Mixing E85 With Pump Gas: Getting Higher Octane For Less Money

Mixing E85 With Pump Gas: Getting Higher Octane For Less Money

There are certain expectations in life and especially in the world of high-performance automobiles. The givens are: horsepower costs money; your engine never makes as much power as you think it should; chassis dynos always hold back on you, and powerful engines need good (read expensive) fuel. Most of the above is true except perhaps the last one.

What if we told you there is a simple way to mix two generally available fuels that would increase the octane rating of the blended fuel while simultaneously reducing the cost per gallon. Would you believe that? The skeptics are already shaking their heads, but it’s true. The only hitch is there are some simple mixing instructions and you will need a gas station that sells E85.

Right away the boo-birds will start whining about all the negatives of E85. But this story isn’t about E85 – it’s about E30 and E50. These numbers refer to the percentage of ethanol in the fuel. As an example, nearly all pump premium fuel sold in this country is E10 and has been for years. By blending pump premium (or 91 if you like) with E85, you can create a custom blend of high-octane fuel.

Cost Savings Through Better Chemistry

Simply put, if you mix 5 gallons of 91 octane pump gas with 5 gallons of E85, you get 10 gallons of a 96-octane fuel that’s very close to E50, or 50-percent ethanol and 50-percent gasoline. (If we’re being pedantic, it actually computes to E48.) The best part is that for most of the country, E85 costs approximately one dollar less per gallon than pump premium.

In California, our overpriced pump premium sells for roughly $3.60 per gallon while E85 sells for $2.20. Blend those two together to create a 50-50 mix and you get 3.60 + 2.20 / 2 = $2.90 per gallon. What we’ve just created is a 96-octane fuel that costs less than 91-octane ($2.90 vs $3.60). That saves $14 for 20 gallons of fuel.

Is there a catch? Of course there is. You don’t get octane for nothing. If we combust a gallon of E10 gasoline, it will create the equivalent of 111,800 BTU of heat. Burning a gallon of pure ethanol will generate 76,100 BTU of heat — or approximately 32-percent less heat than gasoline. This means that we need to burn roughly 35-percent more fuel if we converted to pure ethanol.

If we are considering mixing 20- to 40-percent additional ethanol in with the gasoline (remember, there’s already 10-percent ethanol in most pump gas), this requires delivering more fuel to the engine to compensate for the reduced heat created when we combust the mixed fuel and to create the proper air-fuel ratio.

Blender pumps can be increasingly found in the Midwest where new cars are benefitting from the added octane. Studies have shown E30 can improve fuel mileage in turbocharged four-cylinder engines like the Mustang EcoBoost by increasing the octane rating. This allows the engine to run its more aggressive OE spark map that results in better mileage – assuming you can keep your foot out of the throttle!

For a carbureted street engine, it will need larger jets and perhaps a metering block conversion for a typical Holley 4150 style carburetor. EFI engines will demand an appropriate increase in the pulse-width on the injectors. It’s possible that an EFI engine may need larger injectors to accommodate the required additional fuel volume.

Much of that depends on the initial size of the engine’s injectors. Regardless of injector size, however, all EFI engines running a blended fuel will require recalibrating the tune. An ideal situation would be to have one calibration for E10 pump gasoline and another for E30 or E50 that could be easily switched based on fuel usage.

This idea also requires the user to manually mix these fuels. There are a few blender pumps in the Midwest but they are few and far between. So, for most blended fuel users, this will require some math to come up with the required fuel blend. But you’re a car guy — that stuff is easy. It’s so easy we’ve even created a chart to help with the ratios.

Chart 1

Home Chemistry 101

Now let’s cover some E85 mixing basics. While it might seem that blending fuels for octane would be a straight percentage mix like with gasoline, E85 works a little differently. If we blend 5 gallons of 91-octane gas with 5 gallons of 100-octane race gas (both unleaded) the result is 10 gallons of 95.5 octane gasoline. Ethanol’s blending relationship, however, enjoys a little-known advantage. Small blend percentages, like E30, have a much greater impact on octane rating. This also means that moving from E70 to E85 delivers very little in terms of increased Anti Knock Rating (AKI). This falls under the category of the Law of Diminishing Returns.

