Compression Ratio vs. Grade of Gasoline

I've seen some awesome mods here that I'm considering. But I don't want to buy my gas at the airport. Does anyone know what the compression ratios are that make upgrading to premium or avgas necessary?

IMO, anything in the 10:1 area may require premium fuel, the octane rating of gas has come down over the years.
What are you wanting to build would be the first question?

Most of the pistons come in 10.25:1. This is fine for premium pump gas.
12:1 is only acceptable on a new bike with a down draft,4 valve, water cooled head.

There is an actual octane booster (with lead) still available that can boost octane anywhere from two to six points if you are set on building a screamer. It's called Octane Supreme 130. The stuff available in auto stores will only raise the octane about 1/2 point.

Dont use this CRAP, it dosent mix even with the gas, I know first had what will happen, its called a hole in the piston!!!


bountyhunter wrote:

There is an actual octane booster (with lead) still available that can boost octane anywhere from two to six points if you are set on building a screamer. It's called Octane Supreme 130. The stuff available in auto stores will only raise the octane about 1/2 point.

also depends on your cams and cam timing and other variables,numbers like 10.25:1 and 13:1 are just there to set the hook in your mouth like a fish!:laugh: if you just want to go by that id say 9:1 would be just about right for a good pump gas motor

Yeah, hard to say...

I say do whatever you want to your bike . Someone will usually have an idea of what octane is necessary for whatever mod.
You can start at a suggested octane level, then slowly go down ratings till you start to get knock under hard/uphill accel. then just stick one level above that.

I know a few guys that have had success with this method.

Cheers.

:blink:

BSKZ650 wrote:

Dont use this CRAP, it dosent mix even with the gas, I know first had what will happen, its called a hole in the piston!!!


bountyhunter wrote:

There is an actual octane booster (with lead) still available that can boost octane anywhere from two to six points if you are set on building a screamer. It's called Octane Supreme 130. The stuff available in auto stores will only raise the octane about 1/2 point.

It mixes with my gas. never had a problem, been using it two years.

These guys also recommend it:

www.nowearracing.com/

www.wildbillscorvette.com/prod01.htm

May I ask what elevation you live at? Higher elevations usually have lower octane fuels.

What is your wildest dream for this motor? This is always the easiest question to answer.

What are you really going to use it for? This question is harder.

What do you rate your skill level at, as a rider and a mechanic? Everyone started at the bottom and worked there way up from there. If you have skills then shoot for the moon.If you are new to riding or engine building, keep it simple and give yourself a chance to develp some skills.

There are some good running low compression bikes out there. There isn't anything wrong with a good stock piston 1015cc with some drop in cams, header, carbs., gearing & ignition. It's always a good starting point and a motor that you can hammer-on! It will perform better with a 900 crankshaft in it. It will give you a chance to do it on a budget.

Everyone here will be more than willing, to help you out!
Myself, I would be willing to share with you the specs. of a low compression 1015cc I have built many times.

I have an '81 KZ650CSR (H1) and would like to do the 750-to-810 mod on it. Wiseco lists compression at 10.25:1 and I thought anything over 10:1 needed more than premium.

To answer kzz1p:
My dream for this bike is to plagiarize jordan's engine work and add the 6-speed tranny and oil cooler from the KZ550 donor engine sitting in my garage.

It's my daily driver in lunatic Hampton Roads, VA (when it's running) and I am planning trips to Texas, Oklahoma and Oregon.

I've been riding off and on for 32 years, but have only moderate mechanic experience (carbs, brakes, sprockets, etc). I'm looking for a local mechanic (dealers won't touch it), but will try it myself if I can't find one.

I used to prefer Yamahas, but since I bought my Kaw and found this website, it's personal. You guys rock!

Spot
Me, I, Myself! If I was going to modifiy that motor. I would use the 10.25:1 compression pistons. I would use a cam that has the same intake, opening & closing points as stock or as close as I could get. Yes, a later intake closing would be better but the cam might be very big for your daily driver.

With the 810 mod, or stock 650 w/ 10.25:1 pistons? If I'm going with the mod, I was thinking stock KZ750 heads instead of GPz.