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 Intake / Carb and Fuel Delivery Tech
 Stoichiometry - Gas vs Ethanol blend
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Sparafucile
Sitting Bull

158 Posts

Posted - 01 Apr 2015 :  11:05:53 PM  Show Profile  Reply with Quote
Here's a thought.

Do you know how much ethanol is in your gas tank?

Did someone say that the EGR system gives you a lean fuel formula?
I believe that's incorrect.EGR gases are not a part of the equation. All EGR gas does is take up some cylinder volume reducing your effective cubes. Lean and rich only refers to the Oxygen to Fuel ratio.

The stoichiometric ratio for Oxygen to Gasoline is about 14.7 by weight.

I can't remember how to do the math but I saw some old carburetor guy say for today's gas its 12 to 1. I presume that accounts for the mandatory 10% ethanol. ( Pure ethanol is a 9 to 1 ratio and gives you more water as a combustion product ).

So 14.7 or 12 to 1 is of course for the perfect burn etc....

How do you tune your carbs? To work with the 10% ethanol gas?

Turns out, due to ethanol shortages, gas stations don't always have the 10% ethanol. Gas companies are allowed to buy ethanol credits when they can't get the ethanol, and you get a higher % of gasoline. Cobra Bill can tell you all about this conspiracy between conservative corn growing states and the far left EPA. You will note that the pump says " May contain up to 10% ethanol ". By the way, being forced to blend in ethanol when there is a shortage artificially hikes the price of gasoline.

This means you never know what you are getting, and tuning your car perfectly today may be too lean or rich for tomorrow's tank of gas. Doesn't matter to computer adjusted fuel mixtures in modern cars. They probably run rough a bit and then correct.

Do you guys re tune your carbs when they change between summer and winter fuel blends?

Carberfucile

fbird1969
Sitting Bull

170 Posts

Posted - 02 Apr 2015 :  09:29:21 AM  Show Profile  Reply with Quote
quote:
Originally posted by Sparafucile
So 14.7 or 12 to 1 is of course for the perfect burn etc....

Carberfucile



14.7 means 14.7 parts of air to 1 part of gasoline, completely different from 12 to 1, which is 12 parts of air to 1 part of gasoline.

Also, 12 to 1 is nowhere near the perfect burn ratio...much too rich.

13 to 1 is approximately the best for power because 13:1 is the upper explosive index for gasoline, any more and your just wasting fuel because there's not enough oxygen in the given amount of air to combust with the excess gasoline.

14.7:1 is for best efficiency.
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Sparafucile
Sitting Bull

158 Posts

Posted - 02 Apr 2015 :  7:16:36 PM  Show Profile  Reply with Quote
Perhaps I wasn't clear when I stated that 14.7 to 1 ( by weight) was for GASOLINE and the 12 to 1 ( not my number ) was for GASOLINE ETHANOL BLEND, which ids the salient point of my post.

Upper explosive Index?

14.7 to 1 is the EXACT chemical ratio that uses all the Oxygen and all the gasoline. Of course this is the ideal case where all the oxygen finds all the gasoline, which won't happen for many reasons reasons, like a more explosive burn or too slow a burn or a wet cylinder, etc....

13.1 would be leaner if the old carb guy's 12 to 1 is correct.

It would be richer for straight gasoline. Are you buying straight gasoline?

I was incorrect by saying Oxygen. I should have said air which is mostly Nitrogen. Of course with that correction, I now realize that since the amount of water vapor in air varies between 1 and 5 % this would affect the stoichiometry on a daily basis.

So a computerized fuel injection system which can adjust, is more efficient and can yield more power with less fuel.

At the moment I am using a 5 to 1 ratio of ice cubes to ethanol.


Spara



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Sparafucile
Sitting Bull

158 Posts

Posted - 02 Apr 2015 :  9:20:28 PM  Show Profile  Reply with Quote
I started to do the math, but its too hard, after a double scotch anyway.

My point was that the amount of ethanol in gasoline can vary as much as 10% by volume and the exact ratio is not stated ( or converted to weight ). This means the correct AFR varies, so yesterday's perfect tune is imperfect today. Maybe when you think you might have purchased a bad tank of gas, it's really just the EPA and Iowa corn farmers screwing with you.

I am not sure how the old carb dude came up with 12 to 1 but after doing a little of the math it seemed obviously rich. On the other hand he seemed to have it down so maybe that richer mix just ensures that all the Oxygen is used. That may have been your point fbird1969.

The last time I tuned a carb was around 1981. I'd have to relearn how today.

Sparafucile
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i82much
Sitting Bull

USA
220 Posts

Posted - 14 May 2015 :  11:55:27 AM  Show Profile  Reply with Quote
It is impossible that 12:1 is the proper stoichiometric ratio for 10% ethanol. Consider that a blend of 1470 units of oxygen to 1000 units of gasoline is at the theoretical stoichiometric ratio for gasoline. Now assume you only had 900 units of gasoline and replaced the rest with ethanol. Ignoring the ethanol content completely, you would theoretically need 1323 units (.9*1470) of air to have the correct stoichiometric ratio for just the 900 units of gasoline. So you need at least a 13.23:1 ratio for stoichiometry of 90% gasoline mixed with ANYTHING.

