The Stovebolt Site Forums The Shop Area Engine Shop Carburetor Modifications for E-25 Usage

To All,

A friend of mine in Brazil is running a GMC truck, with a 270 and the stock Zenith 28AV11 carburetor.

Apparently, almost all of the available fuel down there is E-25, so he is looking to make the appropriate modifications.

Main questions to the carb gurus out there: Is it possible to recalibrate his existing Zenith to run on E-25?

Since the stoichiometric A/F for gas is 14.7:1, and ethanol is 9:1, and if my SWAG estimate is correct, I figure that right off the bat the original jetting is around 10% lean. Assuming the appropriate parts are available (or can be made by modifying existing parts) Is the Zenith design robust enough to accommodate the additional fuel flow?

Also, since ethanol has a higher specific gravity than gasoline, will any other internal settings require modification? Float level, for example?

Are there carburetor shops that specialize in these types of conversions? If so, how do the typically ensure that the carburetor is delivering what they expect? Do any use specialized equipment like a flow bench ? Or will I just have to borrow a 5 gas analyzer and ride out next winter down in Rio ?

Even if this particular model can be setup to provide the correct A/F ratio, are there other considerations — like material compatibility with a fuel containing 25% ethanol?

Is he better off just possibly looking for a suitable E-25 compatible replacement carburetor — assuming one even exists?

I would welcome your thoughts on this, especially if anyone has tackled such a conversion in the past.

Best Regards…

I have not done a specific E-25 conversion.

I have assisted with some conversions for E-85 for racing purposes.

So, what is about to follow is some experience, some theory, and some guessing.

First, your 10 percent figure would be mathematically close.

[(0.25 * 67000) + (0.75 * 116700)] / 116700

[16750 + 87525] / 116700 = .893 or 89.3 percent energy content, so 10.7 percent lean.

The problem with this calculation is assuming 100 percent fuel burn.

Ethanol has an AKI of 116. Great for a 15:1 compression race car, not so great for a GMC with what, maybe 8:1 ratio?

All of the ethanol will NOT burn. I currently use a figure of 7~8 percent lean for E-10.

So calibration CAN be made, but it is going to be trial and error for any specific vehicle.

Other considerations:

(1) fuel valve orifice will need to be increased to handle the extra fuel
(2) leather (or in the case of early 1950's Zenith - metal) accelerator pumps will be required
(3) one should start buying spare carbs, as long term, the ethanol is going to eat the bottom of the bowl.
(4) Older brass floats should be used (solder used on the new reproduction floats from the other side of the world doesn't like ethanol in any mixture). I am not a fan of phenolic floats.

Jon.

Back in the dark ages (early 1950's) my father was running a 300 cubic inch flathead Ford V8 on a mixture of Methanol and 20% concentration Hydrogen Peroxide. It was almost impossible to get a carburetor jetted rich enough to run right. I believe we ended up with a fuel burn rate of 12 laps per gallon on a 1/4 mile dirt track. After experimenting with several carburetors including 3 Stromberg 97's, Dad eventually settled on a big single 2-barrel Zenith for a Mack inline six truck engine. The main jets got drilled out big enough that they would try to break off while being tightened, so mixture changes were accomplished by swapping air bleeds instead of jets. An air bleed affects mixture opposite the way a jet does- - - -a bigger hole gives a leaner mixture. It was interesting to confuse the eavesdroppers during practice laps- - - - -"It's a little lean- - - - -gimme a smaller jet!"
LOL!
Jerry

We all know Jon is the Carb King. His reply is further proof!

Thanks Jon,

That's exactly the type of information I was looking for. I greatly appreciate the shared expertise.

I too, was wondering how well the diecast parts would hold up. In addition to extra parts, he might want to think about making sure to run the carb out of fuel prior to any extended storage interval.

I think his best option is to try to locate a "tuner" or other carb specialist down there to help make the required modifications. I would think that there would be other vintage vehicle owners down there in the same boat. He can't be the only one facing this issue.

Plus, my wife has already seen pics of the beaches in Brazil, so there's no way she would let me go down there. At least not by myself.

Thanks again!


Hi Jerry,

I used to share a cube with a guy who once raced alcohol powered drag boats. Can't remember what type of carb he was running, but I remember him also mentioning the challenges of trying to jet it enough to get enough fuel in the engine.

That's funny about confusing people with air bleeds vs jets. A world apart from the fuel injected era we live in now.

Best Regards...

That methanol/Peroxide mix was so corrosive they had to run the carburetor dry between races or it would eat the carb casting, and run a stainless steel pony keg for a fuel tank, plus stainless fuel lines from the tank to the carburetor. A little amyl acetate was added as a surface tension reducer to make the stuff mix well.

Methanol 75%
Peroxide 20%
Amyl acetate 5% - - -by volume.

The fumes from the fuel can would clear sinuses from 5 feet away, and the exhaust was worse!
Jerry

In the late 1970's early 1980's, the EPA wanted the carburetor companies to make "tamper-proof" carburetors.

One of the ploys used was large air bleeds.

Since a majority of average motorists never heard of an air bleed, and an even larger majority of those that had heard of one had no method of changing them, the ploy was quite successful.

Of course, professional racers generally have the capability of machining parts, so this was not an issue for racing with these carburetors.

Jerry - that mixture would burn the redness out of your eyeballs.

Jon.

Hotrod,

The hydrogen peroxide you're talking about; is the final mixture 20% hydrogen peroxide/80% methanol or was 20% aqueous hydrogen peroxide used?

Both- - - -20% concentration peroxide (Medicinal is 2% and hair-bleaching stuff is 4%) is 20% of the fuel mix by volume. 5% by volume is Amyl Acetate, and 75% by volume is Methanol. Back when everybody was using WW II Jerry cans for fuel containers, that mixture had a habit of eating holes in the steel cans. They would mix just enough fuel for a night's racing, and wash the cans out as soon as they were emptied into the stainless beer keg tank. Potent stuff! BTW, nobody ran fuel pumps- - - -they put a little air pressure into the tank to push the fuel to the carburetor!
Jerry

So you had 16 Vol % water in the mix.

Yep- - - -which is a good thing- - - -actually more than that, if you understand chemistry. Peroxide is water with an extra oxygen atom, (H2O2). It's also very unstable. The heat of the combustion chamber makes that single Oxygen atom detatch from the peroxide. "Mono-atomic" oxygen is not a normal state of affairs. If the single O atom finds another one, it combines to form gaseous Oxygen (O2). If it finds some fuel, it's going to be like a sailor on shore leave- - - -out to combine with something, and it doesn't care what! Either way, peroxide makes more oxygen than a similar volume of Nitromethane. The remaining water flashes to steam, and cools the piston crown, the valves, and the spark plug tip. Alcohol-fueled engines run very well at extremely high compression ratios. The old flatheads my family ran were up there around 11:1 or so, and engines with better combustion chamber designs can go much higher on the compression. On days with very low humidity, a couple of 6-ounce Coke bottles of plain water got added to an 8-gallon fuel tank. The slang term for peroxide-enhanced fuel was "fuel tank supercharging"!
Jerry

Most likely hydroxyl radicals and not bare-a$$ oxygen atoms are involved but who cares. That O-O bond is dang weak but again, who cares.

Off-topic about as far-off topic as an off-topic can get off.