how to raise max RPM's

Well I am planning on 810, head work, crank work, built con-rods, FI, and heavy cams.... but then how do you know the "red line"

as for surprising the sport bike guys, I know I will never touch my buddies warmed up r6....not with out a budget of $100,000. I am cool with that. All I want is to build my bike.

I am asking these questions because talk is free and information should be bounced around these forums... but if the best info to be had is; "it's expensive, not common, not worth it..." well then it might be time to stop asking questions on this forum.


As for unrideable.... I am smart enough to keep everything working together. The goal is a nice smooth torque curve on the dyno.

hardr0ck68 wrote:

Well I am planning on 810, head work, crank work, built con-rods, FI, and heavy cams.... but then how do you know the "red line"

as for surprising the sport bike guys, I know I will never touch my buddies warmed up r6....not with out a budget of $100,000. I am cool with that. All I want is to build my bike.

I am asking these questions because talk is free and information should be bounced around these forums... but if the best info to be had is; "it's expensive, not common, not worth it..." well then it might be time to stop asking questions on this forum.


As for unrideable.... I am smart enough to keep everything working together. The goal is a nice smooth torque curve on the dyno.

It seems like you know what you want to accomplish with the your budget, and that's cool. Keep us posted on your progress!

hardr0ck68 wrote:

Well I am planning on 810, head work, crank work, built con-rods, FI, and heavy cams.... but then how do you know the "red line"

as for surprising the sport bike guys, I know I will never touch my buddies warmed up r6....not with out a budget of $100,000. I am cool with that. All I want is to build my bike.

I am asking these questions because talk is free and information should be bounced around these forums... but if the best info to be had is; "it's expensive, not common, not worth it..." well then it might be time to stop asking questions on this forum.


As for unrideable.... I am smart enough to keep everything working together. The goal is a nice smooth torque curve on the dyno.

Geesh....that's reasonable enough. Find a 750 head to start with. They're 10CFM @10" over a stock 650 head before you touch the ports. 69 vs 59. They'll go low 80s with stock valves and runner size doesn't have to be over done. That will meet piston demand for a very high VE number. Much better ports IMO than the KZ900/1000 heads were from the factory.

Web 110 Grind cams will make upper RPM Power.
Web 118 Grind would better suit all around street behavior.

Even the 118 grind with 34mm stock 750 intake valves meets curtain area requirements.

110 has a quicker ramp speed

I'd put new aftermarket buckets in it as well.

29mm CRS Keihins to feed it. Those carburetors rock.

FI is cool but you better have a good map to start with on a fresh built high performance motor or you're rolling the dice in a very risky game

Obviously a good pipe to top it off.

hardr0ck68 wrote “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.”

Higher revs on a stock KZ650 is not the way to make power. According to the Kawasaki Factory Service Manual horsepower begins to drop after 8,000 RPM and torque starts to drop after 7,000 RPM. If one of these engines was running at 11 – 13,000 RPM (without blowing up) it wouldn’t make any more power than if it was running at 5,000 RPM. Perhaps if a bunch of modifications were made, such as larger displacement, different heads, turbo, fuel injection, NOS, different crank, different rods, etc., higher revs might contribute to additional power, but that wasn’t the original topic. Ed

think of how much faster you wear out parts at 8,000 rpms versus 6,000 rpms,now try to imagine 13000rpms.i dont imagine youll find many of the new sportbikes running origional motors at 30,000 plus miles 30 years from now that can still carry themselves down the road :laugh:

hardr0ck68 wrote:

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 havent seen anyone else wanting a 30+ year old engine to rev up to 13-15,000 rpms either..so buy a zx6, put the engine in your kz650, and spin your heart out...problem solved :laugh:

You will know the redline one of three ways,

1) When quits pulling miles per hour.

2) You put it on a dyno and see when it quits making horsepower or torque. It depends on what you are building for. Over reving it, will be a waste of time.

3) You over rev it and it blows up.


