I saw this thread yesterday and my BS flag instantly went up but I wasn't gonna be the first. Apparently if you're the first to call BS too often people think you're cocky.

The stoichiometric ratio refers to a "perfect" burn, IE just enough air to burn all of the fuel and not too much. In chemical terms a stoichiometric reactions has the following results:

all reagent is consumed
there is no shortfall of reagent
no residues remain.

This is a fixed value depending on the two reactants, in this case, air and fuel. Different fuels do have different stoichiometric ratios, but gasoline is ~14.7:1.

The compression ratio has to do with how much the fuel AND air mixture is compressed. The more the mixture is compressed, the more violent the explosion and therefore the more powerful. Since both the air and the fuel are being compressed. The ideal air / fuel ratio remains the same regardless of whether the mixture is compressed 2:1 or if it's compressed 20:1.

Octane rating refers to a fuel's resistance to detonate when it is compressed before being ignited when it is supposed to. The higher the octane rating, the more that fuel can be compressed.

E85 has less energy per unit volume than does gasoline. This means that if the compression ratio is kept the same, you will get less power from E85. However, E85 is much less prone to detonation, IE it has a higher octane rating. This means that you can compress it more without it exploding prematurely and this higher compression will extract more power out of it. IE you can get the same power out of E85 that you can out of gasoline, but you have to increase the compression ratio. Once you do this you will no longer be able to run gasoline because your compression ratio will be too high.

Modern vehicles that can burn both are a compromise . Because it is very difficult to dynamically adjust the compression ratio, any engine that can effectively burn gasoline without detonating is not going to be optimal at burning e85. Sure you could make something that was set up to run e85, but you would HAVE to run e85.

You can throw a turbo into the mix and have a lot of fun. Because boost has a very similar effect to increasing the compression ratio, you can run more boost on e85 than you can on gasoline. This is one way to get around the internal engine modifications. You can have a low boost gasoline tune and a high boost e85 tune.

Based on all the info above you can easily see why an ethanol blend will always make less horsepower than pure gasoline. To get the same power you will have to burn more or increase the compression ratio. Since my '92 can't adjust the compression ratio on the fly, it will therefore get less fuel economy with ethanol blend because it has to burn more to get the same power.

Hope this clears some things up.

 

-100% Gasoline has a stoichiometric A/F (air fuel ratio) of 14.7 to 1 (That means it takes 14.7 lbs. of air to burn 1 lb. of fuel)
-E10 fuel has a stoichiometric A/F ratio of 14.13 to 1
-E85 fuel has a stoichiometric A/F ratio of 9.85 to 1
-Pure ethanol which E10 and E85 are made from has a stoichiometric A/F ratio of 9 to 1
-Alcohol has a stoichiometric A/F ratio of 6.5 to 1

All the A/F ratio means is what amount of air and fuel are needed to have complete combustion of all of the air and fuel in the combustion chamber. In other words the lower the A/F ratio the more fuel is burned to have complete combustion when compared to the same 1lb. of air. It is somewhat related to power and mileage but not a direct correlation because of the different properties of the fuels. This A/F ratio has nothing to do with the octane rating of the fuel which is what the original posted question was about.

Octane rating is a quick way to determine what a fuels resistance to detonation is (Spontaneous ignition of the air and fuel in a combustion chamber, when this happens you hear knock). The higher the octane the more resistance to detonation it has. With more resistance to detonation you can run higher combustion chamber pressures and higher temperatures before detonation occurs. If you have no detonation on 91 octane fuel switching to 93 octane fuel or higher will not provide any power difference. (I am assuming you have maxed out the power potential of your combination on 91 octane fuel and ignition lead has not been compromised to run 91 octane fuel) Since higher octane fuels can resist detonation better they do allow you to build an engine that has higher cylinder temperatures and pressures (pressure can be created by compression and ignition lead) for additional power potential and mileage potential. Every 1 point increase in compression allows you to make about 3.5% more power and some additional mileage if everything else stays the same. Also higher ignition lead can create more power (higher cylinder pressures) as long as you do not exceed the point of negative returns. (You can have too much ignition lead without detonation and power will fall off). (Side note: E85 has an octane rating of 106 to 110, so you can run very high compression ratio’s like 13.5 to 1 for additional power and an increase in mileage over 9 to 1 compression ratios)

