Compression vs Octane, octane boost?

[Martinm210]

Now that I just about got my banshee rebuilt for the second time (just waiting for some head o-rings), I was curious about compression. The bike came with 21CC domes and the previous owner ran premium gas in it with success.

 

But...I've recently just had the cylinders bored out and I can already tell that the compression is going to be higher than before with the much improved ring gap. I plan to check out the compression very carefully and was contemplating running some octane boost. But before doing anything, I wanted to get a feel for what was an acceptable compression level for pump gas.

 

 

Is there a compression limit to where you really need higher the premium octane pump gas?

 

And what about those 104+ octane boost products. Are they safe or any good?

 

Thanks!

Martin

 

1994 Banshee

Chozen Perf, 21CC domes

+4 Ign Advance

Stock Reeds

FMF Fatty pipes

Ported

65.50mm bore, Weisco Pro-lite pistons.

.012" ring gap, .045-.047" squish

Hotrods crank

 

Edited July 6, 2009 by Martinm210

[AKheathen]

octane boosters are junk. will only get you to like 93.4. you are fine with the 21's on stock stroke. our premium is only 90, and im @ the limit with 20's

[Martinm210]

octane boosters are junk. will only get you to like 93.4. you are fine with the 21's on stock stroke. our premium is only 90, and im @ the limit with 20's

 

Cool!

 

I had heard 21s would be fine, but then I read somewhere that you need 22cc's to run pump. My first banshee had a stock head, so this is my first with an aftermarket and domes.

 

I'll just make sure it's fresh 93 pump gas and pull the head again after a short break in as a check.

[mazuullaa]

If your over 156psi on your compression check I would start worry about octane But it should be fine with premium and + 4 timing. Alot of people run that.

[Martinm210]

If your over 156psi on your compression check I would start worry about octane But it should be fine with premium and + 4 timing. Alot of people run that.

 

Cool, that gives me a number to go by. I will check, just waiting on parts right now.

 

The previous owner just had some hole burning in the piston problems, so I'm always a bit skittish about wondering if my fixes are going to fix it. So far, I found a backward head gasket that was covering about 70% of one transfer port that seemed to indicate one side was getting lean/hot, and I also found what appeared to be a partially failing water pump that I replaced. I also replace the case seals.

 

Since then I had one more break down, but that was my fault for changing the crank and not checking squish.

 

Going to run through the carbs once really well and cross my fingers...hopefully the CDI box isn't the source of the hole buring issues and I've nailed it with this one.

 

This weekend running time per rebuild is getting expensive..:eek:

[AKheathen]

165psi on freshly seated rings. it's about the compression ratio and timing. not so much the reading. you should be cranking less than that with your setup, anyways

[jbooker82]

Yea the octane boosters are junk. If you read the bottle closly it will say that it boost octane rating 7 to 10 points. What they dont tell you is that it takes 10 points to go up 1 octane number. IE 90 to 91 octane.

 

A gallon of race fuel is the same price as a bottle of octane booster and it will work 10x better.

 

You mix 3 gallons of 93 octane pump gas with 1 gallon of 110 race gas you will have 4 gallons of 97 octane.