air fuel ratio - Subaru WRX Forum
 
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post #1 of 8 Old 03-25-2005, 09:16 AM Thread Starter
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air fuel ratio

Hey guys...
Just want to clarify something...
The a/f ratio of the wrx is being maintained at around 14.7 by the stock ecu...
So, does that mean even as rpm rises, the a/f ratio is maintained at that level?
Or does it get richer as rpm increases???
Thanks!
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post #2 of 8 Old 03-25-2005, 09:28 AM
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14.7 is the ideal mixture. The stock ECU keeps the mixture lower to prevent detonation. At full boost in high RPMS it common to tune the ECU to 11s.
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post #3 of 8 Old 03-25-2005, 10:44 AM
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11 is too lean for me at WOT. I like it about 10.4 or so.

But, pretty much no car runs 14.7. That is an ideal mixture that only works in laboratory environments. The one stock WRX I put a wideband on ran consistent 12's.
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post #4 of 8 Old 03-26-2005, 10:20 AM Thread Starter
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Ok!
I'm just curious because of the afr data that came out after my dyno run(still stock)
It started almost 15 then dropped to around 12 then up to around 13 then
dropped of gradually to around 10 at redline..
Is this normal???
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post #5 of 8 Old 03-26-2005, 11:11 AM
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Could be. I don't know of many people who have logged the AFR on a stock car, so I don't have anything to compare it to.
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post #6 of 8 Old 05-07-2005, 07:21 PM
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Sorry this is kind of long.. it is from my webpage and I thought you may be interested...


Some key points about Air/Fuel ratio (AFR): During easy cruising our stock ECU uses the stock Oxygen sensor to adjust the AFR to a target of 14.7/1. The reason is like this. At 14.7/1 (this means 14.7 pounds of air to 1 pound of fuel) the amount of air will combine with all of the available fuel. This is the best condition for the catalytic converter to do its job thus providing for a better environment.

For performance most tuners will agree the peak horsepower is achieved with an AFR of around 12.5/1, So .. compare 12.5/1 to 14.7/1 you can see there is less air, which means the mix has got more fuel = Rich.

As mentioned above, our stock ECU tries to get the mix at 14.7/1 during easy cruising. So ... what happens when we mash it to the floor? Well .. it is not so easy to explain in detail but in general this is what happens when we mash it: The stock ECU disregards the oxygen sensor readings and uses a fuel map to determine how to work the fuel injectors.

The fuel map is just a table of numbers that an engineer has established to define the proper injector pulsing for a set of parameters. The parameters considered include: Boost, Intake air temp, Intake air flow, RPM, Throttle position and a few more. The stock ECU adjusts the injector pulsing according to the data in the fuel map. A simple example may be expressed like this:

The fuel map tells the ECU to pulse the injectors 10 times when ever the boost is 1, RPM is 1000, the air temp is 70, the throttle position is 25% open and the mass air sensor voltage is 1.25v.

I have more information on my webpage: http://n8rws.com you will note that most of the AFR and tuning information is pertaining to my Supercharged TOYOTA 4Runner but the same principles apply to our WRX / STI's
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post #7 of 8 Old 05-07-2005, 08:49 PM
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Excellent name Jeff. Cracks me up.

My car is well, it's like 10 lbs of shit in a 5 lb bag...

<------ Click to see what I'm workin' with. ;)
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post #8 of 8 Old 05-07-2005, 11:40 PM
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Smile

... :-)
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