I came across your article on blending E85 with pump gasoline and I have a question. I have a C7 ZR1 that is un-tuned that needs 96 octane to prevent timing from being pulled. I have e85, 93 e10, and limited 93 no E available. I am trying to stay close to 15% E, since my car has no modifications for using a higher ethanol percent. Do you think the 17-18% would be ok to run? I don’t run my car at the track, just some spirited driving.
My car has an 18 gal. tank, assuming it is empty, are my figures below close to correct?
*** Mix #1 E10 93 ***
2 gal E85 + 16 gal E10 93
2 x 105= 210
16 x 93= 1488
1698/18= 94.3 Octane2 x 85= 170
16 x10= 160
330/18= 18.3% E*** Mix #2 No E 93 ***
3.75 gal E85 + 14.25 gal no E 93
3.75 x 105= 393.75
14.25 x 93= 1325.25
1719/18= 95.5 Octane3.75 x 85= 318.75
B.H.
14.25 x 0=0
318.75/18= 17.7% E
The story that B.H. refers to is an article I wrote for the EngineLabs website on mixing E85 with regular pump gas. You can read that article here on EngineLabs.com.
The car that this reader is using is a C7 Corvette with a ZR1 package, which is a supercharged 6.2L LT direct injection engine that Chevrolet rates at 755 hp. It’s also helpful to know that GM rated this engine using 93 octane fuel, a pump gasoline that is not prevalent across the country.
How Gasoline is Rated in the U.S.
Before we get into the details, let’s cover some basics. Gasoline in the U.S. is rated by combining the Research octane number (which is almost always higher) with the Motor octane number (which is lower) and averaging them—which is what you see on the gas pump as (R+M) / 2 = Anti-Knock Index (AKI).
Interestingly, European fuels are rated by their Research octane numbers only, which is a critical distinction that our friends who drive Porches should keep in mind.
Blending Octanes (& Ethanol)
The important thing to know is that when blending gasoline of different octane numbers, the curve is linear. This means if you blend 10 percent 100 octane race gas with 90 percent 91 pump gas, it will increase the octane of the 91 to 91.9—it’s a simple proportion problem. Or, if you mix it 50/50, then you’ll get 95.5 octane.
Blending ethanol with pump gasoline operates much differently. Rather than a straight linear curve, the benefit of ethanol is that the first 10 to 30 percent blend of ethanol with a pump gasoline like 91 will radically increase the octane rating of the fuel. So creating an E30 fuel by blending 91 octane pump gas with E85 would generate a fuel with an octane rating of 94 AKI. Once you get to blend ratios over 60 percent, the octane benefit from ethanol is still there but the rate of benefit slows down—almost like a plateau. Think of this effect as like the Law of Diminishing Returns.
To illustrate this point, standard 87 octane pump gasoline with 10 percent ethanol actually starts as 84 octane without the ethanol. The additional three octane numbers are gained by the relatively minor 10 percent added ethanol.
This is why all the gasoline companies like ethanol—it’s an inexpensive way to increase octane. They just don’t want you to know how easy it is to mix ethanol with gasoline which will increase the octane rating of the fuel and also reduce the cost of the fuel at the same time, because E85 is generally less expensive than pump gasoline.
Other Things to Consider When Blending Fuel
It’s also important to note that one of the major advantages of ethanol is its cooling effect. When alcohol blended fuels are introduced into the engine, the alcohol evaporates much more quickly at lower temperatures. And when this occurs, the alcohol pulls heat out of the air.
An analogy would be when you go the doctor’s office and the nurse swabs your arm with alcohol. That spot feels cold because the alcohol evaporates and pulls heat off your arm.
The same thing happens when running an alcohol blend fuel. The alcohol evaporates and reduces the inlet air temperature. This is important because for every 25 degrees reduction in inlet air temperature, the engine requires one less octane to be detonation resistant. In other words, heating the inlet air also increases octane requirements.
We all know that superchargers heat the inlet air when as it is compressed. So using a fuel with 30 to 50 percent ethanol will radically reduce the inlet air temperature.
As an example, in a test we did years ago a supercharged small block running 9 psi of boost would generally generate around 150 degree F inlet air temperature after the supercharger on gasoline. With E85 fuel, our experience is that the discharge temperature was closer to 90 degrees F—or a reduction of more than 60 degrees F. That will reduce the engine’s octane requirement by roughly 2 points.
Now you begin to see why so many folks are converting to run on ethanol-blended fuel.
Checking Your Octane Blending Math
To address your math as far as blending mixtures go, your numbers are close but our numbers indicate that a blend of pump gas and E85 to produce a splash blend mixture of E30 would be worth 3 octane points. A blend ratio of E50 or 50 percent ethanol will deliver roughly 4.5 to perhaps 5 numbers. In other words, an E50 blend of E85 and pump 91 premium gasoline (that already has 10 percent ethanol in it) would produce a fuel with an AKI number of around 95 AKI.