In Blending Chart Number 1, note how E30 improves standard 91-octane E10 to a 93-octane rating. This is a total ethanol content increase of only 20-percent, because pump gas already contains 10-percent ethanol. Or, in Blending Chart 3, creating E30 bumps that 89-octane gasoline to an inspiring 92 octane. Both of those blending efforts raise the AKI by three full numbers.

Moving up in concentration, like going from E65 to E85, does not generate the same 3-point jump in octane. As a further example, according to Steve VanderGriend, technical manager for ICM who operates ethanol plants throughout the U.S., regular-grade gasoline starts as 84 octane fuel, and with just 10 percent added ethanol, the AKI jumps three points to 87.

Chart 2, 3


Points Vs. Percent

This is also a good place to weave in a paragraph about semantics. Some less-reputable octane boosters on the shelves of the local parts store will claim to raise the octane by three or four points. The operant word in those claims is “point.” A point is defined as one-tenth of a number or “0.10.” By definition, three points represent 0.3 of an octane number. This means that mixing in this mysterious liquid improves the octane rating of your fuel by three points – or moving 91.0-octane fuel to 91.3-octane. That’s not a big deal, especially considering the price of a pint can of even the cheaper stuff.

Our previous example of creating E30 with 91 octane fuel raised the AKI by three full numbers, from 91 to 94 AKI. That small piece of semantics is worthy of the italics added for emphasis. Remember this the next time some snake oil salesman tries to blow smoke up your tailpipe by a cheesy three points.

Concerns About Ethanol

These small blending mixtures of E30 to E50 maintain a large percentage of gasoline. The advantage here is that if the electric fuel pump in your street car is not rated for use with E85, a blending ratio of E50 or less can generally be considered compatible. You should consult your particular pump manufacturer to be sure, but a light blend of E30 should not cause any problems.

Let’s also address the claimed issues attributed to using ethanol in older cars. Our own experiments and investigations have traced the source of many ailments not to the alcohol in the fuel but rather to the high percentages of aromatic additives used in all pump gasoline. These aromatics are often lumped together under the acronym “BETX.” This is chemical shorthand for benzene, Ethyl-Benzene, Toluene, and Xylene.

One of the best ways to know what percentage of ethanol is in your tank is with a sensor like this multi-gauge from Innovate Motorsports (left). This gauge uses a factory GM flex fuel sensor to quickly calculate the percentage of ethanol in the system. The screen as set up here reveals an ethanol percentage of 50.6, the engine is idling at 12.7 inches of mercury (“Hg), Lambda is currently at 0.95 (equivalent to a 13.95:1 air-fuel ratio), and the fuel temperature is 70.6 °F. There are also simple testers out there (right) to determine the percentage of ethanol in E85, which is a good idea since the government now defines E85 as any percentage of ethanol between 51 and 83 percent depending upon the time of the year.

These aromatics are present at anywhere from 20- to 25-percent (and sometimes more) by volume in gasoline. By themselves, BETX aromatics are not necessarily dangerous. But they are responsible for many of the problems in fuel systems, such as killing rubber fuel lines and affecting small rubber parts in carburetors.

Of equal concern is a dangerous by-product created after combusting BTEX, creating what are called ultra-fine particles (UFP). These particles are small enough to enter your lungs and travel past the membranes and directly into your bloodstream. Not good. By mixing ethanol with the fuel, fuel companies are able to reduce the concentration of BETX in the fuel. Conversely, alcohol-free fuels as sold in many Midwestern states contain much higher percentages of BTEX to compensate for the loss of ethanol in the fuel.

