how to raise max RPM's

Here are a few older threads dealing with squeezing power out of the KZ650/KZ750/GPZ750 with an 810 kit.

The first line has good information on page 3 regarding a German bike that put out about 92HP at the rear wheel (about 113HP at the crank).
100hp KZ650

The second has good information from the GPZ750 FAQ
stock cam lift

The third regarding ignition/timing
KZ750 - GPZ750 IC Igniter Mod

Well said.

This is one of the best basic explainations of the evolution of motorcycle engines that I've come across.

kpier998 wrote:

The reason these older bikes don't make horsepower above 10k or so is because of the bore and stroke. The bore of older bikes was much smaller and the stroke much longer for the same displacement.

Additionally the two valve heads do not provide as much air flow capability as the 4 valve heads.

But, the main issue is bore and stroke. As someone pointed out, the rpm limit of most engines is a function of piston speed. Kevin Cameron stated some years ago that 4500 feet per minute of piston speed is a reasonable upper limit for most engines. Let us take a look at how this applies to our motorcycle engines with a specific example in which we compare a Kawasaki 1000 J motor with a late 80's / early 90's ZX10 motor:


Old tech:
J motor Kawasaki with 2 valves per cylinder
Displacement = 998 cc
bore = 69.4mm , stroke = 66 mm
piston feet per minute at redline (8500 rpm) = 3683 fpm

Newer tech:
1989 ZX10
Displacement = 997 cc
Bore = 74 mm, stroke = 58 mm
(I don't know the redline for this engine. Instead, let us solve the fpm equation for rpm rather than fpm. First, assume the same fpm as the KZ1000 J (3683 fpm) Now, solving for 3683 fpm with 58 mm stroke, the equation will yield 9692 rpm. What I am saying is that the ZX10 with it's 58 mm stroke can turn 9692 rpm before its pistons reach 3683 fpm.

This means that although the two engines have the same displacement, the ZX10 can spin to 9692 rpm and be under no more stress than the J motor at 8500 rpm.

Summary - An engine with larger bore and smaller stroke will have more rpm capability than one of the same displacement that has smaller bore and longer stroke.

Now, what else can be said about the larger bore motor? We already see that it has more rpm capability. But, since the bore is larger, there will be more room for larger valves in the head, since the head will have a larger combustion chamber diameter to go along with the larger bore. This is true whether the head is two valve or four valve.

This means that the larger bore motor can be fitted with larger valves thereby providing more valve area to provide air flow to feed the motor at the higher rpms that it is capable of turning. With more valve area to work with, the lift doesn't have to be extremely high to gain adequate flow to feed the motor. This means that the valves are not as stressed at higher rpm.

To help fill the larger bore cylinders the manufacturers switched to four valves per cylinder (Yamaha did 5 valves for a while). Four smaller valves will provide more air flow than two larger valves and will be more reliable at high rpm because they will weigh much less. You won't be dropping valves or breaking valve stems off the valve heads with the smaller, lighter valves - less stress.

More RPM and more air flow equals more horsepower potential.

That is why you can't build the old motor to make the same horsepower and turn the same rpm reliably as the new ones do.

There are probably many other factors including valve angles, intake tract shape and size etc, but the key to it all begins with the larger bore sizes.

These are not my opinions, but those of people more learned than I in the ways of internal combustion. I am just spouting back what I have read over the years - but it does make sense to me.

Hope it helps.

P.S. The formula for piston feet per minute follows. You have to supply the target rpm value such as 8500 rpm:

fpm = (stroke in inches X 2 X RPM / 12
or
fpm = (stroke in inches X RPM) / 6

to convert your bore from mm to inches:
stroke in inches = (stroke in mm) / 25.4


Kirk

kz650 stroke 54.14mm / 25.4 = 2.13inches


(2.13x 11,000)/6 = 3905fps

Well within the reasonable limit of 4,500.

(2.13 x 13,000)/6 = 4615fps

Just outside the reasonable limit... bummer

So it looks like 11k and 12k shifts should be under the hard limit.

:laugh: 3905fps !! That's funny. The hottest handloads I've ever fired in my .300 Winchester Magnum were just under 3300fps. I think you better wear kevlar undies if you plan to fire those pistons through the head at that speed. Ed :laugh:

Serious stuff.

The most knowledgeable people on this site tell you what to do in order to make power with a 650 many different ways,yet even though they tell you the rpm limits, you seem like you will argue your point till you get the answer you WANT to hear..Wow, i cant believe this is still going on...

I am going to use stock KZ750 cams/head and head shaved ..030" in my 810 Kit. I'm thinking It'll give me good torque/streeablility, and save my Tach drive too!! If I don't like it, I'll re- grind the cams to a more aggressive profile. Great Thread by the way !!
What do you guys think??
MAC:)

roy-b-boy-b wrote:

Serious stuff.

thats the magic of a 900-1000 rod,no bolts to break! :S although my turbo crank does have a set of knife rods to make it even stouter.

Did that guy loose his Hand ????

looks like just piston and rod bouncing off track,im sure he'll never forget it though! :S

will61310 wrote:

The most knowledgeable people on this site tell you what to do in order to make power with a 650 many different ways,yet even though they tell you the rpm limits, you seem like you will argue your point till you get the answer you WANT to hear..Wow, i cant believe this is still going on...

No, I will keep asking questions and getting information until i think I know the limits and how to push them.

In his post he stated a "safe" piston speed of 4500fps, in steell's post he mentions 5000fps if i remember right; it is on the first page. It seems to me the heads are the weak part of these motors, and I can either go crazy with TI parts of just use forced induction to get around that.

His post about piston speeds supported the idea of a higher rev limit.

When I send my rods in to get specd and refurbished if needed I will ask them what kind of abuse they are good for... if the rods will hold, the pistons don't melt, and you can shove the air in to the head i don't see why these motors can't go to higher r's.


are you telling me all the stroker 'busa shift at 8,500?


::EDIT:: if you think raising the r's is the only thing I am looking at to make power you must have missed my thread asking about fittin z1 903cc liners to a 650 block. My hope for this motor is 860cc maybe 880... either way i will need custom liners and pistons, but I like the sounds of that anyways!

are you telling me all the stroker 'busa shift at 8,500?


Now you're just being stupid, comparing a kz650 to a busa??:dry: