Here is a good scale to judge which octane to run with what compression ratio. Remember, higher engine timing will result in the need of MORE octane. All listings are minimums and rated @ pure octane levels DO NOT use octane booster!

10:1 93-94
11:1 95-96
12:1 97-99
13:1 102-105
14:1 105-108
15:1 110-115

Hope this helps, if anyone else has an opinion, fill us in!

 

Compression ratios can be used ballpark as KawiKid alludes to. But that tends to pertain a tad more towards the four stroke side of the house. I know more about 2 strokes than 4 strokes, so I may be lacking in specifics for the thumpers.

For two strokes, purely from the math and science side of the house, the mean squish velocity sets the octane requirement. MSV is expressed as meters per second or m/s

What comprises the MSV.

--Compression ratio(sometimes both the corrected and uncorrected, better programs use both)
--Squish width, most often calc'd as a percentage of the bore.
--Squish clearance
--Target maximum RPM, not peak power RPM, but worst case, you miss a shift, break a chain...and over-rev. At 9000 RPM detonation is instantaneous. Severe damage within milliseconds.
--Minor consideration; Bore size. Can play into this some as well, again the better programs factor in alot. Larger bores are more prone to detonation. That's why you see super zoot GP bikes with the displacement spread across several smaller bores.

There's many available programs that calculate the MSV, you enter the data and hit run and it spits out the MSV. 28 m/s is considered the safe maximum for 110 octane gas. That's pushing it tad for long haul motors.

Ignition timing is crucial. Too much advance and no matter what MSV you have set-up, you can create detonation. Advance(static intial) really only helps in lower RPM's and tends to hurt higher RPM's by creating negative work, the piston is still coming up when the spark fires. More heat is created by more advance, the ignited mixture stays in the cylinder longer, before it has a chance to be expelled out the exhaust port/valve.

When you build an engine it is crucial to understand what octane requirements you are going to require. To enter blindly into something and then say, what octane do I need...yikes that's risky, from the standpoint that detonation is very real, dangerous, and catastrophic. If I had my druthers everyone would require minimum 110 octane, there's so much more to be gained in the big picture for the little added cost. If you look at how much fuel you may burn in a given weekend, then race gas may be right for you to set-up towards.

 

Wow, awesome post. I thought I'd post this to help elaborate on "squish" (found it on the web)

Squish clearance is becoming a well known term as two stroke engine tuning interest and knowledge spreads. Still, some may not know what this term actually represents nor understand its importance. This article will answer those questions and tell you how to accurately measure the squish clearance.

Squish clearance refers to the narrowest distance between the piston and the head. Most two-cycle engines utilize squish bands in their head's combustion chamber. A squish band is the flat ring commonly found around the outside edge of the combustion chamber. As might be expected by it's name, the squish band (in combination with the piston) causes a sqeezing action on the air in the combustion chamber.

As the piston hurtles towards top dead center (TDC) in the cylinder, the fuel air mixture between the piston and the squish band is compressed into the center of the combustion chamber. It is this squashing action that allows the combustion process to occur faster than without the squish action. Faster combustion means later (or retarded) ignition timing, therefore higher combustion pressures, less compression work and more expansion work. In other words, more power! All modern two-cycle and four-cycle motorcycle engines utilize squish action to some extent.

While squish is very beneficial, more is not always better. Too much squish action causes extreme turbulence before and/or during combustion. Extreme turbulence can lead to detonation, shown by severe engine knock, engine seizure, or a hole in piston crowns.

Luckily, squish action can be estimated with simple mathematical models. I use a program from Gordon Blair to calculate squish velocities. Mean squish velocity (MSV) is a handy way to gage general squish action and thus avoid extreme turbulence. MSV numbers can safely vary from 0 to more than 25 m/s with average values in the 15 to 20 m/s.

By this time you may be asking yourself, "So what does squish action have to do with squish clearance?" Well it just so happens (as its name implies) that squish clearance greatly affects squish action or MSV. So if your squish clearance is too small, your MSV will be too high leading to catastrophic engine failure.

Mechanical components affect squish clearance limits. If your clearance is too small, the piston can "kiss" the head at high rpm also causing all sorts of problems, (none of which are good for engine performance). Engine components stretch and shrink during operation due to the large forces involved at high rpm. A conservative rule of thumb for squish clearance limits is no less than 0.040" (or 1mm) for cylinders in the 250cc range. Smaller clearance can be used on small displacement engines, while more is required for engines with larger, heavier components.

Squish clearance is easy to measure and can tell you a lot about your engine. To measure your squish clearance, you will need two 5 inch strips of solder, a spark plug wrench and a dial caliper. Remove your spark plug and rotate the engine so the piston is near bottom dead center (BDC). Being careful not to drop the lengths of solder, bend and install them through the plug hole so they reach and touch the cylinder wall. Each length of solder should touch opposite sides of the cylinder with the two solder strips pointed 180 degrees apart. (This eliminates piston rock that can upset your measurement.) Rotate the engine over until the piston "squashes" the solder flat at TDC. Rotate the engine past TDC, remove the solder and measure the thickness at the outer edge closest to the cylinder wall. The minimum solder thickness is your reference squish clearance.

Also found this http://www.hpt-sport.com/blowopen.htm

 

geesh~! yall are gettin jipped on your race gas, around here is 20bucks for a 5gal can to be filled and for a 55gal alcohol drum, it is 80bucks

 

Just an FYI BackCountry I found this...

Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %


Reference 1.
- Chapter 20. K.Owen.

Automotive Gasolines - Recommended Practice.
SAE J312 Jan93.
SAE Handbook, volume 1. ISBN 1-56091-461-0 (1994).