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Discussion Starter · #1 ·
Came across this writeup related to a/f ratios >>>> article .



It was giving a few parameters, although very general in nature, do they apply to a Viper. Here is one part of the writeup. Notice the first couple os sentences.



"The optimal a/f ratio for non turbo cars is in fact 14:7. This corresponds to a 0.50 o2 voltage output.

However turbo cars require a much more rich mixture for proper and reliable operation. Depending on

who you talk to the optimal a/f ratio under boost varies. Also how much boost you are running can

have an effect on what ratio you would want. For general street conditions you should shoot for

something in the neighborhood of 12.5:1. This is a generalization and you should do your own research

to find out the best ratio for your purpose. Also if you are reading o2 voltage, he best figure we have

been able to attain is 0.82 volts under Wide Open Throttle while boosting. You can see by the gauges

listed below it would be rather difficult to find .82 volts OR 12.5:1 a/f ratio. Being that the o2 sensor

outputs different voltages at different temperatures, an a/f gauge should not be used solely for tuning."




----I know most only use a/f readings with boosted car, but are there any N/A cars tuned to 12.5 or higher?
 

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J DAWG said:
Came across this writeup related to a/f ratios >>>> article .



It was giving a few parameters, although very general in nature, do they apply to a Viper. Here is one part of the writeup. Notice the first couple os sentences.



"The optimal a/f ratio for non turbo cars is in fact 14:7. This corresponds to a 0.50 o2 voltage output.

However turbo cars require a much more rich mixture for proper and reliable operation. Depending on

who you talk to the optimal a/f ratio under boost varies. Also how much boost you are running can

have an effect on what ratio you would want. For general street conditions you should shoot for

something in the neighborhood of 12.5:1. This is a generalization and you should do your own research

to find out the best ratio for your purpose. Also if you are reading o2 voltage, he best figure we have

been able to attain is 0.82 volts under Wide Open Throttle while boosting. You can see by the gauges

listed below it would be rather difficult to find .82 volts OR 12.5:1 a/f ratio. Being that the o2 sensor

outputs different voltages at different temperatures, an a/f gauge should not be used solely for tuning."




----I know most only use a/f readings with boosted car, but are there any N/A cars tuned to 12.5 or higher?


John, the article you are quoting is for a narrow band O2 like the one that comes on a stock car that reads the air fuel ratio in the 0-1 volt range.


This is a totally different animal than the wide band air-fuel O2. The wideband O2 reads from 0-5 volts therefore allowing for more resolution to accurately read an air fuel ratio from 10:1-20:1. I do not know the exact voltage correlation in the 0-5 volt range that corresponds to a particular A/F ratio, but it is this resolution that allows the 10:1-20:1 range. The narrow band O2 that comes on the car really only reads that the A/F is richer or leaner than the 14.7:1 and the stock ECU adjusts its preset parameters accordingly.


On the average boosted application, 12.5:1 is considered to make the most power. Many people run a richer A/F like 11:1 or 10.5:1 to cool the mixture. This is definitely safer, but not really the best way to go as the specific heat absorption of gasoline is not all that great as compared to that of water or methanol. Water has a specific heat absorption 7 times greater than gasoline and methanol's specific heat absorption is roughly 3 times greater that that of gasoline. That's why water/methanol injection can allow for more aggressive timing and boost while quenching quite a good amount of heat that could lead to detonation.

I remember some of the guys at the AEM tuning class saying that the Mitsubishi EVO is really prone to detonation so those guys typically run a tad richer than 12.5:1 on the A/F to make sure they're safe. I do not know if this is the case but I will take their word for it.
 

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Discussion Starter · #3 ·
Thanks PC for catching the "narrow" band part. Interesting info on heat and liquids. Always wondered what the real benefits of water/meth. came from.
 
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