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Motocross Star/Valli Yamaha 250F's putting out over 50hp.....WOW

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I can see why teams aren't going to publish numbers, but the guesstimates vary widely. I wonder what the true number are, more so how are they achieving them? I've heard the Yamaha teams use GYTR heads / cams, cosworth pistons, crank works? Cranks....so is there more massaging going on or is it all bolt on now?

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Well my 2009 yz250f is pushing 50 hp at a guess from how much hp the after market parts should give. Cams 2 hp ported head 4 hp 14.1.1 HC piston 3 hp fmf pipe 1 hp yz450 carby 3 hp back for screen removed half a hp so about 49 and a half hp it also has boysen quick shot oring mod and boysen power wing not sure if that adds power or not.it is reliable as. But it has a carrilio conrod boysen high flow water pump and over size radiators so it runs pretty cool and the rod is heaps lighter than a stock rod and is stronger. I've still got to put a vortex X10 ignition in Wich adds 4 hp so it should be up around 54 hp at a guess but got to Dyno it for exact numbers

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It doesn't work like that, each mod doesnt add intop of the other, some male power ar low end, some add power at overrev 

 

IMO the 2009 wasn't very pierful std, on a UK dyno it would only be about 33hp , modded it would probably only get to 38

 

Mxa dyno gives very high readings 

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2 minutes ago, mog said:

Mxa dyno gives very high readings 

They are like the butcher that keeps his thumb on the scale

You never really know what your getting 

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Had a  2008 CRF 450  years back with a modded pro motor. Cant remember the team. I think it was Paul Carpenter's bike . These motors had an estimated 3-4 hr life span. It had one race on it and I got maybe 2 more hrs out of it. It didn't have anything super exotic in there. HC piston, one off cam, Pipe design for the mods, etc. However the porting work was the issue. The failure was a broken intake valve. On inspection it was easy to see why. They removed ALL of the valve guide and supporting casting that protrudes into the ports. This leaves about 1/2 the valve stem support and was fully responsible for the short life span. Stock head with all the other work lasted as long as a stock motor except the cam chain needed more frequent replacement.

 

 

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18 minutes ago, lowmass said:

Had a  2008 CRF 450  years back with a modded pro motor. Cant remember the team. I think it was Paul Carpenter's bike . These motors had an estimated 3-4 hr life span. It had one race on it and I got maybe 2 more hrs out of it. It didn't have anything super exotic in there. HC piston, one off cam, Pipe design for the mods, etc. However the porting work was the issue. The failure was a broken intake valve. On inspection it was easy to see why. They removed ALL of the valve guide and supporting casting that protrudes into the ports. This leaves about 1/2 the valve stem support and was fully responsible for the short life span. Stock head with all the other work lasted as long as a stock motor except the cam chain needed more frequent replacement.

 

 

Paulie was/is one fast mofo!!

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Elevation is the best normalizer. A 50hp motor at sea level might actually be 50hp, but a dyno . has a "corrected" hp in the software, as with all the corrected configurations, and crank vs rear wheel hp, etc. Same with temperature, RAD (relative air density) guages will change jet sizes every couple of degrees for perfect jetting. So if you start racing in the morning in cool weather, and the weather warms up considerably over the day without changing carb settings, youre jetting is "off", or rich, and start losing considerable hp. High comp vs stock compression ratios make this even more critical.

Example, did a CR500 test on a dyno the rider was really happy with. But the dyno said it was 48hp. Leaned out jetting by 2 jet sizes, and the dyno was at 62hp. He was a lot happier with corrected carb jetting

There is an online jetting calculator, that can you can use anywhere, ...will post it when i re-find it

 

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On 6/29/2020 at 9:20 AM, mog said:

It doesn't work like that, each mod doesnt add intop of the other, some male power ar low end, some add power at overrev 

 

IMO the 2009 wasn't very pierful std, on a UK dyno it would only be about 33hp , modded it would probably only get to 38

 

Mxa dyno gives very high readings 

I don't see the 09 throwing down 50hp w/o some serious mods, beyond the bolt-on stuff. I had a '10 that was pretty decked out with GYTR head/piston, eleven10 carb mod etc and it was nowhere near 50hp. I'm sure it's a strong running 09 but I just don't think you could reliabily see 50hp w/o running that ragged edge and rebuilding after every race or 2.

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On 6/3/2014 at 11:49 AM, harrperf said:

 

 

On what dyno???

That's always the "question"

On an engine dyno it's possible to be at 50 with what we call 45-46 horse bikes.  On a buddies rear wheel dyno the a GP winning 250f last year made 38hp.

On a different "rear wheel" dyno it makes 47hp. 

It's pissing contest to see who can claim a higher number.  Whats more important is which bike wins starts and turns fast lap times.  Big hp number dont accomplish this - I build 45 horse two stroke 144's that make more power down low and rev farther than stock and are up 10hp from stk.  Sounds great on paper.  But over 5 laps with identical riders a 40 horse 250f will kill it in lap times. 

 

The statement above of more low end is why they are getting such good starts and so fast around the track.  the hardest a bike pulls in ANY gear ISN'T at peak hp.  It's at peak torque.  So if you make peak torque better and/or earlier - the bike accelerates faster.

 

Likely wasn't @mog's orange is best dyno...:busted::cheers:

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On 6/3/2014 at 6:10 AM, Kev_XR said:

Spinning at speeds turbines run. (well not really)

12,000 rpm or 200 revolutions per second. Mind boggling really.

Any oil break down and the engine is gone.

Detonation from lean gas air mixtures.

Not to mention valve float.

Does the AMA have a rev limit yet? I can see that coming when 250F bikes start blowing up like the old 125s.

You don't know what you are talking about.

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