Some Stage 3 MAF math - (say that three times fast (discussion))
12-08-2005, 07:08 PM
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Some Stage 3 MAF math - (say that three times fast (discussion))
(Obviously there are generalizations made in the below math and they should not be considered absolutes)
RS4 K04 turbo characteristics:
K04s at 1.6 bar (23.52psi(70% efficientcy)) flow approximately 0.15 m3/s (or approximately 318 cfm)
There are two turbos in the S4 so the total potential CFM should be 0.30 m3/s (or approximately 636 cfm)
MAF characteristics:
Stock S4 MAF 73mm
Common conversion 88mm
MAF reading offset 1.45 correction factor.
Topic for discussion:
If 1 g/s = 1.77 cfm (approximate average given temp modulation) then an 88mm MAF reading 250 g/s = 362.5 g/s (when correction factor is applied) would approximately equal 640 cfm.
(I realize that there are temperature range air density factors in the MAF conversion but, I chose to simplify the discussion by leaving that out since a 40-100 deg range assumes similar basic air density of modest significance)
Question for discussion:
How can a 1.6 bar, 636 cfm potential max boost system provide a MAF reading of greater than the total potential cfm?
Is my math wrong here?
No particular car is implicated in this example as this is a math topic for discussion.
RS4 K04 turbo characteristics:
K04s at 1.6 bar (23.52psi(70% efficientcy)) flow approximately 0.15 m3/s (or approximately 318 cfm)
There are two turbos in the S4 so the total potential CFM should be 0.30 m3/s (or approximately 636 cfm)
MAF characteristics:
Stock S4 MAF 73mm
Common conversion 88mm
MAF reading offset 1.45 correction factor.
Topic for discussion:
If 1 g/s = 1.77 cfm (approximate average given temp modulation) then an 88mm MAF reading 250 g/s = 362.5 g/s (when correction factor is applied) would approximately equal 640 cfm.
(I realize that there are temperature range air density factors in the MAF conversion but, I chose to simplify the discussion by leaving that out since a 40-100 deg range assumes similar basic air density of modest significance)
Question for discussion:
How can a 1.6 bar, 636 cfm potential max boost system provide a MAF reading of greater than the total potential cfm?
Is my math wrong here?
No particular car is implicated in this example as this is a math topic for discussion.
12-08-2005, 07:20 PM
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I want to stay focused on the....
..."how can it be higher" rather than the "how much".
Because the topic could digress. I think my math may not correctly account for g/s (air flow) vs cfm (volumetric measurment) in the presented example.
Side note: The case of the vehicle in question is considerably higher.
Because the topic could digress. I think my math may not correctly account for g/s (air flow) vs cfm (volumetric measurment) in the presented example.
Side note: The case of the vehicle in question is considerably higher.
12-08-2005, 07:49 PM
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How do you figure out maf correction factor?
shouldnt it be:
73mm vs 88mm maf should be 1.2055
73mm X 1.2055 = 88 so if 88mm maf reading is 250g/s wouldnt it mean 301.375? then the CFM would be much lower!
73mm vs 88mm maf should be 1.2055
73mm X 1.2055 = 88 so if 88mm maf reading is 250g/s wouldnt it mean 301.375? then the CFM would be much lower!
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12-08-2005, 08:01 PM
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Most likely the airbox outlet/air intake is turbulent.
You could also have a boost leak on the pressurized side... metering more air then is actually being used, so you may be flowing 1.7bar worth of air to get your 1.6.
It's pretty accurate at the airflow speeds we're seeing.