which is faster- VP C-11 or C12

[Alan Roehrich]

Originally Posted by FastbackTom

I attended a fuel seminar at Sema about 10 yeas ago... the guy was the NASCAR liaison from Unical, and offered up some real good advice (for us non-chemist-types) on race fuels in general. I've also learned a lot from my own experience with dynoing my own engines, talking with engine builders about various compression ratios and cranking compressions. I also have a fair amount of experience with VP's C12, and a little with C14 and C14+. My current engine is built around VP's C12, the mandated spec fuel for this other class I run (outside of NHRA SS).

-Quality gasoline contains the same number of BTUs per gallon regardless of octane. Higher octane does not mean more power (a common misconception)

-Octane is a measure of burn rate, or the way I view it, burn retardant. All things being equal, the higher the octane number, the slower the burn.

-Higher dynamic compression speeds up burn rate. A high compression engine with a cam that has lots of duration my have a lower dynamic compression ratio than you think. Cranking compression, measured with a traditional (quality) compression tester, is a decent way to determine dynamic compression on a normally aspirated engine.

-The simple way I look at octane; the best octane for my engine is the lowest that doesn't allow it to it knock.
-Too high an octane will make it lazy, and not want to rev up
-use sealed containers, drain your fuel after a race weekend... The chemicals that increase octane by retarding burn rate, are the lightest in the mix. They tend to 'boil off' at room temperature, lowering it's octane rating.
-UV light also has a negative effect on race gasoline. Those opaque poly fuel jugs allow UV to get through, I've been advised to use the colored ones that block UV, and store them out of direct sunlight. Also, poly jugs are not a perfect seal, the lighter chemicals will seep through poly jug material, when heated pressurize the cap and make it leak, and the vapor escape when you open the cap.
-The 2-valve wedge head engines I play with will not run on VPC12 with 300psi on cranking compression without rattling, but when brought back to 250psi, they will run on C12. Realize that's only one data point for one type of engine, but it may be an indicator of octane required for 250psi cranking compression.

The problem with the "burn rate, burn retardant, octane" theory is that there are a lot of Stock and Super Stock racers who keep a 5 gallon sealed pail of VP C25 in their trailer for heads up runs, and switch to that from C11 or C12 for a heads up or class run. If you look at the octane ratings, if the "burn rate, burn retardant, octane" theory held true, every one of those guys would slow down, unless they were retarding the timing and/or detonating on C11 and C12. C25 will also show better on the dyno as well. And C25 is capable of supporting 15:1 compression.

At the risk of over simplifying it, octane merely prevents detonation or pre-ignition. Burn rate is actually a separate function, and the ideal burn rate depends upon RPM, chamber design, and bore size, among other factors. Detonation is basically two flame fronts colliding, and pre-ignition is the start of the combustion process before the ignition spark occurs, which is when/where combustion should begin.

C25, as an example, is a fuel formulated/blended for high RPM engines, such as Comp Eliminator, and Pro Stock. Further, it is also formulated with a burn rate for the higher RPM of those classes, as well as the large bore size of Pro Stock, where most, if not all engines are well over 4.625" bore size. It has a high octane, to deal with 15:1 and higher compression ratios, but also a relatively fast burn rate, in order to work with large bore sizes and high RPM.

Another function of a fast burn rate is the lower ignition advance requirement. Once you advance the timing beyond a certain point, you begin to create "negative torque", where you are building cylinder pressure from combustion before the piston even reaches TDC on the compression stroke, so you're actually trying to push the piston back down the bore. If the burn rate of the fuel is relatively slow, you will need so much ignition advance in order to burn all of the fuel and air before the "power stroke" is over, and the exhaust valve opens, that you will create a considerable amount of "negative torque". Fuel and air burned after the exhaust valve opens is pretty much wasted, other than possibly creating enough exhaust gas velocity to assist in scavenging.

You need enough octane to prevent detonation and pre-ignition with the static and dynamic compression ratio you run, the dynamic compression ratio being a function of the static compression ratio and the cam timing events. Dynamic compression, to a degree, can be measured by cranking compression, but is best calculated using various formulas or engine dyno simulation programs.

What burn rate you need depends on displacement, bore size, rod : stroke ratio, RPM range, as well as piston design and chamber design.

It should be noted that you cannot simply change fuels and make a direct comparison. Especially not with a carbureted engine. The changes in specific gravity will require jetting and air bleed changes, the changes in octane and burn rate will require adjustments to timing, and it will be necessary to determine what A/F ratio each fuel requires for best performance.

You simply cannot just switch from one fuel to another while not making adjustments and declare one better than the other. Not if you're interested in the truth, and going faster.

This is a subject upon which volumes of material have been written, and about which lot of misconceptions exist. It would be impossible to adequately address it here. I've glossed over and grossly over simplified a lot of it for the purposes of getting it in a post in a thread on the Internet. I'm sure many will find it clear as mud, and some will misunderstand or disagree. Take it for what it is worth.