Air to fuel ratio
11-24-2002, 10:10 PM
#1
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Air to fuel ratio
Getting tuner cat for christmas and I was wondering why you want more than 14.7:1 afr when at WOT. Is there an ideal afr are there other perameters... I know its a little rich from the factory.
11-24-2002, 10:26 PM
#2
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14.7:1 is the "stoichiometric" air/fuel ratio.... in theory, exactly the 14.7# of air required to burn 1.0# of gasoline, and produce only water, carbon dioxide, and of course the nitrogen that goes along for the ride. In practice, the combustion is incomplete, but this A/F ratio also produces the optimum overall combination of the pollutants carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx). If you run any richer than 14.7:1 you increase the production of CO and HC, and if you run any leaner than 14.7:1, you increase the production of NOx...
So, the auto industry standardized on 14.7:1 as the optimum mix for minimum total emissions, and designed the catalytic convertor to operate with the exhaust produced at 14.7:1. This A/F ratio also produces decent fuel economy, although something in the mid- high- 15's would be more economical.
But all of this has NOTHING to do with producing power. Classic carb engine design as a rule-of-thumb tuned to 13.2-12.8:1 for maximum power and maximum torque. The modern EFI setup alters those numbers somewhat, but in general you will still find you make more power in the range of 13.5-12.5:1 in a normally aspirated, fuel injected engine.
So..... for low emisisons and fuel economy, you tune for 14.7:1. For making torque and HP, you need to tune richer.
The PCM uses a formula to calculate the "target" A/F ratio, and in general, you will often see 11.7:1 as the target A/F ratio in "power enrichment mode"..... high load, high throttle openings. Yes...."the LT1 runs rich".
So, the auto industry standardized on 14.7:1 as the optimum mix for minimum total emissions, and designed the catalytic convertor to operate with the exhaust produced at 14.7:1. This A/F ratio also produces decent fuel economy, although something in the mid- high- 15's would be more economical.
But all of this has NOTHING to do with producing power. Classic carb engine design as a rule-of-thumb tuned to 13.2-12.8:1 for maximum power and maximum torque. The modern EFI setup alters those numbers somewhat, but in general you will still find you make more power in the range of 13.5-12.5:1 in a normally aspirated, fuel injected engine.
So..... for low emisisons and fuel economy, you tune for 14.7:1. For making torque and HP, you need to tune richer.
The PCM uses a formula to calculate the "target" A/F ratio, and in general, you will often see 11.7:1 as the target A/F ratio in "power enrichment mode"..... high load, high throttle openings. Yes...."the LT1 runs rich".
Last edited by Injuneer; 11-24-2002 at 10:30 PM.
11-25-2002, 02:49 AM
#3
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Location: Buffalo, New York
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Fred said it well, as usual. There are a number of reasons why richer than 14.7:1 at WOT produces the most hp. The most basic is that you want to be sure that there is a molecule of fuel to combine with each oxygen molecule in the combustion space. Since the air and fuel don't mix completely, and combustion is a process which occurs over time, you need an excess of fuel to ensure that each oxygen molecule meets up with a fuel molecule in time to combine.
Every different combo has a slightly different optimal AF ratio. Supercharged cars can benefit from an even richer AF ratio than NA, SC LT1's for example seem happiest in the 12.5:1 range or a little leaner. The excess fuel helps cool the intake charge.
Rich Krause
Every different combo has a slightly different optimal AF ratio. Supercharged cars can benefit from an even richer AF ratio than NA, SC LT1's for example seem happiest in the 12.5:1 range or a little leaner. The excess fuel helps cool the intake charge.
Rich Krause
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