Are you talking about the ratio used to charge the compressor cylinder, or
are you referring to the motor that drives the compressing mechanism?

In any case, I don't know of any air compressors that use high octane fuel?

 

I dont care what the octane rating is it takes more energy to build a higher
compression such as 10-1 than it will to make 9-1
there is more work needed to compress the higher than the lower
so if you take 2 engines built exactly the same but 1 has higher compression
it will require that engine to use more fuel to overcome the extra compression
think of it this way 2 guys lifting wieghts both are as strong as the other
one guy has to lift 10 lbs more than the other who will run out of energy first????

 

I'll try again 'out:

You are looking at "pumping losses" not brake hp output.

From basic engine theory, "brake" power, or power you can use, aka "flywheel hp" is "indicated hp" - "friction hp" - "pumping losses". The higher compression engine gets more "indicated hp" from burning the fuel. The friction is virtually the same and the pumping losses will be fractionally more for the higher compression engine, but the extra power from the higher CR will more than compensate for the pumping loss. This is especially true at idle.

Unfortunately guys lifting weights aren't a perfect analogy to an IC engine. However, our engine "guys" aren't equally strong: 11:1 guy is stronger than 10:1 guy. That analogy is a little closer.

I'm taking side bets...

 

Maybe another analogy that we can all try in real time is:

Take a low compression motor (9:1). Run 94 octane fuel take some 1/4 mile
passes at the track, or stick the car on a dyno.

Repeat the tests with proper octane for that motor.

Also monitor hydrocarbons HC% in the exhaust during each test.

http://gmthunder.com/tino/reardyno.jpg
http://gmthunder.com/tino/dynomonitors.jpg

 

Truth be told, the reason I even initiated this discussion is because I've been looking rather closely at this new E85 fuel. I've seen numerous reports of people who have converted their cars to be E85 friendly, and just about all of them said that they got an average of 2-4 less mpg than with standard fuel. The compression ratios of today's engines seem to fall between the 8.5:1 and 10:1 range, so it makes me wonder if the reason people are getting worse gas mileage is simply because there aren't any modern production engines that can exploit the full potential of E85's 105 octane rating.

There also seems to be a shortage of info about the compression threshold of any given octane level of gasoline. As in, if one had a motor with 10:1 compression, what's the lowest octane he could run before running into detonation, etc. Hard to say whether this is an issue of liability, or if nobody has had enough time and money on their hands to afford to blow up a few motors to test this out. The reason I'm concerned about octane threshold, is because as tempting as it would be to build a 13:1 motor, at this stage in the game I don't think that would be wise due to the relatively scarce availability of E85. The 13:1 motor might be the ideal match for this fuel, but I'm afraid that with a compression ratio that high, it would be limited to using E85 exclusively and that could end up being a problem at some point. I've searched around to see if there is a mathematical formula for calculating compression tolerance for different octanes, but so far I've come up short.

Anybody have any insight about this?

 

This is all hypothetical, right? You aren't in the process of trying to convert your SBC or anything are you? I'm still trying to figure out why you asked the first question.

This would have been a good first post if this was your real intent. Ask us what you really want to know. It sure saves bandwidth.

Search around a little about running ethanol (or methanol) as a fuel, and also search around here or other places for "dynamic compression ratio". This might give you some insight. It's not just about compression ratio.

Doesn't it take more (mass) alcohol (even 85% alcohol/15% gasoline) to produce a given amount of power than 100% gasoline? That might account for the few miles per gallon. Isn't miles per $ what you are looking for, not miles per gallon? Or is it Hp/$ ?

There's so much to this, that a forum isn't the place to re-engineer an engine. Some of the OEMs are offering FFVs. You might look there. Of course they are not optimised for either gasoline of E85 or anything in between. That's the rub. Compromise.

There is tons of research that has been done on octane, detonation, etc. Spend some days researching this. Ethyl Corp. did a lot of basic research decades ago. Google, the SAE among others are your friends here. There really isn't a shortage of information, but perhaps a shortage of finding it.

Good luck!