This was written by Rich Rohrich, and is some of the best info on octane boosters you'll find on the 'net:
I've never liked octane boosters as a substitute for race gas, but given the number of people who send e-mails asking about them it's apparent that people want to know more. They will never replace the real thing, but octane boosters can be useful at preventing knock if you only need to raise the octane a few points, but they come with a host of caveats.
- Conveniently located at your local dealer or auto parts store
- Easy to transport
- They are cost effective if your octane needs are minimal
- Most boosters raise the RON octane more than the MON octane, which
makes them fairly useless for most race applications, or high load
high heat situations
- Most of the better additives at this point are either Toluene or MMT
based. Toluene has the advantage of being relatively cheap, and fairly
effective at raising the octane even in fairly high percentages. Plus
it has good resistance to pre ignition which is an important consideration
in a two stroke. The downside is as you raise the percentage of Toluene
(or most aromatic hydrocarbon based additives) , the throttle response
quickly becomes flat and mushy. The snap goes BYE BYE.
So the engine won't knock, but it probably won't run worth a damn either.
- MMT based boosters (the most common today) quickly reach a point of
diminishing returns (i.e. the more you add the less it helps)
- Organometallic boosters like MMT can cause a huge increase in engine
deposits, and increase the risk of plug fouling and pre ignition
- Most boosters tend to kill the throttle response and make it
difficult to jet. Mix in the constantly changing base fuel your mixing
it in and you have a constantly moving target.
- Most octane boosters (especially toluene based) have high solvent
properties which can raise hell with reeds and crank seals in two-strokes
- If you need to raise the octane a significant amount, octane boosters
are much more expensive and much less effective than race fuels
There's more to it than this but I'm sure you get the point. Basically
it boils down to this, if your engine only needs a small increase in
octane as a safety margin then using octane booster is still better than blowing it up, but you'll likely give up throttle response for the
convenience. If you need a large increase in MON octane you won't get it from octane booster without paying more than you would for real race gas in the first place, plus it will run BAD. No snap, and an impossible to read plug.
During some testing with VP "Octane Power Boost" (MMT based) on my YZ417 (13.25 :1 CR). I've also noticed a reluctance to start when the engine is cold, although this may be application specific. The throttle response isn't as good as running
VP-C18 or C-12 but it doesn't knock, and the power you give up on the top end would only be noticeable to guys running flat out all the time.
Even though I hate to recommend octane boosters, if you have to use them here's some things you can try in order of my preference based on my testing on a 92mm stock compression YZ400, and a 94 mm 13.25 compression YZ417. I used Amoco Premium MTBE based (non-alcohol) as my base fuel in all tests, which were done in fairly warm 80-95 degree weather. My results probably aren't really useful to anyone who isn't running a similar combination, but they hopefully will shed a little light on the subject.
VP - "Octane Power Boost" - MMT based mixed 1 OZ./gal.
Pretty good throttle response once you get the jetting sorted out which tended to lean out on this combination. This was probably the closest to good race gas that I've seen. Raising the percentage of VP quickly killed the throttle response and made it much more difficult to start cold. This would be a reasonable choice if you can't get race gas or just feel cheap. Street price $10 for a 16oz bottle.
Snap "The Outlaw Racing Formula" - MMT based mixed .75 OZ./gal. Reasonable throttle response not quite as sharp as the VP additive but close. Jetting tended to lean out on this combination. Raising the percentage killed the throttle response and made it much more difficult to start cold. This is easy to find at most auto parts stores and speed shops. Street price $10 for a 16oz bottle.
PJ1 - "Gas Energizer Octane Plus" Toluene based mixed 2 OZ./ gal.
Throttle response is fairly soft when mixed at 1 OZ./gal and gets worse as you increase the percentage, jetting tended to less sensitve to this additive. Top end pull was a better than the MMT based additives at 2 OZ./gal. I've heard that this product has been discontinued, but no confirmation at this point. Street price $6 for an 18oz bottle.
Maxima - "Hi-test Octane Booster" mixed 2 OZ./ gal.
This appears to be a Toluene based additive, but Maxima doesn't list the specifics in their literature. Like the PJ1 additive throttle response is fairly soft when mixed at 1 OZ./gal and gets worse as you increase the percentage, jetting tended to be less sensitve to this additive. Top end pull was a better than the MMT based additives at 2 OZ./gal. Street price $4.50 for a 16oz bottle.