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Old 05-16-2012, 01:10 PM   #21 (permalink)
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Quote:
Originally Posted by trooper Tdiesel View Post
odd.
the local E85 pump says 105 octane

and in winter its a 70% blend.
That was using the average between the octane of the Ethanol and Gasoline. Which is not how to calculate octane. It needs to be run through a test engine. Like this.

Although it certainly doesn't act like 96 octane, from what I've heard. There is a guy with a 12.5:1 turbo charged Honda K-series running E85 and he got pretty close to the same MPG after the build as he did on Gasoline.

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Old 05-17-2012, 02:42 PM   #22 (permalink)
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Sorry if I missed this part, but I've been skimming at work (distracted a bit as I go).
I have yet to see anyone mention a change in tune. Tuning is key to running safely with heavier modifications, like running 100% e85/ethanol mix, as well as increasing your compression.
If you don't offset significantly higher compression by lowering timing and altering your fuel map, you'll risk detonation, causing you much more headache (and cost) than you'll save.

In terms of 100% ethanol, doesn't it use something like 20-30% more to run the motor than just using regular gas? This is why I mix it in (about 15% mix) in my fun car, to take full advantage of the detonation proofing (or at least, resistance) that it provides, without increasing consumption significantly - I can safely run fairly high boost, make plenty of power, and not worry about putting a rod through the side of the block.

I'm asking, because I don't know for certain, but I don't think it's as simple as increasing your compression and running pure ethanol - I think you need to alter your fuel curve as well right? Timing can stray into the advanced, but you still need a proper tune for it to run "right." Also with older cars (pre 1994 or so), you should probably consider replacing your fuel lines as well, since older ones are more prone to breaking down and falling apart with a little ethanol action.

Just my .02 cents, and since I'm not too knowledgeable about the fine details of tuning, but it seems that you'd want to get a wideband, and see where you're running (at the very least).
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Old 05-17-2012, 03:19 PM   #23 (permalink)
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I'm sorry, I didn't mean to turn this into an Ethanol discussion. Too high of compression is when you have to pull timing to run Gasoline. Since we're not talking hotrods it should okay to pull timing off the top if you keep advance under cruise and moderate acceleration right? That's what Honda does to their economy engines, the Fit has 10.8:1 and runs fine on regular. That's not 1:1 since it's an aluminum engine with a carefully designed combustion chamber but the basic idea translates.
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Old 05-17-2012, 07:11 PM   #24 (permalink)
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I'd imagine that'll work just fine. Low load and throttle for cruising is a great time to lean it out, which is what people are doing (a little differently) here quite a bit.

I'm questioning more the tuning side of things - because you can get more power by advancing timing, at the risk of pinging. All other things being equal (are they ever?) More power to get up to and maintain speed at the same rpm/load/fuel level means the driver doesn't need to press the pedal down that much more for the same effect. Sorry, that's difficult to word clearly.

To the original question - I think increasing the cr that much would be dangerous without a proper tune. But then, I've never tried it, just the impression I get. That's basically what a turbo does - increases the compression ratio fairly significantly (turbo dependant), and you need to add fuel and pull timing to keep it running safe. Otherwise, you trigger a knock sensor (where present), and the car will pull timing and add fuel to compensate - which counters the effect you're looking for.

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