1976 KZ900 "old school Superbike"

Question for updating my mind....

When researching my piston kits, I found some with the same cc depiction but they had different compression ratios (10.5:1 or 12:1)...

And some of these where from the same manufacturer so I really got confused as you may have imagined. Ok Plummen, I know you realize that isn't hard to do (confused)... LOL!

What exactly does this due for performance (drag racing) going one way or the other?

Or if the plan is to use super charged/turbo, the higher compression versions are required or something to that effect...

OMR

Basically the higher that the fuel air mix is compressed the bigger the explosion when the plug fires. the bigger the explosion the more force that is applied to push the piston back down. Therefore more power or HP.
The higher the compression the more heat that is developed and the fuel wants to ignite spontaneously or early, uncontrolled.(pre ignition, detonation).
higher octane fuel is harder to ignite (ignites at higher temps) , therefore can be used with higher compression motors to control pre ignition. The higher the compression the higher the octane needed..
So higher compression = more HP but its harder to run on the street due to the fuel availability issue. A high compression motor will ware out the rings faster as well.
Turbo’s actually use lower compression piston because the compression is supplied by the turbo, stuffing the cyl to raise the compression.
I am no expert here so I’m sure some of the others will help out .
Hope this helps.

I really appreciate the feedback I'm getting from you guys.
I think I'll just go w/10.5:1 to keep it simple. Like I said, I'll get the hp rush from my future KZ turbo project.

Hey, Mark1122 - I agree with all you said, but some newer sportbikes are running over 13:1 cr. How do they get away with it? Is it more a function of ignition timing or...?

I wish i knew more about this myself. Some things the new bikes have that help them run higher compression are there computers. These can map the ignition timing just right. with the kz's mechanical advancer it can only change timing in a linier fashion. computers allow the timing of the spark to be set were ever and when ever it is needed.
generally in a 2 valve kz motor the spark comes on too late at mid rpm when u add compression. The new dyna 2000 ignition allows u too write your own map so u could retard the timing slightly at any rpm, similar to the new bikes. It also has a preset curve that does retard the timing a bit in the mid rpm’s. This also goes hand in hand with the fuel delivery.
A lean mixture will promote heat and pre ignition. a computer controlling the ignition timing in concert with the fuel mapping is a big help.
At higher rpm's, the motor is going so fast that the spark must be started later (fewer degree's before tdc) since it doesn’t have as much time for the piston top reach tdc as it does at slower rpm's. Remember that the spark is timed to fire the fuel so that the fuel will start to burn just before tdc and reach max burn as the piston is just past tdc so it pushes the piston back down. Can u imagine how fast that happens? can u imagine that the timing would be impossible to get perfect at all dif rpm's. and then add the fact that with a spring adjusting the advance that this is actually impossible. This is where a new bikes computer comes in handy.
Then there is cams. We can only set the timing once manually , by degreeing the cams on our KZ's. If we set the lobe centers to low numbers we can increase compression and improve the low end torque. If we set them higher we can improve top end. Well, what if u could automatically adjust the cams on the fly for both? Some new motors can.
Then there is the cams duration. the longer the int and exht valves are open at the same time (duration overlap)the more fuel air mix will be charged into the cyl., but only at high rpms. At high rpm's extra fuel is actually sucked into the cyl. at low rpm's the piston will actually lose fuel mix due to the exht valve closing late and the piston pushing the fuel back out(and losing compression).
The longer the valves stay open at the same time the more compression u lose at lower rpms, therefore u could run higher compression piston with longer duration cams. But the cost is low end torque.
How about the exhaust pipes primary tube, and the baffles, length and diameter ? Again these thing affect where the power is at its premium. Do u want the torque or power at high or lower rpm’s? Some new bikes have a valve in the exhaust to adjust the back pressure.
I’m know there is a lot more. I have just scratched the surface here. That is as far as my knowledge goes, just the basics. I would love to here more about this from the others here.
I hope this helps.

WOW!!! Mark1122 - I'm really impressed and thankful for the depth of your response! I agree that the computer and f.i. make it all possible to use high compr. ratios.
Since, my KZ is intended to be just a good running streetbike and our riding season is short (Mar-Oct.), I don't want to spend more time tuning than riding, other than some dragstrip runs for fun. So...simple is the key word for this project.

I run a 1075 kit with 10.25:1. I keep a couple of 2 oz bottles (6 hr energy shot)under the seat filled with Amisol octane booster. I add 2 oz each 2.5 gal fill up. Got rid of the occasional ping I had when I snapped the throttle open while cruising.

most of the latemodel sport bikes can run more compression because of a few different reasons.
1. 4 or 5 valve cylinder heads
2. advanced ignition timing methods
3. advanced self compinsating fuel injection
the only way you could run high amounts of compression on pump gas is with a second spark plug per cylinder to promote a complte burn in the combustion chamber with a reduced chance of detonation on todays pump gas and also reduce the amount of ignition timing you would be able to run.

timebomb33 wrote:

most of the latemodel sport bikes can run more compression because of a few different reasons.
1. 4 or 5 valve cylinder heads
2. advanced ignition timing methods
3. advanced self compinsating fuel injection
the only way you could run high amounts of compression on pump gas is with a second spark plug per cylinder to promote a complte burn in the combustion chamber with a reduced chance of detonation on todays pump gas and also reduce the amount of ignition timing you would be able to run.

The significant differences that allow the new motors to run higher compression:

#Much less dome on the piston with flatter combustion chamber yields an easier path for the burn in the chamber.

#Liquid cooling

#Computer controlled ignition advance & Fuel Injection

Hey Plummen, don't forget....

Add to the list... Wiring Mod!
Fixes everything, so you should be good to go....

Has anyone ever made an aftermarket KZ head with more than two valves per cylinder?