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Discussion Starter #1 (Edited)
What is the best air/fuel ratio for our Hemi's. I've heard 12.5 is the key number ( I think I remember hearing that ) but upon reading it says 14.7 is optimal.

also is a value of 16 leaner or richer or say 10 leaner or richer.

Lower number = richer AFR & higher numer = leaner?



For
gasoline-driven engines, the theoretically optimal air fuel ratio is 14.7
pounds of air for every pound of fuel. At this ratio, theoretically, all available oxygen in the air
combines with all available fuel. This ratio is called the stoichiometric ratio. Stoichiometric for
different fuels are as follows:

Gasoline 14.7
LPG (Propane) 15.5
Methanol 6.4
Ethanol 9.0
CNG 17.2
Diesel 14.6

The measurement Lambda is the actual air fuel ratio over the stoichiometric ratio. A Lambda
measurement of “1” equates to the air fuel ratio of 14.7 (for gasoline engines). When Lambda is
less than 1 the engine runs “rich”, i.e., unburned fuel exists in the exhaust stream. If lambda is
greater than 1 the engine runs lean, i.e., free oxygen (0

2) is present in the exhaust. Depending on
the engine, maximum power is typically delivered when the engine runs slightly rich (for example
at lambda values of 0.8 to 0.9 for most engines). This instrument provides a means to measure
the actual air fuel ratio or lambda in the engine in operation directly from the exhaust. For this a
special wide-band oxygen sensor is used to measure the lambda value derived from the oxygen
content (or lack thereof) of the exhaust gases.

anyone got the math to wang out the numbers to convert into Lamda? I think we just mult or divide 14.7 by the lambda value?


I'm just doing alot of reading on tuning, trying to get as much info as possible that way I can just learn to tune it myself after a pretty basic tune.

Next I'll be asking about advanceing and retarding spark?

 

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I just started playing with Predator tweaks and have been trying to learn this stuff as well, I missed this thread somehow. You might have have gotten more replies in one of the performance categories or the Predator section, but anyway, I think you have it correct. Based on what you posted above: Stoich = perfect AFR (14.7 for a gas engine). So if a Lambda value of 1 = stoich and any value less than 1 = rich or any value above 1 = lean, than anything below 14.7 is rich and anything above is lean. I've read that the "optimum" AFR for the Hemi is 12.4, which would be (mildly?) rich based on the above.

When I started trying to decipher the Predator graphs I logged during 0-60 pulls (after adding a C&L CAI and having the suitcase muffler removed) I was seeing the throttle position jump from 0 to 100% and the "Desired AF Cyl 1" drop from ~14.5 to ~11.4 and ST and LT KR spike up to 5.5/6.5 at certain RPM's along the rev range. The KR readings made it obvious that the motor is leaning out, which makes sense given the freer breathing of the CAI and exhaust, but I made the mistake of thinking that the AF CYL 1 reading was showing the actual drop. I was surprised that the AF didn't change after I increased fuel in the rev ranges where ST/LT KR appeared, even though the change reduced KR. Then I read a little about wide band AFR instruments and realized that "Desired (keyword) FA CYL 1" must be what the ECU thinks the engine NEEDS...not what it's GETTING. I think I strayed from the topic...

Being new to this EFI tuning stuff, I reserve the right to be COMPLETELY wrong about what I said above. :4-dontknow:
 

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On the 5.7 13.5 is what I believe is optimal and on 6.1 12.5. some tunner's run just below these #s to be more on the safe side and the fuel helps cool the combustion chamber. The 5.7 react's different then the 6.1. There's a lot that goes along with this to get the best out of tunning, your LT and ST
 

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On the 5.7 13.5 is what I believe is optimal and on 6.1 12.5. some tunner's run just below these #s to be more on the safe side and the fuel helps cool the combustion chamber. The 5.7 react's different then the 6.1. There's a lot that goes along with this to get the best out of tunning, your LT and ST


If we are talking about WOT Fuel, I think 13.5 is high even on the 5.7.

If you forged the 5.7 it could take a leaner condition but to me, its still too lean for my liking.

I have seen cars like it from 11.9 - 12.6 to 1 ...... every car is different
 

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It depends, are you trying to get, better gas mileage or more power? 14.7 is the atmospheric pressure, and its the ideal for any car. 12.5 is Ideal if you want more power and 18 is ideal for better gas mileage. It all depends on what you want. What are you looking for? I run the diablo trinity, so its diff. than the Pred. Ive been running my car leaner for better mileage, around the 18% mark.
 

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It depends, are you trying to get, better gas mileage or more power? 14.7 is the atmospheric pressure, and its the ideal for any car. 12.5 is Ideal if you want more power and 18 is ideal for better gas mileage. It all depends on what you want. What are you looking for? I run the diablo trinity, so its diff. than the Pred. Ive been running my car leaner for better mileage, around the 18% mark.

Tuning with a Diablo lets you adjust the WOT table cause part throttles are in open loop and controlled by the computer. If you are leaning out your ride by 18%, its under WOT and will not do much for you on Fuel economy, but you could really be running unsafe
 
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