Please teach me about engine compression.

the compression numbers they list with pistons is just bait on the hook,the numbers you really want to know is the cranking pressure of the assembled engine.pistons and cam timing/size both play a part .

Yes I agree the cranking compression is Reality, Ratios are theory, it is controlled by atmospheric pressure, as well as cam timing and valve overlap. Get the Automotive math book suggested, its very good. Don't forget that as conditions/ engine parameters change, Ignition timing and total advance are big players.
You should think about some cylinder head work, if not done already, along with exhaust and ignition advance curve/ total advance. Don't go overboard with compression, pump gas is CRAP these days, 91 yea the day it was made, not after its been in a tank, either in the ground or you garage. Additives become regular purchases or some good AV-GAS for the extra ratio building. I found at sea level I could get a 175lb cranking compression with a lot of cam, on a water cooled motor, it would be less on an air cooled, and less timing as well.

Ohhh the crank goes round and round...the piston goes up and down...the air gets sucked inside...then squeezed up nice & tight..:silly:


In general, more compression = torque. A 1075 kit with 10.25:1 rated compression ratio pistons is fine for the street.

You want strong midrange?......Then keep the turbo pistons out of the equation.

Punch it to 1074 and get some drop in cams like the Web 118's. It will work fine on pump gas and you can keep your airbox. I promise you it will not disapoint you or be tempermental

I'm running exactly this (minus the WebCams) with a K&N replacement filter for the airbox.

Runs great on trash gas and fun. I can't wait to be able to afford a pair of the WebCams, though...

Plummen, Larry Cav and Motor Head are right (at least insofar as I know anything about this)that cylinder pressure is really the issue here. As noted, a cam with lots of overlap will actually reduce cylinder pressure without touching pistons or anything else. That's PART of why we see modern street legal sport bikes with 13:1 or more able to run on pump gas, while older engine designs or engines with less overlap would ping like crazy. This is also why turbo and super charged engines use lower compression pistons. The forced induction creates higher pressures, and pistons with lower domes, or none at all, allow for a larger combustion chamber which equals a larger BANG.
Like others here, I find this all rather fascinating and surprisingly complicated.

TexasKZ wrote:

Plummen, Larry Cav and Motor Head are right (at least insofar as I know anything about this)that cylinder pressure is really the issue here. As noted, a cam with lots of overlap will actually reduce cylinder pressure without touching pistons or anything else. That's PART of why we see modern street legal sport bikes with 13:1 or more able to run on pump gas, while older engine designs or engines with less overlap would ping like crazy. This is also why turbo and super charged engines use lower compression pistons. The forced induction creates higher pressures, and pistons with lower domes, or none at all, allow for a larger combustion chamber which equals a larger BANG.
Like others here, I find this all rather fascinating and surprisingly complicated.

Thanks for the Turbo explanation. Makes sense to me.

DoubleDub wrote:

I'm running exactly this (minus the WebCams) with a K&N replacement filter for the airbox.

Runs great on trash gas and fun. I can't wait to be able to afford a pair of the WebCams, though...

you are running with 10.25:1 on reg gas and no ping?

Skyman wrote:

What would be the difference between the 8.5:1 kit and the 10.25:1 kit of the same size in terms of performance?

On my CSR, I installed a Wiseco 1135/10.25:1 kit. It's a very "spirited" bike, and really fun to ride. But frankly, I find that I ride my LTD more often because it has a "smoother" personality, and is more comfortable on longer rides. And, of course, I have to use premium fuel in my CSR, while my LTD still drinks regular.

Thanks.

what head work is done to the 1135, and what cams?

the dif between 8.5 and 10.5 would only be a few hp.

Yes, 87/89 octane w/ethanol (depends on the gas station) and no pinging at all. All stock except for the K&N replacement filter and the Wiseco kit (well, and the KZ1000 jugs).

Where this is going is actually an understanding of the difference between dynamic and static compression ratios.

Piston kits are rated in "static" values.

Camshafts and valve timing are the elements that affect "dynamic" ratio...eg. cylinder pressure.

Lot's of information on both on the web. Search a bit and you'll find it.

eeeeeeeeeeeeeeeeeeeeeeeeeeeek more technotalk,save yourselves boys! :laugh:

mark1122 wrote:

Skyman wrote:

What would be the difference between the 8.5:1 kit and the 10.25:1 kit of the same size in terms of performance?

On my CSR, I installed a Wiseco 1135/10.25:1 kit. It's a very "spirited" bike, and really fun to ride. But frankly, I find that I ride my LTD more often because it has a "smoother" personality, and is more comfortable on longer rides. And, of course, I have to use premium fuel in my CSR, while my LTD still drinks regular.

Thanks.

what head work is done to the 1135, and what cams?

the dif between 8.5 and 10.5 would only be a few hp.

On my CSR, no head "work" was done, other than just lapping the stock valves. I replaced the stock cams with '84 GPz cams bought on EBay.