If you are considering running a higher than E30 percentage of ethanol in your carbureted engine, you might want to consider these green E85 metering blocks (left) that are calibrated for E85. If you’re only running E30, it’s likely you can get away with simply increasing the jetting slightly. . We calculated that running E30 should require a two to perhaps three jet size increase to compensate for the leaner mixture compared to E10 pump gas. It might also be a good idea to change the idle and high-speed bleeds slightly smaller to maintain the relationship. Ethanol has a higher specific gravity than gasoline so adding E30 may affect the float level slightly (right) so pay attention to this. A denser fuel will make the float sit higher on the fuel which effectively lowers the float level.

What’s Cooler Than Being Cool

A further benefit of increasing the ethanol content in fuel is something called latent heat of vaporization. A major advantage of mixing ethanol with gasoline is an increased cooling effect created when the ethanol vaporizes. When water changes state from a liquid to a gas, it removes heat during the conversion. When sweat evaporates off your skin, it cools your body by removing heat.

The same thing happens inside the intake manifold. Ethanol is more than twice as effective as gasoline at removing heat from the intake tract. We’ve listed the latent heat factors in Chart 4. You can see that water is really good at pulling heat out of the air. That’s why water injection on supercharged engines works so well.

Chart 4

Ethanol’s high latent heat ability cools the inlet air which lowers the engine’s octane requirement. This may not sound like much but consider a study completed by the OE’s back in the 1970s. The test determined that for every 25 degrees reduction in inlet air temperature, the engine’s octane requirement was lowered by 1 full number.

If we run a gasoline/ethanol mixture of fuel that can reduce the inlet air temperature by 25 degrees, this means the engine could now make the same power with one less octane number of fuel. But adding ethanol also increases the octane rating which is why blending fuel is such a positive step.

RON, MON, and R+M/2

The fuel industry first designed an octane test for gasoline way back in 1932. For early, low-compression engines that were not very efficient, Motor Octane Numbers (MON) seemed to carry more weight. But late model engines, especially direct injection spark ignition engines, tend to place greater emphasis on the Research Octane Number (RON) testing. Modern fuels ratings in the US average the two numbers by adding RON and MON, and then dividing by 2 (R M/2). What follows is a description of each test.

Research Octane Number (RON) Test

This evaluation uses a single-cylinder engine operating at 600 rpm. This test uses a fixed ignition timing of 13 degrees and a preset inlet air temperature of 125 degrees F (52 degrees C). The compression ratio is varied for the fuel. The octane rating is determined by mixing n-heptane (0 octane) with iso-octane (100 octane). Mixing the two fuels creates the RON. As an example, mixing 90 percent iso-octane and 10 percent n-heptane will deliver a 90 RON fuel.

Motor Octane Number (MON) Test

Some consider this as more representative of loaded use. This test also uses a single-cylinder engine where the ignition timing varies from 14 degrees to 26 degrees BTDC. MON testing pre-heats the fuel temperature to 300 deg. F (149 degrees C).

This chart lists the Research, Motor, and AKI numbers for both pump premium as well as blends of ethanol and gasoline. These are not always going to be hard and fast numbers as pump fuel and percentages vary quite a bit across the U.S. But these numbers will offer a relationship between RON and MON and what can be created by blending pump premium with ethanol. Some of this data came from the Urban Air Initiative.

Premium E10 E30 E50 E85
RON 92.1 100.5 105 108
MON 84 87.5 88 92
AKI 88.1 94 96.5 100

Practical Applications

So let’s apply this to a typical naturally aspirated and carbureted street car on the drag strip. Let’s say we have a normally aspirated 383ci small-block Chevy with the whole package of 10.75:1 compression, good heads, a nice hydraulic-roller cam, a solid-flowing intake, with a 750 cfm mechanical secondary carburetor.

For this example, let’s assume it runs fine on 91-octane pump gas but we have to pull the timing back three degrees of total timing (from 36 to 33 degrees) to prevent the engine from rattling (detonating) on hot days. Let’s also put the jetting at 75 primary and 83 secondaries with no power valve in the secondary side.

