how to raise max RPM's

I have been thinking about this for a bit, what is the limiting part of the kz650 design that sets the redline at 9,000.


Is it valve float?

Crank bearings?

Piston speed?

I have read that some folks say higher RPM's are not what it takes to make power... I have always been under the impression though that with these inline 4's higher rev's is the way to go.





**I just want to hear this discussed, I have no current build for which this information is needed but I do wonder what is needed to make it to 11,000-12,000 R's **

here is what it takes to get 11,000 rpm. bigger carbs,bigger cams, portwork[lot's of it] ,stiffer valve springs, bigger valves,more compression, better exhaust, electronic ignition shall i go on.

:lol: Well then how about 12,000


I understand that the work to get a motor to turn like that won't be cheap...

Why is higher compression on the list? Wouldn't more squish slow the motor sown?

Do you think a motor with this work could be a street bike and reliable while still shifting at 11g?

no i think a combination like that would only be good one place,the race track, actually if you change your ignition over to electronic [im not saying this because i have stock in dyna] you can significantly raise your rev limit because your not dealing with points bounce [the point springs can't keep up] all that other stuff i was mentioning would help the bike but it's not nessesary to make it rev .i would be putting a rev limiter on it for sure. i figured out a long time ago the stock redline was only a sugestion. i do caution you to use a rev limiter. i was ripping around on my 1000 with a buddy on his tl1000s, i missed a shift with the throttle pinned in 2nd time motor reved to the moon and snapped the crankshaft in half[no rev limiter ] bit of a shock to the system.

more cubes and more air is the secret !

revs are one thing making power high up in the rev range is another.
stopping the valves from floating and getting enough fuel/air in out a 2 probs. stopping the thing from throwing its component parts all over the floor is another
have the same probs with the aircooled vw race engines i build, you can make them rev and make power but the faster you make it rev the more likly it will all end up on the floor in a big pile of broken alloy, have hit 9000rpm with my own vw motor.
it is fun doing it though.
cheers rob

sooo a great port job to get the air in, heavy valve springs to keep the valves shut and some fat cams to get enough air in and out.... It's all in the top end?

electronics, rev limiter any thing else?


Will the cam bearing surfaces (or crank bearings) need any coating to keep them from puking?

Someone I know told me to empty all the rotating mass out of the bottom end you can to make it spin up faster and he claimed that rotating weight in the engine robs you of HP.

Someone I know told me to empty all the rotating mass out of the bottom end you can to make it spin up faster and he claimed that rotating weight in the engine robs you of HP.

I was thinking this also. I don't have ANY experience building bikes, so anything I say is a guess.

I was just wondering, do they make lightweight pistons and rods to help you rev more quickly? If they do, are they suitable for street use? Is the cost so much that it isn't feasible for most people when compared to the gain you get?

The cams and head work are what limits your rpm's. The cams for KZ's will only get you a couple thousand rpm's. I have a big bore cams head work a pipe and smooth bores and my bike peaks out at 10,700 rpm so spinning it tighter doesn't help any. I also have a hopped up ct70. They regulate the power for the lights by how fast the enginea spins. With the cam I have in it I now have to use 12 volt bulbs as the 6 volt bulbs just blow when the motor starts to really spin.

Some interesting responses above.

Maximum rpm is limited by piston speed and speed of airflow through the ports.

Maximum piston speed on the KZ motors is around 28 m/s (and that's really pushing it).

Max air speed through the ports is limited to the speed of sound.

Those are the "hard" limits, although maximum piston speed can be increased if you throw enough money at it (a la Formula One).

650/750 stroke is 54mm, 54mm x 2 = 108mm
108mm = .108m
.108m x 10000 rpm / 60 = 18 m/s

So, assuming you utilize the best alloy for the pistons, top of the line machining on the bore, and that nothing breaks (rods/valves/crank/etc), lets see what 28m/s will do.

28m/s = 28000 mm/s
28000mm/108mm = 259.259 x 60 sec = 15555 rpm.

So, the piston speed hard limit on the 650/750 motors is 15,500 rpm.

Above that limit engine life will decrease dramatically.

Although, if your pockets are deep enough, most things are possible.

I came up with almost the same figures but I think I forgot to carry the naught.

Is a 810 kit and stock rods shot peened enough to handle all that steel?


Soo with a hard limit of 15,500R's what do you think it would take to make a 6/750 handle 13,000 on the streets? Would 2,500 off of the hard limit be enough to keep the motor from meltdown?

Could it be done with the stock liners and wisco pistons or would the build need something more exotic?


Does anyone on these boards ahve a 6/750 that commonly take above 11,000?