how to raise max RPM's

I take mine up to 11,000 ALL the time. It's going to be hard to have a street motor that goes much higher. I did up the bottom end once but ended up spinning a bearing anyway. I had the rods peened and polished, 1 thousands taken off each side of the big end, the small hole bushed and everything micro balanced. The big end has pretty tight clearances to squirt oil at the small end through the little notches at the top of the big end and it builds a little heat so opening up the sides a little cools things off a bit. It's a lot cheeper to have a back up bottom end waiting for a rod to go.

I think Lorcan runs Carrillo rods in his turbo 810. I don't know what rpm he is turning at 211 mph, but I think it might be over 11k.

hardr0ck68 wrote:

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.

The design. It is the total design that limits the usefulness of the kz650 engine. Now, you can look at the GPz750 engine, and see that it can be taken a little further, but not much. Here's the design characteristics that are limiting:
1. two valves per cylinder. This very much limits the design. Yes, you can push the envelope upwards with porting, larger valves, and more radical timing, but this narrows the usefulness of the engine, which is a sign you're trying to stretch the design past where it works well.
2. Wide included valve angle. With the valves splayed apart as wide as they are, you end up with a very open combustion chamber, so when you begin to use more radical cam timing you are forced to raise the compression ratio by doming the pistons more radically, thus stretching your actual combustion chamber out to a very ugly and inefficient shape. But you have to have this wide valve angle because
3. it's aircooled. Now air cooling has lots of advantages, none of them performance. Low maintenance, low weight, easier to manufacture and work on, but you have to have a high included valve angle to let air down around both sides of the exhaust valve if you want the engine to survive.
So, you have a light, simple aircooled 4 cylinder engine, that gets as much horsepower as it can within its design limitations. Plummen's right. Your best bet is bigger, not faster. Find a 750 cylinder block and head, or buy an 810 kit, if you want to enjoy your bike. Otherwise, the horsepower might be there, but it just might be a pain to ride. The only reason racers do what they do is because the rules say they can't make it bigger.
As for me, I'm looking at a set of 1260 pistons and sleeves before I get my GPz1100 done. :woohoo:

Splandman wrote:

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?

No. Just, NO. Rotating weight does NOT rob you of horsepower. Now, it DOES rob you of acceleration, so if you're talking about dragracing, this matters. Two things about this: lighter parts for racing are one thing. But racers use parts that make them faster at the expense of knowing they'll have to replace them, too. Fer instance, aluminum con rods are really cool, but racers know that they have to check them for stretch every so often, 'cause aluminum does that. How often you want to tear down your engine to check it?
Also, the rotating mass of the crankshaft is a GOOD thing for riding on the street. I don't care how much street racing you do, you'll eventually have to pull away from a stop sign in front of a cop. Flywheel weight is good then!

steell wrote:

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.

Man, you like math! But, again, this doesn't take into account what the shape of your combustion chamber has become, which I don't think would sustain anything close to complete combustion at 15.5k. Maybe if you put in a second spark plug. I've seen somebody do that on a kz1000, but I dunno about a 650. And it doesn't take into account the valve timing you'd have to run to get it to breathe at 15.5k. So, what your number prove is that if somebody took a kz650, made a water-cooled 4 valve head, it MIGHT make it to 15.5k. But the $64 question would be, "Is it still a KZ650?"

Pstrbrc, the original poster asked:

**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 **

So I answered the question he asked. He didn't say he wanted more power at that rpm, just said he wanted to turn it faster

Thread topic is

how to raise max RPM's

, so that's exactly what I answered.

Your posts do a fine job of covering the power angle, nothing for me to add, so I don't

steell wrote:

Thread topic is

how to raise max RPM's

, so that's exactly what I answered.

Your posts do a fine job of covering the power angle, nothing for me to add, so I don't

Oh, yeah, wasn't saying you were wrong, :ohmy: just wanted the original poster to understand that rpm was all you were addressing. Wouldn't want him to take what you said in any way shape or form as an affirmation of his idea, just answering his question.
And I was just trying to help him understand that his question might not really be the question he meant to ask. :blink:

bringing a thread back from the dead.... kinda a new old question


If raisin the max rpm does not help you build power on these motors, then how could you:

A: benefit from a raised RPM?

B: Increase the power in high (11-13K) RPM range?

C: Make bigger power on a 650/750 motor?

I was getting into Ford 302 engines a while back and i was looking at all the radical things i could to run against 350 chevys.

351 winsor ctank,etc,etc, Boy i was going to build a 302 out of this world.

And then one day it hit me. I had a 351 winsor and it only needed to be removed and installed and cammed up a little and i have saved $3000.00.

Thes bike are what they are and you can do anything you want with them but as the questions asked,"Is it still a 650".

I remember when the 650 was taken to the firestone proving ground a long time ago and run for 24 hours. They broke over 120 records. Was stated it could never happen again.

As the stated limiting factor for the higher RPM is the two valve head you are going to have to get a better head.:laugh:

Or, You could buy a bike that will turn the big rpms and wonder what the benifits would be to own a bike that would pull from 3500 rpm to 5000 and not have to shift.

hardr0ck68 wrote:

: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?

Im pretty Sure the zx6 doesnt redline till over 12 or 13,000.. just buy one of them and keep your 650 reliable, then you can have both..

If I wanted to buy a zx6, or an R6 or a cbr600 I would own one now...and be blissfully ignorant about what it takes to make an engine work and work well.


You folks never get on a gpz1100 guys telling them to just buy a busa, or zx1400 and not bother with adding a turbo or NOS.


Yeah it will be questionable if it will still be a kz650, but it will be MY bike.

I really miss Steell cause that guy knew stuff other than "buy a new bike"

I think you need to keep it all in perspective. These motors are ancient technology and were never designed to turn the kind of rpms you desire. Sure it's fun to kick around the idea of a 13,000 rpm KZ650 surprising modern sportbike guys at a stoplight, but like others have said, unless you are willing to spend an outrageous amount of cubic dollars on a hard to ride, unreliable hand grenade then you should probably stick to more conventional mods and rpm limits.