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Some Calculations & Pointers for Mixing Gasoline Octanes - Pak Auto Services

Some Calculations & Pointers for Mixing Gasoline Octanes

Some Calculations & Pointers for Mixing Gasoline Octanes


I got here throughout your article on mixing E85 with pump gasoline and I’ve a query. I’ve a C7 ZR1 that’s un-tuned that wants 96 octane to stop timing from being pulled.  I’ve e85, 93 e10, and restricted 93 no E accessible. I’m attempting to remain shut to fifteen% E, since my automotive has no modifications for utilizing the next ethanol p.c. Do you suppose the 17-18% could be alright to run? I don’t run my automotive on the monitor, just a few spirited driving. 

My automotive has an 18 gal. tank, assuming it’s empty, are my figures beneath near appropriate?

*** Combine #1 E10 93 ***

2 gal E85 + 16 gal E10 93
2 x 105=   210
16 x 93= 1488
1698/18= 94.3 Octane

2 x 85= 170
16 x10= 160
330/18= 18.3% E

*** Combine #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 Octane

3.75 x 85= 318.75
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 web site on mixing E85 with common pump gasoline. You may learn that article right here on

The automotive that this reader is utilizing is a C7 Corvette with a ZR1 package deal, which is a supercharged 6.2L LT direct injection engine that Chevrolet charges at 755 hp. It’s additionally useful to know that GM rated this engine utilizing 93 octane gas, a pump gasoline that isn’t prevalent throughout the nation.

How Gasoline is Rated within the U.S.

Earlier than we get into the small print, let’s cowl some fundamentals. Gasoline within the U.S. is rated by combining the Analysis octane quantity (which is nearly all the time increased) with the Motor octane quantity (which is decrease) and averaging them—which is what you see on the gasoline pump as (R+M) / 2 = Anti-Knock Index (AKI).

Curiously, European fuels are rated by their Analysis octane numbers solely, which is a crucial distinction that our mates who drive Porches ought to have in mind.

Mixing Octanes (& Ethanol)

The essential factor to know is that when mixing gasoline of various octane numbers, the curve is linear. This implies in the event you mix 10 p.c 100 octane race gasoline with 90 p.c 91 pump gasoline, it’ll enhance the octane of the 91 to 91.9—it’s a easy proportion drawback. Or, in the event you combine it 50/50, then you definitely’ll get 95.5 octane. 

Mixing ethanol with pump gasoline operates a lot in another way. Somewhat than a straight linear curve, the good thing about ethanol is that the primary 10 to 30 p.c mix of ethanol with a pump gasoline like 91 will radically enhance the octane ranking of the gas. So creating an E30 gas by mixing 91 octane pump gasoline with E85 would generate a gas with an octane ranking of 94 AKI. When you get to mix ratios over 60 p.c, the octane profit from ethanol continues to be there however the price of profit slows down—virtually like a plateau. Consider this impact as just like the Regulation of Diminishing Returns.

For instance this level, customary 87 octane pump gasoline with 10 p.c ethanol really begins as 84 octane with out the ethanol. The extra three octane numbers are gained by the comparatively minor 10 p.c added ethanol.

This is the reason all of the gasoline firms like ethanol—it’s a cheap method to enhance octane. They only don’t need you to know the way straightforward it’s to combine ethanol with gasoline which can enhance the octane ranking of the gas and in addition cut back the price of the gas on the identical time, as a result of E85 is usually inexpensive than pump gasoline.

Different Issues to Contemplate When Mixing Gasoline

It’s additionally essential to notice that one of many main benefits of ethanol is its cooling impact. When alcohol blended fuels are launched into the engine, the alcohol evaporates rather more rapidly at decrease temperatures. And when this happens, the alcohol pulls warmth out of the air.

An analogy could be once you go the physician’s workplace and the nurse swabs your arm with alcohol. That spot feels chilly as a result of the alcohol evaporates and pulls warmth off your arm.

The identical factor occurs when working an alcohol mix gas. The alcohol evaporates and reduces the inlet air temperature. That is essential as a result of for each 25 levels discount in inlet air temperature, the engine requires one much less octane to be detonation resistant. In different phrases, heating the inlet air additionally will increase octane necessities.

Everyone knows that superchargers warmth the inlet air when as it’s compressed. So utilizing a gas with 30 to 50 p.c ethanol will radically cut back the inlet air temperature.

For instance, in a take a look at we did years in the past a supercharged small block working 9 psi of increase would typically generate round 150 diploma F inlet air temperature after the supercharger on gasoline. With E85 gas, our expertise is that the discharge temperature was nearer to 90 levels F—or a discount of greater than 60 levels F. That can cut back the engine’s octane requirement by roughly 2 factors.

Now you start to see why so many of us are changing to run on ethanol-blended gas.

Checking Your Octane Mixing Math

To deal with your math so far as mixing mixtures go, your numbers are shut however our numbers point out {that a} mix of pump gasoline and E85 to supply a splash mix combination of E30 could be value 3 octane factors. A mix ratio of E50 or 50 p.c ethanol will ship roughly 4.5 to maybe 5 numbers. In different phrases, an E50 mix of E85 and pump 91 premium gasoline (that already has 10 p.c ethanol in it) would produce a gas with an AKI variety of round 95 AKI.

Whether or not that general AKI quantity is adequate to permit your engine to not pull timing again is past our means to foretell—however in our humble opinion it might be very near best.

Don’t Overlook About Your Injectors

One last caveat is that this mix ratio assumes the engine’s present gas injectors are massive sufficient to produce the extra gas quantity. It is a actual requirement as a result of ethanol solely produces about 75 p.c of the warmth of gasoline, so it’s important to run extra gas to make the identical quantity of warmth.

If the injectors are too small, the responsibility cycle on the injector approaches or hits 100%. Usually, injectors don’t function constantly above 90 p.c responsibility cycle and might really ship much less gas than at decrease responsibility cycles so it’s finest to keep away from working the injector above an 80 p.c responsibility cycle, particularly with a supercharged engine the place working lean could cause critical engine harm in a really quick time period. 

Gasoline Octane Mixing Pointers

E85 Gasoline Complete Mix Ratio Octane AKI
1 4 5 E25 93
3 7 10 E33 94
4 6 10 E40 95
10 10 20 E48 96

There are blender pumps starting to make an look within the Midwest that may supply E30 proper from the pump. Manually mixing the gas just isn’t tough however take into account that most pump gasoline is already combined with 10 p.c ethanol. (Picture/Jeff Smith)

Jeff Smith has had a ardour for automobiles since he started working at his grandfather’s gasoline station on the age 10. After graduating from Iowa State College with a journalism diploma in 1978, he mixed his two passions: automobiles and writing. Smith started writing for Automotive Craft journal in 1979 and have become editor in 1984. In 1987, he assumed the position of editor for Sizzling Rod journal earlier than returning to his old flame of writing technical tales. Since 2003, Jeff has held varied positions at Automotive Craft (together with editor), has written books on small block Chevy efficiency, and even cultivated a formidable assortment of 1965 and 1966 Chevelles. Now he serves as a daily contributor to OnAllCylinders.


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December 2022