would worry that you may have to give up torque because of the short stroke. It should be a bulletproof setup though.

dave you have it right on from what I have read. In this engine anything smaller than 77 MM bore makes the 5 valve engine inefficient because all of the surface area of all 5 valves can not be used inside of the cylinder.

My question I belive has been answered. This is going to be a high HP low torque engine for a 4 stroke. Maybe we should call it a 3 stroke. A compromise between the two :)


because an engine is a short stroke, doesn't mean it won't have a good spread of power.

possibly a four valver would be better, but if you remember the fzr's came out in 1985. so i'll back Yamaha.

have you ever smiled at the fact that we use a single exit exhaust, or that the brake lines take a different route.

fudging design, marketing & patents have more to do with all this than the last one BHP.

if the world superbikes produce 150BHP at the rear wheel that means each 500cc cylinder is producing 73BHP. the WR/YZ's are producing 40-45!

There are a few errors above me here....I'm just going to clear these up if you don't mind.....This is coming from a Yamahead.......1st..the Yam R6 does not have 5 valves, it has 4.........2nd..I also thought the 250F might be one cylinder off the R1 1000cc 4 cyl., but it isn't, the R1 has a 74mm by 58mm bore and stroke, while the 250F has a 77mm by 53.6mm B/S.....which is a little shorter stroke than the YZ125's 54.5 stroke.......3rd..the Yamaha FZR's did not come out till 1989, the first 5 valver was the 1984 FZ750.........And last, we are just going to have to wait and see what kind of power these motors are going to make, I figure Yamaha wouldn't have made a WR (woods) model that made top end horsepower only, it'll probably have a good spread of usable power. You know nowadays they can make motors make whatever kind of horsepower they want to, computers and data aquisition, and digitally programmable ignition mapping, make just about anything possible, the next step will be fuel injection on dirt bikes, like Cannondale is doing.

As far as the street bike stuff goes I dont know a thing. I dont get into that area to much. However you made some really good points on how they can make about any type of engine perform however they want it to perform with new technoldgy. I guess we will just have to wait and see. However I am impatient!!!

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