Really? Lowest octane? Always thought the other way around. Seems that the higher the octane the longer a tank will last me. Hmmmm? Looking forward to the debate.

Lowest is best just look at the 500EFI 87 oct and it runs great. If you want you can just send me the $$ you would have spent on 93.

First,
Steve is correct. Toluene is the easiest way to boost octane. Or Xylene. Both are expensive and toxic.
Toluene has a research octane rating of 120 and a motor octane of 115, therefore it comes out to 117.5.
Now, if you really want to do it right, the best way to do it is to use toluene and MTBE. It will very soon be no longer available due to the "tree huggin' hippies" out in CA. All oil refineries are now having to reformulate due to the regulations against MTBE.

Now, the reason why it is true that the lowest octane with out detination works best, and is most cost effective, is because octane slows the burn. If you (for example) use 100 octane in your motor designed to run on 87, the fuel is still burning when the piston is past bottom dead center. (that statement is not completly accurate, but I'm trying to give you an idea of what's happening in the cylinder)

Simply put, you have X degrees of crankshaft rotation, which translates to piston travel for gas to completely burn. The amount of time it takes to burn depends on the amount of heat in the cylinder, more pressure = more heat (compression), the faster the burn, the need for higher octane to slow the burn to keep the fuel burning within the X degrees of crankshaft rotation.
If the fuel is burning past that point you're wasting it, (cost effective) if it's burning before that point it causes detonation.

Thats why you have advance in your timing. The faster the motor spins, the sooner the plug needs to ignite the mix so the fuel will burn in the needed parameter.

hope this helps
DAVE

Oh, BTW, the unleaded race fuel is made the way I described up to the 104. The 104 is oxygenated however, so keep that in mind if you plan on using it.
Everything past the 104, octane is raised by using lead.
DAVE

I guess that a lower octane is for a stock engine. If you make modifiation to your engine, like and increase of horsepower, like 330 to 500 hp, a higher octane is better. Yes? No?

88,
the answer is in my second and third paragraph above
DAVE

So beside compression. Is there any other reason to run race fuel . What about Superchargers or blowers? My Buddy mix's 20% of CAM 2 with 93 octane . Stock compression 454 mag Supercharged motor about 600 hp running 5 to 6 pounds of boost Engine builder told him it would help.

It's not a simple subject, but the simple answer is:

If you have a given motor and parts combo, it will have a "optimum" amount of spark advance at an "optimum" rpm for making best power. This can only be found for sure on a dyno.

Now we have KNOWN values for the peak power rpm and the most effective spark advance for that combustion chamber design, piston crown profile, and all the other variables that affect the outcome.

You will make the BEST power with the LOWEST octane fuel that allows you to run that optimum spark advance at peak power rpm without crossing over into detonation. Be advised that this elusive OPTIMAL combination of fuel octane, spark lead, etc. also required that the jetting be optimized for that fuel as well.

I will be happy to remind you that engine temp, antifreeze concentration (zero for us raw water cooled guys), ambient air temp, and a host of other hard-to-control issues will also have some semblance of an effect on the outcome.

REAL WORLD: On a given engine, if you can run between 30 and 34 degrees advance at WOT on 87 octane, then you should run 87 octane. If you get evidence of detonation on your plug insulators at 30 degrees, then you go up to 91 and see what you got. If you run 30 degrees and 91 octane with no detonation, then you stop there. OF COURSE YOU TEST FROM THE OTHER DIRECTION, starting with 30 degrees and 93 octane, then bumping to 36 degrees 2 at a time, then dropping fuel quality till you start hinting at detonation. Stop with that fuel quality, drop the timing 2 degrees and call it a day.

Race gas, however, is formulated with a KNOWN set of volatiles and no weird additives to prevent freezing or to clean junk out of your station wagon's carb circuits. For this reason, it is the best thing to use for jetting carbs and giving consistent plug reading. Be aware that RACE GAS is available in "NORMAL" octanes as well as high octanes. Low octane race fuel is just EXTRA HIGH QUALITY normal gas.

If your boat does go faster on high octane gas, then it can only do so if it has a knock sensor with auto timing retard. There are many cases in modified engines where 87 octane DOES lean into detonation and the computer chops the ignition lead. This will cause a loss of performance.

Hope some of this makes sense.

Dave and mcollinstn

Very excellent posting Guys that is what this place is all about.

FFO,
A supercharger adds compression. You have something called static compression (compression ratio w/o boost) and when the boost kicks in it's called effective compression ratio.

Your buddy is most likely being helped by boosting his octane because if he didn't he would probably would have to retard his timing to around 28. With adding the race fuel he's able to run some more timing. I don't know about your buddy's engine but thats my guess.

DAVE