You talked about a smooth torque curve on the dyno. You did not state the rpm range, that you wanted the torque in?

I did not know what to think, after reading your opening statement:

"I am asking these questions because talk is free and information should be bounced around these forums... but if the best info to be had is; "it's expensive, not common, not worth it..." well then it might be time to stop asking questions on this forum".

NOTE- If you are looking for a special, one of, engineering project, this might not be the site for you. If you are looking for a performance or racing build, we can help with that. Information or ideas are free here, disrespect or wasting other peoples time, is not free.

We want to help, will you allow us to help you, is the question?

No I am totally in for advice, and I do listen...

It is just frustrating seeing all the comments along the lines of "if you want to o that fast, buy something else"

We are all here because we are into these old bikes, I don't want to play with a new bike, I like playing with mine.


I know that on the stock motor more rev's will not help. My goal is 100hp on a dyno with a smooth torque curve...so I think I will need to get a bit exotic. If someone has built a 100 kz650 before then awesome, just let me follow that pattern. I know that Dave Slone got about 85hp on a dyno with a 810 kit and FI. So I am going to need more than that.

I know for sure I am not the only one who wants to build a 650/750 block. Dave did it, Steell was all about the small bikes, Mark Wing has a hot 650.... so should they have all just bought zx6's?

will61310 wrote:

I havent seen anyone else wanting a 30+ year old engine to rev up to 13-15,000 rpms either..so buy a zx6, put the engine in your kz650, and spin your heart out...problem solved :laugh:

Well I only want to spin those r's because I thought that that is how these motors made power... what is so different in the design of the newer bikes that makes them like high r's??

The thread has gotten to much into what I want for my bike. What I wanted to do with this thread is to find out what limits the rpm's of these motors? What blows up first? Why don't they make power in the high r's?

I just want to learn about the operation here, I know it's a bit of theory but I build this bike to learn....

Old bikes are good, I fully understand what you are saying.
I know sometimes things get off topic but most people mean well.

100 hp is hard for most 900/1000 to do. It doesn't sound like alot, but it is. It's okay to push the envelope, it makes for some good inventions.

You might think of trying to reach your goal in steps, not big leaps. That could benifit you and others with kz650's.

I have not seen a dyno chart for a 810, as of yet. That alone, might just help others.

You know that you will need a good bottom end for your project. You know the cylinder and pistons. Your biggest problem is the head! That will be the key or limiting factor. Buy cams that work well with the head, I strongly suggest that you start with carbs. Why?, this would be a good base line for you and others to look at.

A good motor with carbs and a dyno print out. Then move to fuel injection, check your improvements. There will be a need for dyno time along the way, no getting around it. You will be able to keep step by step records of your build.

It's not all about the bike, it's about your learning curve also!

Have you ever seen guys that could build any kind of motor, and make it go fast? The secret is, it's the small things! Not the big fancy parts.

Vance and Hines, blew up 65 test motors, before they got a good one. After that it still wasn't fast enough to qualify for a race event. Walk before you run and things will get better, and last longer.

hardr0ck68 wrote:

will61310 wrote:

I havent seen anyone else wanting a 30+ year old engine to rev up to 13-15,000 rpms either..so buy a zx6, put the engine in your kz650, and spin your heart out...problem solved :laugh:

Well I only want to spin those r's because I thought that that is how these motors made power... what is so different in the design of the newer bikes that makes them like high r's??

Newer bikes have smaller valves, less weight to open and close. Shallow valve angle. Twin swirle combustion chamber, it fills the cylinder better. The carbs sit higher on the motor, the gasoline is being dumped down hill. It looks like a big funnel to me..............

To expand on another side issue----

More rpm will equate to more hp at a given torque production. Suppose we have an engine that makes 50 lb/ft of torque from 5,000 to 12,000 rpm. That engine will make MUCH more horsepower at the higher rpm than the lower one. Dynomometers measure torque and calculate horsepower. Horsepower is basically a mathematical relationship between torque and engine speed (rpm).

asbestos suit on.