Now to power production, E10, E85 and alcohol all have the ability to make more power than straight gas if everything else is the same. They have this ability because the amount of fuel that is burned to create complete combustion contains more energy than gasoline, (they will use more fuel since they contain less energy per gallon but since they pack in additional fuel so the total energy released at combustion is higher).
-E10 would be .3% over straight gasoline (Not even detectable)
-E85 3% to 5% more power
-100 % ethanol 6% more power
-Alcohol 14% more power
The above listed numbers are possible when fuel and spark is adjusted to optimum for each combination, but no mechanical changes are addressed. If you mechanically adjust compression ratio’s to optimize power and mileage for each combination the power advantages of E85 and Alcohol are even greater. (3% additional power for each point of additional point of compression as a rough rule of thumb but there are decreasing rates of return one the CR exceeds 12 to 1) (E85 will run at any compression ratio that gasoline will run at but is really shines when the compression ratio’ s are at 12 to 1 or higher, especially for pump fuel)

Now for mileage:
-E10 will require about 3% more fuel to cover the same distance as gasoline
-E85 will require about 33% more fuel to cover the same distance as gasoline
-E100 will require about 40% more fuel to cover the same distance as gasoline
-Alcohol will require about 100% more fuel to cover the same distance as gasoline
(This assumes the engine was designed for street gasoline with no mechanical changes made to make it run on the other fuels) (If you raise the compression ratio of the engine to 12 to 1 you will reduce the amount of additional fuel needed when compared to gasoline but then you cannot switch back and run 91 octane gasoline)

The short summary to the original posters question is run whatever fuel allows you to create maximum power without compromising ignition lead. (As originally posted)

 

If there's only .3% (1/3rd of one percent) power difference between E10 and gasoline, then the power issue seems moot.

That would bring use back to my comment about 3% cost difference vs 3% usage difference. In that regard, it would seem to be a wash.

I believe mine will run on 91 w/o retard. OTOH, I haven't scanned it at WOT with less than 93. 91 certainly didn't create any audible ping when I've been forced to run it.

For normal cruise and MPG situations, I'd bet the 3% usage vs cost difference dictates a lack of advantage with either option. One of these days, I should run scans with lower grades to find out how well it turned out. With good quench and heads, I might be surprised. Maybe I could run mid-grade.

Really, the biggest unexplained item from this thread (from my perspective) is if PLRX really got 4MPG better mileage (which is way more than 3% improvement). It would seem unlikely OR that it was a temporary lean condition after switching from E10 to gasoline. Maybe the trims hadn't re-adjusted yet????

 

E85 is not 14.1/1 for stoich. I bet you mistyped?

E85 can be run at 10.xx/1 or more. Its value is high octane 100+. Resists detonation. That is what alcohol does,

OOPS... I did not see page 2.

I believe the low compression seen in late 70's early 80's were to reduce emishions.

 

Dont forget that stoich ratio and maximum power ratio are typically different. Although stoich for gas is 14.7:1 maximum horsepower is usually in the 12.5 - 12.8 range. The extra fuel is actually wasted, there is no air for it to combust with, it cannot burn.

I believe the extra fuel just helps to prevent detonation but I wouldn't swear to it.

 

Interesting....IDK. Seems like there's gotta be more to it than that. For example, even if all the "extra" fuel for WOT mixture doesn't get burned, enough add'l might/must get used to increase power per stroke, right?

I would also find it interesting to know if there's any real difference in emissions between gas, E10, and E85. Though, by definition, stoich mixture would imply complete incineration of the fuel, we all know there's some that remains. Otherwise, why the need for converters?

Another question in my mind is about the true nature/ability to hit stoic mixture. As Nathan points out above, WOT commands a richer mixture. For normal cruise, I thought converters had to "see" a mixture that favors the rich side -- in order to avoid burning up the converter. This adds to my question about residual elements.

 

I believe there is a cooling effect when runnin richer than stoich in WOT-PE.

when I ran EGT's it was clear richer A/F = lower EGTs.

 

I think this is the big thing. Remember the car is always guessing / adjusting the fuel based on the feedback from the O2, it goes from slightly rich to slightly lean and back and forth, over and over again.

I'm going off of memory here, so take it with a grain of salt, but:

Theoretically, in a perfect stoich burn the only emissions would be CO2 and H20. If memory serves it's the impurities in the fuel and variations in atmospheric conditions as well as variable combustion chamber temps and such that creates the in-efficiencies and thus the emissions. A lack of oxygen is what creates CO instead of CO2. All these imperfections are what leads to the imperfect exhaust. The catalytic converter is simply a cludge to cause these imperfect compounds to react and break or "burn" into less harmful components.

