What is a general safe effective compression ratio on E85? The internet results in varying opinions from 13:1 up to an extreme 20:1.

Jeff Smith: This is an interesting question that came out of last month’s discussion about E85. To quickly refresh, E85 is 85 percent ethanol (grain alcohol) and 15 percent gasoline. It has a commonly accepted octane rating of 105 although that may vary depending upon the quality of the 15 percent gasoline. According to Tim Wusz of Rockett Racing Brand fuels, the quality of that blended gas may not necessarily be as good as 87 octane. So with that as a variable, we might safely assume the octane to be closer to around 100. Rockett sells its own brand of race E85 listed as 112 octane. All gasolines use an anti-knock index (AKI) for the octane number created by averaging the fuel’s detonation-suppressing capabilities based on two very different tests. The first is a calculated number called Research octane and the second is Motor octane generated by actually testing the fuel in a variable compression ratio single cylinder engine.

In the case of Rockett Brand’s E85, the Research number is 116 and the Motor number is 108. Adding the two and dividing by two ((R+M) / 2) produces an AKI of 112. Evaluating these numbers, the “real-world” Motor number is always lower, but the goal is to have the two numbers be relatively close. Wusz says that of the two numbers, greater emphasis should always be placed on the Motor octane number.

Before I get too deep into this, let’s try to quickly answer your question with some actual dyno testing as opposed to just theory. As I mentioned last month, we tested a small block Chevy with a static 10:1 compression ratio with 9 psi of boost using E85. This was run on normal E85 and we checked to make sure the fuel was actually 85 percent ethanol and according to our test it was very close. The engine ran perfectly and we experienced no detonation problems. If you look at our Effective Compression Ratio Chart (we’ll explain what that is in a moment) this would be like running a normally aspirated small-block with a 15:1 static compression ratio. The engine made 600 flywheel horsepower using a Magnuson supercharger.

One way to estimate the actual static compression ratio is by using a simple formula that produces a result referred to as Effective Compression Ratio (ECR). This simple formula converts the amount of boost to additional compression ratio as if the engine were normally aspirated. This formula does not take into account the increase in inlet air temperature that occurs when you compress air – we’ll get to that later. Given that warning, the formula looks like this:

Effective Compression Ratio (ECR) = [(Boost / 14.7) +1] x Static Compression Ratio

Let’s plug in our 10:1 compression engine with a psi of boost into this equation:

ECR = [(8 psi / 14.7) +1] x 10

ECR = 15.4:1

This reveals our engine was running an effective compression ratio of over 15:1. That means that you could reasonably build a 14:1 compression ratio engine and run it on E85 and not expect problems with detonation. This also reinforces Rockett’s listing their E85 fuel as compatible with a normally-aspirated engine with up to 16:1 compression ratio.

To answer your question directly, it appears both from our ECR chart and our own testing that you could run E85 on a normally aspirated, static compression ratio of 15:1. The issue becomes the ability to generate that much compression. My guess is that a huge domed piston would hurt combustion efficiency more than it would help. We did a quick scan of the JE piston catalog and the highest compression off-the-shelf piston for a big-block Chevy, for example, was 14:1 with a 112cc chamber.

According to Wusz, even given the cooling effect of E85, his recommendation would be to limit the static compression ratio to 13:1 on a 4.00-inch bore small block and slightly lower in a larger bore engine like a big block Chevy. His recommendations will be conservative since we were talking about pump E85 as opposed to his company’s version of E85 which has a significantly higher octane rating than normal pump E85. There’s still plenty of advantages to building a street engine with high compression that will take maximum advantage of the fuel’s reduced energy content.

Remember that cylinder pressure is directly affected by cam timing. This is because compression cannot begin until the intake valve closes. This means that a late closing intake point (with a long duration cam) reduces the engine’s dynamic compression ratio. If you put a mild cam in a 13:1 compression engine, the dynamic cylinder pressure might be equivalent to 15:1 with a larger cam. This is certainly a bit of a no-man’s land since there probably aren’t too many people with practical experience along these lines, so you should perhaps be conservative and keep the compression in a range where everything is manageable. Then there are other considerations such as how difficult it will be for the starter motor to crank over a hot, 15:1 compression engine.

The big advantage that E85 has over gasoline is alcohol’s excellent latent heat of vaporization. This is a fancy term for alcohol’s ability to absorb heat when it evaporates. If you’ve ever swabbed your skin with rubbing alcohol, it evaporates quickly, leaving your skin feeling very cool. The alcohol in E85 does exactly the same thing in the intake manifold, cooling the air as it evaporates. This reduces the inlet air temperature. This is important because cooler air is more dense, which means it packs more oxygen into the cylinder for the same volume of air.

Next, because of the cooling effect, we can safely assume that the inlet air temperature for our normally-aspirated engine will be dramatically cooler using E85 fuel. This number is a little harder to predict, but I think it would be safe to estimate the inlet air temperature entering the cylinders could conservatively be 25 to 30 degrees cooler than ambient air temperature. This is important because Wusz says that based on OE testing, for every 25-degree reduction of inlet air temperature, the engine’s octane requirement will be reduced by one full octane point. So if our 14:1 engine really was at the knock limit of the fuel with an inlet air temperature of 80 degrees, reducing this by 30 degrees would mean the engine is now “safe” by roughly one full point of octane – which is a significant drop. Cooler air is denser, which should produce higher cylinder pressures and nudge the engine towards detonation. But as we’ve mentioned, cooler air also reduces the tendency for an engine to detonate. So colder air is always better and E85 is very good at cooling the air entering the cylinders.

We’ve included a couple of charts that may help make all this a bit easier to understand. As you can see, there are a ton of variables, but the bottom line is that I think a 13.0:1 big block to perhaps 13.5:1 small block engine would be very responsive when running on E85 and that extra compression will return some of the lost mileage from E85’s reduced BTU heat output compared to gasoline. Think about that – a 13:1 compression street motor on E85 that makes excellent power that could also deliver acceptable mileage. Very few people believe that’s possible  – but I think it is.

Anti-Knock Index Chart

The AKI number is calculated by adding the Research and Motor numbers and dividing by 2.

 AKI = (R + M) / 2


Effective Compression Ratio (ECR) Chart

This chart is a just a guideline, but if we assume we can safely run an ECR of 16:1, then a static 10:1 compression with 9 psi of boost is an ECR of 16.1:1 and that’s exactly the combination we safely ran on the dyno.


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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.