Steve
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2barrelpower
Sitting Bull

USA
135 Posts

Posted - 14 May 2015 :  12:48:14 PM  Show Profile  Reply with Quote
Read that ethanol is 9 to 1. The link goes to a vizard chart on tuning a carb for ethanol blended fuel.

https://books.google.com/books?id=_pE30rLFql0C&pg=PA122&lpg=PA122&dq=stoichiometric+ratio+10+percent+ethanol&source=bl&ots=DHCGdH8NTY&sig=4_SqnfPX4Sprz5lYy--ktn9TmWU&hl=en&sa=X&ei=MdBUVaH4I4GvogST-IDQBQ&ved=0CCUQ6AEwAg#v=onepage&q=stoichiometric%20ratio%2010%20percent%20ethanol&f=false

69FB, 350P, 9 to 1, Comp 260H, Stock Valve Train, Performer Intake, 1406 Carb, Hooker Headers, Points Style, Crane XRI, Crane PS20, TH350, stock stall, 3.08 gear, and 235/60/15 Firestones.
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i82much
Sitting Bull

USA
220 Posts

Posted - 14 May 2015 :  12:55:55 PM  Show Profile  Reply with Quote
So you are probably looking at 14.3 or so with 10% ethanol.

Steve
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2barrelpower
Sitting Bull

USA
135 Posts

Posted - 17 May 2015 :  1:50:06 PM  Show Profile  Reply with Quote
"For street gasoline without any ethanol added, the value is 14.7:1. That is 14.7 parts of air to one part of fuel. When the gasoline has 10 percent ethanol, the stoichiometric value drops to about 14.3:1."

Read more: http://www.hotrod.com/how-to/engine/hppp-1206-how-to-fix-idle-problems/#ixzz3aQ3lAOPj


69FB, 350P, 9 to 1, Comp 260H, Stock Valve Train, Performer Intake, 1406 Carb, Hooker Headers, Points Style, Crane XRI, Crane PS20, TH350, stock stall, 3.08 gear, and 235/60/15 Firestones.
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Blued and Painted
Chief PONTIAC

USA
3406 Posts

Posted - 17 May 2015 :  5:25:42 PM  Show Profile  Reply with Quote
An injector atomize's the fuel a lot finer than a carb. It like comparing a Sauna to a rain storm.
EFI has a lot more inputs and can adjust in ways and @ speeds that a carb just cant keep up with.
The carb is good at keeping the engine at a good average fuel ratio without having any smart capability. Then consider raw fuel coating the intake plenum and runners
If you tune a carb to 14 to 1, you will get a lean miss at idle. If your crusing speed ratio, for some reason, gose to 15.5 then you could burn a piston.
Carberated idle mixture should be around 12.5 and lean out to about 13.5 in the power range and high rpm's to avoid problems.




Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 17 May 2015 5:35:21 PM
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Bentwings
Tribal Scout

USA
28 Posts

Posted - 30 May 2015 :  12:51:21 AM  Show Profile  Reply with Quote
Hi, had to step in here as I use an AFR gage built into my blown sbc Willys. With today's gas it's tough to tune accurately without referencing AFR. My car is supercharged with a 6-71 blower and 2 x 750 carbs. Normal cruise is 14.0 to 15.0. I see as high as 16 to 17 under very light throttle on the highway.
Normal idle is 13.2. Sometimes dropping to 12.8. I use metering blocks with replaceable idle feed restrictions a well as other adjustable metering jets. I drive this car a lot. It has 20k on the clock so far.

The blower makes this combination extremely sensitive to correct AFR or it will backfire specularly and possibly disastrously. To correctly tune this system or any other NA system you need a good understanding of how the carb works. A majority of the time the motor runs on the idle or transition circuits. You can clearly see this with the AFR gage. It's easy to change metering jets and watch the AFR. In my case an entire day, a tank of gas and dozens of changes by .001 plus adjusting idle mix screws a micro amount got the idle and transition mix nearly perfect. It does change day to day but not enough to worry about. WOT was difficult as you can only use full throttle a few seconds before grossly exceeding speed limits. The data recorded worked great here so a couple jet changes and power valve changes fixed the power circuit.

With everything correct now I can run 87 gas and cruise all day rain or shine. I wish I could get 20 mpg with this car but it just won't happen. I've averaged 15.1 for 650 miles on vacation and up to 17 on occasion but average cruise town and highway is 13 mpg.

This what the EFi does automatically but you still have to understand what the motor requires under different conditions in order to make the system work. They are capable of refining the AFR exactly for any condition so a well tuned system is hard to beat.

We have 10% ethanol mandated here. I hear lots of numbers but no hard data. We do have non oxy premium available but I don't have experience with it. It would be easy to try a couple tanks just to see what it did with no mechanical changes. My target value is 14.7 but 14.2 seems to work better as anything over 15.2 produces lean surge cruising. It hard to feel but it's there. More so when it s cold outside...like in the 30-40 degree range.

I don't know the exact number for E85 but it is much richer. I'd guess around 10. I met a guy with a blown bbc running E85 and he said 9.0was his target. Yet he banged the blower showing off.

Anyway serious tuners really need to permanently install an AFR gage. It's a couple hundred bucks and another couple hundred for the data recorder but it makes tuning really easy. Without this its just guessing.
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Bentwings
Tribal Scout

USA
28 Posts

Posted - 30 May 2015 :  01:01:21 AM  Show Profile  Reply with Quote
By the way I'm getting ready to field a 61 B body 455 so that's why I'm here hopefully to learn more about these cars.
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