Bump to help clear up some timing Questions people might have when switching to e85. Quote is from a e85 tuning forum i'm on:
[quote=""hotrod""]Quote »4. When you say e85 burns faster than gas at power mixtures are you comparing it to 105 or 93 octane gas? I was under the impression that e85 burned slower than gas (93 pump).[/quote]That is a common belief but is not supported by a couple lines of evidence. E85 will allow the use of very large ignition advance numbers but that does not mean it needs lots of ignition advance.
In tests University of Michigan did some tests on small engines where they adjusted them for MBT timing (minimum best torque timing) while running the engines on E85. At low loads the engines wanted exactly the same ignition timing that they did on gasoline. They showed a very slight tendency to use less timing under high load.
To find MBT timing, near torque peak advance timing until you hit your max number, then start backing off the timing until you can just detect about a 1% drop in torque. That is you MBT timing. Because of ethanol's high detonation resistance it will allow you to advance the timing way beyond that number but you gain very little other that causing huge increases in cylinder pressures and stress on bearings and head gaskets.
On a 400 hp engine you would be giving up only 4 hp to gain huge increases in safety on the tune, and protection against things like unplanned lean outs etc. It is a tuning principle that has been in use for over a hundred years by professions (you know like the guys that designed supercharged WWII fighter plane engines, where maximum performance was a matter of life and death).
Quote »Under light loads does e85 burn slower? I was told you should add alittle timing in that area to help get some mpg's back.[/quote]It burns about the same speed as gasoline under light load. Lots of folks do that and it is harmless as at light load you can't hurt the engine with extra timing (within reason). What you are actually doing it creating an artificially high compression ratio under light load by starting the burn early and raising cylinder pressures, and cylinder temperatures for a cleaner burn. Some engines like it others don't care, I think it depends on cylinder head design and quench and the mixture motion in that particular engine.
Ethanol's burn speed changes much more drastically than gasoline does as fuel air mixture changes. At rich max power mixtures it burns faster, when too rich or too lean it burns at the same speed as gasoline. If you change your fuel air mixture by any significant amount you need to revisit timing at that load setting too, because you have changed the mixtures burn rate.
Larry[/quote][quote=""hotrod""]Quote » The way it was orginally explained to me is that since e85 is a higher octane fuel it burns slower and needs the extra timing.....consider that erased from the memory banks[/quote]Yes that phrase should read higher octane fuel will allow more ignition advance, but it may not need more ignition advance.
Where people go wrong is many engines cannot run ideal ignition timing for the fuel due to its low octane. When they put high octane fuel in it then for the first time, they can run proper timing which in their case is more timing than is possible on low octane fuel. They are mixing up the cause and the effect. The ideal ignition timing is determined by the mechanical design of the engine. How much mixture motion it has, how the cam and valve timing events are timed, cylinder head design, squish in the combustion chamber, etc. and is largely a fixed number. You then try to find a fuel that will allow you to run enough timing to get to that ideal timing to put max cylinder pressure in the area of 12-14 degrees after top dead center. They also forget burn speed changes with fuel air mixture. A rich max power mixture burns quite a bit faster than a lean idle fuel air mixture, so the ideal timing changes with fuel mixture too.
That is why entire engine families have very similar ignition timing. You talk to two small block Chevy builders that live 3000 miles apart and I bet their ignition timing recommendations for similar engine builds is within about 2 degrees of timing, frequently in the low mid 30 degree range. The new fast burn heads will be on the low end of that range and the classic muscle car era heads will want in the mid 30 degree range. The old Chrysler Hemi's had less mixture motion due to the hemispherical cylinder head and obstruction of the flame front by high dome pistons, and some of them wanted 40 degrees of advance to make big power. It was not the fuel it was the engine design.
Larry[/quote]I have also finished up my winter modifications and even got a set of coilovers last week. Once i run the tank empty of this old gas from last season the e85 or should i say e70 will be going in with the 1000cc injectors. I already have made a base map for it.