which is faster- VP C-11 or C12

[Alan Roehrich]

Originally Posted by FastbackTom

Good stuff, Alan. So help me understand what's going with C25 octane 113 versus C12's 108 octane...

Why would a guy be able to just throw in C25 and pick up power in a stock or super stock engine, with relatively lower CR? I believe you, it just doesn't add up inside my brain.

Tom, it evidently has something to do with burn rate.

I would not find it easy to believe if I had not seen it repeatedly. I've seen it done dozens of times, and seen the dyno sheets. Most guys keep a carburetor tuned to run the C25, as the different formula requires changes to the air bleeds, jets, and emulsion circuits. They have a bracket mode carburetor, tuned to go as fast as possible on "regular" fuel, then they have a "heads up" carburetor tuned to use C25.

Back in the day, so to speak, most of the octane was often gained by adding tetra-ethyl lead. That does retard the burn speed. These days, they use various "high end" or "light aromatic" chemicals to increase octane, those do not necessarily retard the burn speed, and in some cases, actually speed it up.

Remember that octane is simply a measure of resistance to detonation and pre-ignition only, and NOT a measure of resistance to actually igniting and burning under the correct conditions. Detonation and pre-ignition occur due to heat and pressure, where as normal ignition occurs due to the properly timed spark.

It is true that if you are using a fuel with an octane higher than you need to prevent detonation and pre-ignition that you are "wasting octane". But there may be other advantages to that fuel. I know guys who use C14 and C14+ because it runs better and is more stable and consistent than C12, even though they do not need the increased octane.

The days of octane being the single all important factor in fuel choice are gone. You simply need enough octane to prevent detonation and pre-ignition with your engine combination and tune up. Whether or not you can go faster with a higher octane fuel depends entirely on whether or not you can make good use of a different burn rate, and tune the fuel to work with your carburetor or injector system.