Rebuilding for flex fuel

teazer · Post by **teazer** » Sat Jul 02, 2011 6:24 pm

As has already been covered, E10 is 10% ethanol, E85 is notionally 85% ethanol.

Ethanol is less energy dense than gasoline and it burns at a different air:fuel ratio, so jetting has to be significantly increased.

using high ethanol content on a motor not modified for it will generally result in worse consumption and slightly less power.

BUT

Ethanol is a great knock resistor and can stand much higher CR.

That's why the turbo boys like it. they can crank up the boost, add more ignition advance and make slightly more power. It's slightly less of a power boost than methanol so you could expect to see 5-8% power increase with say 14:1 or more CR but you can't ever run that motor on regular gas.

65ShelbyClone · Post by **65ShelbyClone** » Fri Jul 29, 2011 1:06 pm

Assuming all soft parts in the fuel system are resistant to ethanol, it's mostly just a matter of fuel tuning to get an existing engine to burn E85. I did it briefly with my XR600 as an interim way to get pinging under control. The exact ethanol content of E85 varies between mixed batches. If it happens to have a genuine 85%, then you're looking at 105 octane. Rockett brand racing fuels offers high quality E85 now that is guaranteed 85% and is blended with race gasoline so the octane is even higher than 105.

The downside is that it sucks water out of the air(and through the walls of plastic containers) quite readily and does not stay fresh very long. Come to thing of it, E10 doesn't stay fresh very long, either.

Lefty Loosey · Post by **Lefty Loosey** » Fri Jul 29, 2011 5:10 pm

Lefty, what are the modifications you eventually want to make where high ethanol will be the least problematic?

Given that ethanol has a much higher octane than gasoline, I expect that I'll have to advance the timing quite a bit in a carbureted system. It will be quite a bit higher with woodgas (assuming I get that far).

I'm really looking too far ahead--I'm still trying to get the engine torn down (I don't have a place to work on it right now).

teazer · Post by **teazer** » Sat Jul 30, 2011 11:51 pm

Octane and flame propagation rates are not directly or inversely proportional. While some race fules with high octane numbers are indeed slow burners, that's not really the reason that they burn slowly. Ethanol and methanol both burn slower than gasoline but increasing the CR causes teh mix to burn faster and if you optimize the system for 85% ethanol you will end up at most with a couple of degrees more advance.

There is no better fuel for automotive use than gasoline in terms of energy density apart from diesel and that's a whole other discussion.

65ShelbyClone · Post by **65ShelbyClone** » Sun Jul 31, 2011 12:42 am

The open-air flame speeds of ethanol and methanol are actually higher than gasoline...

teazer · Post by **teazer** » Sun Jul 31, 2011 9:48 pm

Open air, but what about in a compressed gaseous mix at the right proportions of air:fuel?

65ShelbyClone · Post by **65ShelbyClone** » Mon Aug 01, 2011 8:08 pm

That depends on what the "right" proportions are. The literature I have read generally indicates that stoich and leaner ethanol air/fuel mixtures maintain a higher laminar flame speed than gasoline while richer mixture flame speed decreases. Turbulent burn rates are much faster for bother gasoline and ethanol, although they are harder to model.