Whether that overall AKI number is sufficient to allow your engine to not pull timing back is beyond our ability to predict—but in our humble opinion it would be very close to ideal.
Don’t Forget About Your Injectors
One final caveat is that this blend ratio assumes the engine’s current fuel injectors are large enough to supply the additional fuel volume. This is a real requirement because ethanol only produces about 75 percent of the heat of gasoline, so you have to run more fuel to make the same amount of heat.
If the injectors are too small, the duty cycle on the injector approaches or hits 100 percent. Generally, injectors do not operate consistently above 90 percent duty cycle and can actually deliver less fuel than at lower duty cycles so it’s best to avoid running the injector above an 80 percent duty cycle, especially with a supercharged engine where running lean can cause serious engine damage in a very short period of time.
Fuel Octane Blending Guidelines
| E85 | Gasoline | Total | Blend Ratio | Octane AKI |
|---|---|---|---|---|
| 1 | 4 | 5 | E25 | 93 |
| 3 | 7 | 10 | E33 | 94 |
| 4 | 6 | 10 | E40 | 95 |
| 10 | 10 | 20 | E48 | 96 |
On the other hand I have a pickup that will run e85 from the factory. In several tests after I purchased it even at 40 cents a gallon cheaper, it was less expensive to buy e10 because of the the loss in mileage. Buying none ethanol boosts the highway mileage 3 to 4 miles per gallon over e10.
So with gas prices like they are now the loss in mileage doesn’t seem to be worth the price unless you are racing all the time.
Need input on mixing unleaded 91 octane pump gas with 110 leadec octane racing fuel. What effect does mixing the two have on performance or need to change jets in holley carb. This is for drag car.
I just converted my K24 Accord coupe to flex fuel with eFlexFuel’s Pro kit and can only run a 50/50 blend according to them but wasn’t sure what ethanol percentage I was looking to maintain or what duty cycle even meant. So this article was EXTREMELY HELPFUL! Thank you guys so much for providing such a clear and detailed breakdown of how fuel ratios work!
Hello to all the team! I purchased Race Gas Ultra, my question is, whow do I get 96 octane with 91 pump gas? What is the mixture? How many ounces of Race Gas Ultra do I need per gallon? It is very complicated and confusing. I really need your help on this. On the can it only say’s to mix one can of Race Gas Ultra with 4 gallons of 91 to make 112 octane, this mixture is way to high for my use. How do I lower that mixture?
You all have a great weekend!
Fabian.
Hey Fabian, I checked with Race Gas, and here’s what I found out:
…
“Race Gas Ultra is specifically designed to make either 112, 110, or 108 octane. We do not recommend blending to lower octane level. If your engine requires less octane than 108 please try our original formula RACE-GAS Race Fuel Concentrate.”
…
If it helps, visit the Race Gas website and you’ll see a handy blending guide that’ll give you some common (and easy) mix ratios.
What about blending 100 Low Lead Aviation gas with 91 octane car gas. I want to do this to get 93 octane car gas for my Rotax aircraft engine with turbo. What ratio would i use ?
Your Rotax 914 Turbo will run absolutely just fine and perfect on USA 91 Octane Premium. My only caveat is this….. when Spring time comes around with warming days, there is the possibility of getting Winter grade 91 Octane Premium with a very low Reid vapor pressure, that might possibly cause vapor lock if you shut down hot, and then come back in 30 minutes to an hour later. This is not a problem in my Van’s RV-12, because it has an electric fuel pump in the fuel tank, as well as the mechanical pump on my 912 ULS, so positive fuel pressure and no possibility of vapor lock is possible.
That said, the best solution is that only in the spring time, add a gallon of 100LL AvGas to every 9 gallons of Premium unleaded. That pretty much always eliminates the possibility of vapor lock. I’ve heard this from flight schools, 5 of them, that run Light sport Rotax powered airplanes, as the best solution. Once summer grade gas gets in the auto fuel tanks at gas stations, it again becomes a non issue.
Rotax manual https://www.rotax-owner.com/pdf/OM_914_Series_ED3.pdf Page 2-8 states European Standard of a RON of 95.
https://www.afpm.org/issues/fuels-vehicles/95-ron/95-ron-octane-standard
2nd paragraph… 95 RON = 91 Octane, US standard.
So there’s your answer, run straight Premium. Keep the lead out of the ring lands, gearbox for prop, and engine as well as the cases. Rotax designs these engines to run on Unleaded Mogas, leaded fuel in Europe, Germany and Austria, where these are made, is Verboten.
What about 100 Low Lead Aviation gas. I want to blend it with 91 octane car gas to get 93 octane that my Rotax aircraft engine with turbo needs for 180 hp. What ratio would i use ?