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Octane compression ratio calculator11/7/2023 This means that a late closing intake point (with a long duration cam) reduces the engine’s dynamic compression ratio. This is because compression cannot begin until the intake valve closes. Remember that cylinder pressure is directly affected by cam timing. 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. 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. 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.Īccording 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. My guess is that a huge domed piston would hurt combustion efficiency more than it would help. The issue becomes the ability to generate that much compression. 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. This also reinforces Rockett’s listing their E85 fuel as compatible with a normally-aspirated engine with up to 16:1 compression ratio. 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 reveals our engine was running an effective compression ratio of over 15:1. Let’s plug in our 10:1 compression engine with a psi of boost into this equation: Given that warning, the formula looks like this:Įffective Compression Ratio (ECR) = x Static Compression Ratio 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. This simple formula converts the amount of boost to additional compression ratio as if the engine were normally aspirated. 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). The engine made 600 flywheel horsepower using a Magnuson supercharger. 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 ran perfectly and we experienced no detonation problems. 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. 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. Wusz says that of the two numbers, greater emphasis should always be placed on the Motor octane number.īefore I get too deep into this, let’s try to quickly answer your question with some actual dyno testing as opposed to just theory. Evaluating these numbers, the “real-world” Motor number is always lower, but the goal is to have the two numbers be relatively close. Adding the two and dividing by two ((R+M) / 2) produces an AKI of 112. In the case of Rockett Brand’s E85, the Research number is 116 and the Motor number is 108. 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. 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. Rockett sells its own brand of race E85 listed as 112 octane. So with that as a variable, we might safely assume the octane to be closer to around 100. According to Tim Wusz of Rockett Racing Brand fuels, the quality of that blended gas may not necessarily be as good as 87 octane. It has a commonly accepted octane rating of 105 although that may vary depending upon the quality of the 15 percent gasoline. To quickly refresh, E85 is 85 percent ethanol (grain alcohol) and 15 percent gasoline. Jeff Smith: This is an interesting question that came out of last month’s discussion about E85. 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.
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