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ngnetzky

Ideal Compression Ratio

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I'm just curious to know what the ideal compression ratio is for e85. My engine has a 9:1 compression, and I'm just trying to gauge how my engine will react to running on e85, once it's converted.

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For naturally aspirated most often TALKED about ideal compression would be in the 12.5-14:1 range depending on heads and setup. Most guys do not run this because of the expense to set it up and the inability to run gasoline in a pinch so they back way down and use boost instead. You do not have to be that high unless you are going for thermal efficiencies- but E85 should handle it well if you do. Your 9:1 compression will likely act just like any FFV would since they are low enough compression to burn 85-87 octane gasoline rather than 100-109 octane E85.

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9:1 is kind of low. 9.8:1 is kind of that crossover point. Too far south of 10:1 causes problems.

 

Just 10:1 or 11:1 does great. Pushing the edge of pump gas just starts to get to get good with E85. An optimum flex fuel point is right about there to be efficient with both gas and E85. Good luck running 85 octane, though. Low octane gas still hates it. E20 would work fine, though.

 

14:1 compression for any street car makes too much torque for street tires, transmissions, and some people. I could never put my grandma into my car, largely out of sympathy. We can optimize the engines for the fuel, but you're looking at a major leap in power and efficiency that most people aren't ready for. It's the "unleashing" part that worries me. Cars have speed limiters to keep most average joes from doing anything too stupid. Some people shouldn't have a 175mph street car just handed to them on a silver platter even if it is capable. 

 

A "reprocussion" effect would be a high-end boutique market for biofuels. Supercars like the E85 CCXR making 1000hp+ are going to be the new standard of supercar. A 1200hp E85 Corvette with twin turbos and "mild" boost settings would rock anyone's world, too. They can also be clean and get 20+mpg at cruise! What's not to like? The repair and maintenance costs of blown drivelines, bald tires, shot brakes, frequent fluid changes...oh, and don't sneeze or you'll do a 360 on the highway! The fuel can be pretty clean but the power is so violent the rest of the car pukes.

 

Sorry to rant. I've been having fun in class ;).

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I know several people with NA engines running around 12:1 compression ratios and they each wish that they had gone a bit higher. Best efficiency for conventional engines seems to be in the neighborhood of 13.0 - 13.2 from what I've seen posted.

 

In the turbocharged engines, with a base compression ratio near 8:1, they can run up into the low 30 psi boost range before they have to worry about detonation if they are running proper rich max power fuel air mixtures.

 

Larry

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