If you could find a way to get 100% burn efficiency, then you wouldn't need a catalytic converter at all.

 

EGTs.....

Exhaust gas temps ===OR===
Engine Gestisational Turmoil?

 

I also run E85 and in an effort to assist bjankuski to right this thread here is my two cents:

IME E85 does make marginally more power (maybe 5%) than gas on a stock engine, though that is burning upwards of 50% more E85 (by mass) than gas. The big advantage though is that E85 acts about like a 105 octane fuel so when using a power adder you can do things with E85 you simply cannot with pump gas.

As for stoich, that is an ideal mixture of air and fuel with the key word being ideal. Unfortunately there is nothing ideal about how an engine burns fuel so to maximize power requires burning more fuel than stoich to consume as much of the oxygen as possible while also cooling the charge, slowing the combustion process and reducing the combustion temperatures. This is not an exact science which is why different engines create MBT (mean best torque) at different AFRs (air fuel ratios).

As for the OP question I agree with what others have said: use whatever makes you happy. All cars made after the mid-1980s have been designed for at least E10 so there is no harm to you car to use it.

 

A more extreme example...

A guy in another car forum posted http://forums.turbobricks.com/showthread.php?t=73061 link to why it's easy to run E85 in all gasoline engines.

I read thru it and believe the well-touted author (guest) LOL, truely believes EFI cars can easily adapt to, and run more efficiently on E85.
While his goal is clearly leading to boosted applications, he makes this conclusion about all EFI cars...

"Any modern EFI can, and if you let it, it will adapt to any fuel regardless of what AFR it takes to reach Lambda=1. But they are not programmed that way.

You can extremely easy program it do run on gas, then on E85 without even doing anything else than just filling up with E85 at the pump. The programming is the easy bit. The EFI just reads the O2-sensor and corrects the base fuel mixture and stores how much is needed to get to Lambda=1."
.
.
.
"But... all modern EFI's have a built-in adaptation already. Not as big as the step between gas and E85, but big enough to fix the car if any error with the fueling should occur."

Due to this post, someone concluded and is posting (in that other car forum) that Mystery Motor Oil (MMO) is all you need to run E85. He says add 1/2c to a tank of gas and you're set.

People from that "other forum" may be looking in here to see what we think. That's because I linked to this thread in an attempt to convey how significant an E85 conversion would be -- and is MUCH more complicated than "just letting the ECM learn how to run it from 02 sensor data".



Let them know what you think!



EDIT: BTW...the one claiming MMO is all that's needed said it was to avoid the corrosive effects of alcohol in the fuel system. (Course, he used the word 'Dude' 5-times in one sentence....dudes.

 

Hey I'm gonna try coal... he did say it will run on any fuel.

 

Or better yet...

Try running this:
http://www.youtube.com/watch?v=FyHFdLdCdkY

 

When I tuned my car for E85 I disregarded "stoich". With the wideband, lambda is lambda. It reads O2, not the fuel. The alcohol allows me to give the ignition a lot of advance fast without getting knock counts. Thats what the octane is all about.

 

here is a good link that compares the BTU value of each fuel,
also click on the talk tab on top for more info
http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent

 

If my friend ever gets all the bugs worked out of his turbo-charged Talon he's going to play with E85 some. Because it's turbo charged he can make more power on E85 because he can turn up the boost, but because the compression ratio of the engine is otherwise fixed, when he is NOT under boost he is going to get less fuel economy. He also has to upgrade his injectors to be able to flow sufficient fuel at WOT for 30 or 40 lbs of boost.

The single biggest drawback to E85 around here is that nobody sells it! They have it in Nashville, they have it in Louisville. They even have it in Franklin, KY, but there is not a single E85 pump in Bowling Green. As soon as a station around here picks it up he'll tune his car to run E85 exclusively. He's willing to give up the fuel economy for the increased power.

 

Last time I went to Beech Bend, I was a bit concerned about that. Searched the web looking for a station that sells it. Driving 120 miles, several passes down the track and 120 miles back on one tank. I always carry a chip with a premium gas tune, just in case. The highway mileage on the E85 was ok so I made the trip with fuel left over.

 

The oil companies don't really want to sell it, because only 15% comes out of the ground in Saudi,85% comes from Iowa. (or Somerset in my case).