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Old 09-27-2008, 12:15 AM   #1
Joe@Trick Tuners
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Default What should my Air Fuel Ratio (AFR) be?

I'll start off be explaining the difference between lambda and AFR. Lambda is the same for all fuel types. At the stoichiometric point (the perfect amount of air to fuel for a "equal" burn) lambda will be 1.0. Lambda can be converted to AF for a given fuel type my multiplying the lambda by the stoich AFR for your fuel type. The air fuel ratio is the mass ratio of fuel to air, so for gasoline its 14.7lb or air for 1lb of fuel. A "rich" (sometimes referred to as "fat") AFR means low numbers, and a "lean" AFR means higher numbers, or less fuel. What exactly is rich or lean depends on the car and current load. See below for more info on the proper AFR ratios and you can call them rich if they are lower, or lean if they are higher.
Code:
Diesel          14.6:1 (diesel gets more complicated so i won't go there) 
Gasoline        14.7:1
Methanol         6.4:1
Ethanol          9.0:1
Propane (LPG)   15.5:1
Methane (CNG)   17.2:1
Hydrogen        32.4:1
(Please note that this is not the oxygen to fuel ratio, but the air to fuel ratio.)

All O2 and wideband O2 sensors really only measure lambda so they will work correctly for all fuel types as long as the lambda is displayed. To convert lambda to AFR simply multiply by the stoich ratio for your fuel type. For example if lambda is 0.85 then the gasoline AFR will be 12.5, but it will also be 5.44 for methanol. It may be helpful to display lambda if you are running a mix of different fuels at an unknown concentration (like meth injection). From here on i will discuss the AFR ratios for gasoline motors.

During idle and steady part throttle operation the AFR should be at 14.7 (some say 14.64, and yeah it rounds to 14.6 but that’s what the industry does). In reality the AFR can be between about 13 and 16 at part throttle without causing any issues. During thottle transitions it can sometimes swing wildly. At closed throttle decel there is something called coasting fuel shutoff that will turn off the injectors to save gas. You will see and AFR of 18-20 during this time, and this is totally normal. At WOT (wide open throttle) the AFR is very important for keeping the motor happy (hence the need for a wideband). Here are some typical values for gasoline WOT AFRs.
Code:
Mild NA          12.5-13.5
Wild NA          12.0-12.7
mild boost       11.5-12.5
heavy boost      11.0-11.7
nitrous          11.0-11.5
Sometimes its common to go richer than these values for reasons such as detonation and cylinder temperature control. Many times cars that runs at sustained boost for long periods of time (Texas mile and top speed runs) will require an extra rich mixture to keep the temperatures and detonation in check. Running at 10.5:1 is fairly common for these cases. Many motors will make peak power at a AFR of 12.5:1 as long as there is sufficient octane for detonation control without retarding the timing too much. A richer AFR has HUGE anti knock properties due to the fuel's cooling effect, but there comes a point where its better to decrease the timing, than keep richening it up.

Adding Meth
Remember that meth has a much lower stoich point than gasoline. If you do not know the actual quantity of meth and the quantity of fuel injected, its better to use lambda than AFR. If you do know the quantity then you can use this equation to calculate the AFR to match your lambda.

[desired lambda*% gasoline in decimal form*14.64]+[desired lambda*%meth*6.4]= AFR

For 20% meth at a 0.8 lambda (11.7) the calculation will be
[0.8*0.8*14.64+0.8*0.2*6.4]=10.4






Please post up if you have questions or comments.
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Old 03-26-2012, 11:19 PM   #2
OneQwkStang
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What would mild boost and heavy boost be?
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Old 03-28-2012, 02:19 AM   #3
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Quote:
Originally Posted by OneQwkStang View Post
What would mild boost and heavy boost be?
I guess its more related to IAT temps than actual boost. If I'm seeing over 150F I'll shoot more near 11. So intercooled vs non-intercooled would have different AFRs for the same boost. But I also like to keep things safe so its pretty rare that I would ever tune a car at 12.5:1 under boost especially if its knock limited, and never over email. If I could run near MBT, and had the car on my dyno, then I might lean it out a touch and see if it's worth more hp than the extra safety margin.
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Old 04-01-2012, 06:41 PM   #4
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Thanks, i'm currently running 11 psi and havn't seen IAT over 125 degrees with AFR at 11.5 WOT. Mind you I havn't got into hot weather since the upgrade.
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Old 04-02-2012, 12:16 AM   #5
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11.5 is exactly what I would shoot for with those specs.
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Old 04-06-2012, 01:14 PM   #6
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11.5 is exactly what I would shoot for with those specs.
Thanks !!
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