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Yamaha MX & Off-Road Dirt Bikes
How much HP can Yamaha's 426 produce
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[QUOTE="Rich Rohrich, post: 154020, member: 16241"] Chris - To answer your question. With the stock valve area I believe the 400/426 engine is good for about 50-54 reliable sustainable crankshaft horsepower using standard leaded race fuel. Going to bigger bores and longer strokes will fill in the area on either side of the torque peak but the small valve area is a bit of a limiting factor. I think it's possible to raise the peak horsepower revs with changes to the appropriate hard parts, but IMO there is little use in doing this with the existing valve area. Lots of people are selling "big valve" mods but I've yet to see evidence that they flow substantially better than the OEM setup. If you are looking for big peak horsepower numbers look for a four-valve engine with valve area in the 32-34% of bore area range. If you want an engine with a BIG wide table top flat torque curve then the 5 valve Yamaha is IMO the best starting point. If I were interested in doing a project like you suggested I would look to the 600 Rotax engine. The good flat track tuners can get HP numbers well within the range you are looking for and they can do it with a high degree of reliability. You can purchase an engine for a reasonable price. OEM and performance parts availablility is exceptional and general knowledge of the engine's quirks is excellent. There is a huge used market for these engines as well. [/QUOTE]
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Yamaha MX & Off-Road Dirt Bikes
How much HP can Yamaha's 426 produce
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