1976 KZ900 "old school Superbike"

depends mostly on how big your cams are/overlap

I run 10.5:1, so i can run pump gas. A fresh 10.5:1 set up can run 225 psi compression
If the cly psi gets too high u will get pre ignition or detonation.. So u need to retard the timing more and this adds heat. Higher octane allows less timing. By using more cam duration or retiming the cams u can lower the cyl psi but u will loose bottom end and gain top end. So its all good at the track, but not so good for the street.

"So its all good at the track, but not so good for the street."... No doubt, what a pain in the ass that must be.

Seems you would be constantly tweaking this for optimum operations.

Thanks for the info...

OMR

I'm not a track guy, but i think that tweeking is what its all about when you are a track guy. U are allways trying to get a little more. the more u tweek the more u learn about what makes your setup fly or not.
Like the cam timeing. as u alow the valves to stay open longer u get more top end power but there is a fine line befor your big lobes hit something.
Some guys will take the heads off and see if the valves are hitting the pistons yet. Thats how tight they tune there tallerances.

Well, here's what I know. My buddy and a few of his friends (w/KZs,LTDs,Z1-Rs)all ran 12:1s on the street with any number of different cams/carb/exhaust set-ups. They never had any problems with running pump fuel on a daily basis. I figured if its good enough for them, then its good enough for me.
One big factor - this all took place years ago when there was hi-octane leaded fuel available everywhere. Could it be that today's fuel can't handle the high compression?

mark1122 wrote:

I'm not a track guy, but i think that tweeking is what its all about when you are a track guy. U are allways trying to get a little more. the more u tweek the more u learn about what makes your setup fly or not.
Like the cam timeing. as u alow the valves to stay open longer u get more top end power but there is a fine line befor your big lobes hit something.
Some guys will take the heads off and see if the valves are hitting the pistons yet. Thats how tight they tune there tallerances.

the best motors are always the ones where you can just see the witness marks on top of pistons!:laugh:

the altitude you are at could effect the ability run a little more compression.just a thought.

Funny you mention the altitude. Because of the dense, cool air up here in the frozen North, My friend was able to run fatter jets than most guys in the lower 48 states.
I know its a no-brainer to go w/ 10:1, but I'm "old skool" where more is better. I have no problem running octane boost,if needed. We're talkin short day trips here, anyway.
Still, I'd like some feedback from guys that are currently running 12:1 pistons on the street. Good or bad. Thanks.

with the fuel quality the way it is today any two valve motor that is running more than 11-1 is really pushing it as far as detenotion goes you will have to retard the timing back quite aways and definitly run some sort of octane boost and if you try to run something like e85 that has alot of alcohol in it then you will run into all kinds of carb and petcock problems, try a little experiment soak a oring in the octane boost that you might run and 1 in some ethenol and see what happens to them. you will probaly be suprised at how they react. if i were you i would try to keep the compression down to around 10or 11-1 no more do a bit of port work and a decent set of cams and you will have alot more fun and the motor will be alot more reliable.

Sounds like good advice, Timebomb33.
As far as I know, we don't have E85 up here. But, I agree that 10.5:1 is probably a safe way to go. I'm not looking to squeeze every last oz. of hp from the motor. I just want a good, strong-running machine.
My next KZ project will have a 1075 turbo kit, 'busa forks, TL1000R swingarm,etc. so I'll get my hp fix from that one.

Just an added note that promotes pump gas. These KZ's can only go for about an hour and a half on a tank of gas . This mean that u can only stray away from home for 45min before u have to turn around, unless u can run pump gas.

also make sure the town you live in has premium gas before you build the high compression motori live in a small town in sask. and they nearest big center is an hour+ away. also we only have one gas station in town. so when i built my last big inch high compression street motor i installed in the frame hooked every thing up and went looking for gas so i could start it. guess what no premium in town so i drove an hour away got some gas came back fired up the bike rode it around a bit came back to the house and pulled the motor out and built a smaller lower compression engine and now i have a very nice spare motor for one of my drag bikes.