which is faster- VP C-11 or C12

[Jack Matyas]

C-11 has always been faster for me and has never failed fuel check in one of my cars .............The fuel check issues with C-12 have always been with fuel that came from 54 gallon containers .Yes , I know that sounds odd but I've not heard about problems with fuel from 5 gallon pails .

[art leong]

I've used C12 since I built my car (over 5 years) and I've never had a problem with fuel . I even left about 3 gallons in my fuel cell for 4 months and just figured it was bad but had it checked anywaycheck. much to my surprize it was fine. That was without capping the vent, and in an unheated garage.

[Rich Biebel]

When I raced my last Stocker......

I used some Sunoco Purple, C-12 and C-11at various times

I felt like the C-11 was a little better but not really anything more than a couple ticks at most....

I brought some fresh Sunoco to one race....It passed on 1 run and failed on the next?? I even had the fuel in the jug tested before I ran the car. It was ok.

I flushed the fuel system as good as I could....bought a new jug and C-11....It passed.

Bought C-12 at Cecil County from the tracks supplier and it was right out of his sealed drum..... On the very next run it failed and so did a lot of other racers that day using C-12 from what I saw. I showed the NHRA fuel check man the receipt as I used a CC to buy the fuel an hour earlier. I think they let everyone slide on that issue at that race. Seemed to me like they knew there was some kind of problem there.

After that event I drained my fuel system and cleaned out my fuel cell before every event. Bought fresh C-11at the start of the race and never had it fail.

The C-11 stinks bad in a trailer if you have a cell with an open vent.
I wanted to add a valve on the fuel cell vent to close it and reduce that issue as it can't be good to let the light ends of the fuel vaporize like that....

Last time I ran that car.....I ran Sunoco at a local bracket event and switched back to C-12 for an NHRA race and it checked fine both times it was checked......The fuel check issues with racing a Stocker is not fun.....especially when the stuff costs $10-12 a gallon!

[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.

[Mike Croley]

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 speaker you heard at SEMA was likely Tim Wusz from Union 76 , one of the smartest fuel blenders ever .

[FastbackTom]

Originally Posted by Alan Roehrich

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.

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.

[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.

[Mike Croley]

In a low compression normally aspirated race engine , you will sometimes find that very high octane race gas will make more power than the proper race gas . With a tune up matched for the fuel of course . I know of a racer who won back to back Stock Eliminator titles using 118 octane race gas . The burn rate is the key , and the chemical makeup of the race gas determines the burn rate .

[Mike Voth]

Originally Posted by Ed Wright

I may be doing something wrong. After some tuning my LT1 likes C11. Never one issue with fuel check. I don't drain or seal anything. Your results may vary. LOL

Same with our Monte Carlo. More power with the C11, no failed fuel checks.

[Mike Croley]

I guess the folks at Renegade Racing Fuels were reading this thread . The opening page on the Renegade site renegadepro1.com has been changed to reflect a lot more of their automotive race fuels .