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A stock yz250f on my dyno cranks out around 40 hp and 20lb ft.
Same for a kx250f (actually kawis' hate my dyno or like magazine dynos...it's always usually a bit shy of 40 and around 19.5lb ft)
On my dyno our best effort ktm is in excess of 50 and 24 lbft, yamaha similar torque numbers and a tick off peak hp, kx similar to yamaha etc.
stock ktm gearing 14:51 - ratio is 3.64
stock yz250f gearing is 13:50 ratio is 3.84
Now - this doesn't account for internal gearing - both stock bikes could be IDENTICAL at the tire in ratio depending on internal ratios - which a factory transmission CAN change - but often wont need to. The factory services ktm 250 trans is std ratios...
When making more power you have some options. Keep the gearing you have and get more acceleration - or reduce teeth on the back and trade some of the additional pull you COULD have had - for more width in usable power. AKA less shifting.
So - lets assume the factory yz250f makes 20 percent more torque (the only number that matters for acceleration) That puts it around 24 lb ft - lets be really generous and say its 25 and stock is 20.
You could then re gear the bike 20 percent TALLER and have the SAME pull stock has but HUGE width to power.
What is a 20 percent change?
3.84*.8 = 3.072
Working backwards - 13*3.072 = 39 tooth rear or 39/13 = 3.072.
Going to a 47 from a 50 is a 5 percent change in final drive ratio.
So if the team IS making 20 percent more torque - you would have 15 percent more pull - and 5 percent more width than a stock bike - assuming stock internal ratios. Commonly mis understood - changing sprocket teeth DOESN'T"T change gear drops. When you shift - you will drop an identical amount of RPM using a 50 or 47 rear tooth sprocket.
The funny part I laugh at with privateers and excuses from riders - assuming you have a decent built bike - you shouldn't be more than 10 percent off a factory effort in torque or hp. When you do all the math behind what that means in real world differences of acceleration - it's hardly anything. It's actually an argument against mod engines for amateurs and budget allocation on lower budget efforts.
That 10 percent is best made up with lots of chassis and suspension testing - where most privateers are more likely to be 40-50 percent off a factory effort.
The BIG difference from a factory effort is in RPM. A top team will run well over 15k for sx - I'd guess 16-16.5 on some teams based off what I hear.
We can reliably go to 15 on some builds but past that is playing with a money pit. It takes cubic dollars to chase RPM - and lots of testing. That's not even accounting for making good usable power at that RPM.
So a private effort at 14k - vs a 15.5k motor that pulls properly that high - is night and day in on track performance for a host of additional reasons.
At the ultra high rpms - case integrity becomes a problem - valve train stability and life is a challenge - and piston/rod durability is of serious importance. I personally think timing chains are a massive weak link in performance at those speeds as well.
So im not "all that impressed" by running a 13:47.
on a ktm a 13:48 is nothing far off a 14:51
And to anyone who ever wonders why a stock 250f and stock 450 at a local track in the hands of a good local rider turn similar times - and the 450 isnt just WAY faster feeling - it's all final gear ratios. a stock 250f vs a stock 450 in 3d gear can be 30 percent different in final drive ratio - aka knocking off 30 percent of torque at the tire for acceleration of the 450 - but trading it for 30 percent more width - aka why you don't shift a 450 much.
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