There are many older fuel pumps that are not rated for E85. But with blends at E50 or less, often these pumps will work with the higher lubricity present with a greater percentage of gasoline vs ethanol. This is a Holley Ultra XP E85 carburetor that could be used with E50 or higher concentrations of mixed fuel. If you ran this carburetor with E50 it would certainly require leaner jetting and perhaps some bleed changes to create an accurate fuel curve.

Now let’s add a splash of E85 to our 91-octane pump gas to create an E30 fuel. This will effectively lean out the mixture slightly. We did the math and this should require adding perhaps three jet sizes to both the front and back jets. In our above example, this means 78 jets in the primary and 86 jets in the secondary. With a higher percentage of ethanol, this fuel can be run richer without reducing power, so keep that in mind. It’s always better to be slightly rich rather than slightly lean. This will be a good starting point for further testing.

One way to easily test the fuel percentage to make sure we mixed it properly is with Innovate Motorsport’s gauge package that reads ethanol content as well as delivering the air-fuel ratio from an oxygen (O2) sensor. Here’s where it gets tricky because the numbers from the O2 sensor will not be accurate.

Chart 5

In our accompanying chart, you’ll see that any addition of ethanol in the fuel (including the 10-percent added to nearly all pump fuel), affects what is called the stoichiometric air-fuel ratio of gasoline. Pure gasoline has a stoichiometric ratio of 14.7:1. Add 10-percent ethanol and that ratio drops to 14.1:1. Even a mild E30 change moves the stoichiometric number from 14.7:1 to 12.9:1. This can get confusing very quickly.

This is important because most (if not all) air-fuel ratio meters use 14.7:1 as their reference number for gasoline. But, if you’re using E30, that displayed number will read much richer than it will for straight gasoline. We won’t go into all the details as to why this happens because the explanation would double the length of this story.

Generally speaking, fueling engines with E20 to E50 doesn’t require changes to ignition timing. Research indicates that low mixing percentages might improve with 1 or 2-degrees less timing.

For an easy fix, what you can do is switch the meter to read in what is called lambda. Lambda is an engineering term for air-fuel ratio, just expressed differently. The stoichiometric air-fuel ratio number (Lambda) for all fuels (gasoline, methanol, ethanol, and others) is 1.0. Any mixture richer than Lambda is expressed as a number smaller than 1.0 – like 0.85. Any air-fuel ratio leaner than stoichiometric is expressed as a number larger than 1.0, like 1.15.

So to make this easy, just remember that a good place to start your tuning with a mixture of E30, E50 or any other mixture would be to convert your meter to read in Lambda and shoot for a WOT mixture of 0.80 to 0.85. Once your tuning has reached this point, you can then fine-tune to determine what your particular engine wants. When tuning at the drag strip, always watch the trap speed. If a tuning change increases the trap speed, you are making changes in the right direction.

This carbureted centrifugal blow-through LS package using a Vortech supercharger would be a perfect candidate for an E30 / E50 fuel blend. We made 500 hp at the rear wheels using 100 octane race gas in this application but it would likely make even more power on E50 due to lower inlet air temperature.

We’ll illustrate how Lambda operates using straight gasoline. If we multiply the stoichiometric air-fuel ratio of gasoline (14.7:1) times a Lambda of 0.85 we get 12.5:1 which is a safe air-fuel ratio to start with gasoline for WOT tuning. Remembering that Lambda is always 1.0 for any fuel makes part-throttle tuning far easier than trying to remember the max-power air-fuel ratio of different fuels like E30 or E50.

We’ve touched on a ton of material in this story regarding the advantages of blending ethanol with pump gasoline. Many details will remain to be answered such as calculating jetting increases in carburetors, calculating injector sizing to see if your current injectors have enough “headroom” to deliver additional fuel required by the added ethanol. If there’s enough interest in this tuning approach, we can return with more information and perhaps even some hands-on tuning tests with a carbureted engine to show how all this plays out.

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About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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