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Old 02-20-2006, 05:57 PM   #1
moby4dick
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Default Significance of A/F ratios, vs RPM...

A Noob question for this...

Can someone explain the significance of the above?

As a reference, I recently had my car tuned and the A/F ratio was 11.8 from around 3700 RPM to redline; from 2K to 2750, it was around 13.8, and then slopes down to the number I mentioned.

While numbers are numbers, I just don't really know what that means.

Thanks!
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Old 02-20-2006, 07:07 PM   #2
mcowger
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AF = Air:Fuel Ratio.

Combustion of gasoline has a certain ratio that is 'stoichiometric', meaning all the inputs (air and fuel) are used up in the combustion. That ratio is 14.7:1. In turbo cars like ours, we like it to run a bit richer during hard throttle conditions (meaning more fuel than needed) for extra cooling and knock resistance/prevention reasons.

So, from 3700RPM on, your ration of air to fuel was 11.8:1 - thats about right (maybe a bit lean, but it depends on other factors) and indicates a reasonable safe tune.

Enjoy.
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Old 02-20-2006, 07:25 PM   #3
flatthump
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Quote:
Originally Posted by moby4dick
A Noob question for this...

Can someone explain the significance of the above?

As a reference, I recently had my car tuned and the A/F ratio was 11.8 from around 3700 RPM to redline; from 2K to 2750, it was around 13.8, and then slopes down to the number I mentioned.

While numbers are numbers, I just don't really know what that means.

Thanks!
Well I think you were asking signifigance of AFR vs. RPM. Well...RPM is much more significant to ignition timing than AFR. Load and boost have detrimental effects if the AFR isnt tuned accordingly. I guess im trying to say that when getting a protune or what not, your tuner is looking at many factors to determine and perfect the AFR, RPM is easy to equate. Because as an air pump's (engine) speed increases, its flow of intake charge can be predicted easily, in a linear fasion, through the basic laws of geometry/physics. This is the job of the MAF sensor.
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Old 02-20-2006, 11:06 PM   #4
mtxfrk2k1
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man i wish i knew all this stuff by heart.

i would love be able to start tuning my car.

i have a S-AFC and MBC.

i have egt, boost, oil temp guages along with the built in a/f guage on the AFC.

is there any good tutorials on self tuning?

or any on just learning what all is involved in tuning.


right now i just know the basics of like 3 major factors are ignition timing, boost, and a/f.

i know my S-afc and boost only have 2 of the 3 covered.

i have a super 16g getting ready to go in and i think i can get a safe enough tune with these 2 until i can afford a utec or something of that nature.

any input?
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Old 02-20-2006, 11:54 PM   #5
mcowger
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Quote:
Originally Posted by mtxfrk2k1
man i wish i knew all this stuff by heart.

i would love be able to start tuning my car.

i have a S-AFC and MBC.

i have egt, boost, oil temp guages along with the built in a/f guage on the AFC.
If this guage is reading from the stock sensor, its worthless for tuning your car
Quote:

is there any good tutorials on self tuning?
Seatch for the tutorial by micktheginge - its good.
Quote:
or any on just learning what all is involved in tuning.
Forced Induction Performance Tuning is good
Quote:

right now i just know the basics of like 3 major factors are ignition timing, boost, and a/f.

i know my S-afc and boost only have 2 of the 3 covered.

i have a super 16g getting ready to go in and i think i can get a safe enough tune with these 2 until i can afford a utec or something of that nature.
You can't do this safely with the SAFC, as it screws with your MAF signal to get the fueling changes. You NEED something more advanced.
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any input?
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