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Old 11-19-2012, 10:56 PM   #1
rtodnem
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Default AFR math problem

Hey guys,
I have been struggling with an AFR calculation. I want to know how many CFM it takes to match up with 650 cc injectors.

The RC Fuel Injection web site calculator for 350chp shows 66 lbs /hr or 1.1 lb/minute which is slightly over the 80% IDC for the 650cc injectors.

Based on a 11.5 AFR it would seem like 1.1 fuel x 11.5 AFR = 12.65 lbs air.

My info shows 1 lb air = to approx 13 cubic feet. So 12.65 x 13 = 164.45 CFM.

That CFM seems way too low based on the FAQ turbo list. What am I missing ??? Thanks in advance for your help.

Richard
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Old 11-19-2012, 11:03 PM   #2
wrxlaunch
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350C with what kinda fuel, and turbo
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Old 11-19-2012, 11:10 PM   #3
ppayer38
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To simplify this 650's will get you over 300whp and be able to keep up with any turbo that flows under 47lbs/min. I'm on the safe side as most don't like taking injectors past 85% even know in at 94%idc haha
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Old 11-19-2012, 11:20 PM   #4
eg33GC
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Quote:
Originally Posted by ppayer38 View Post
To simplify this 650's will get you over 300whp and be able to keep up with any turbo that flows under 47lbs/min. I'm on the safe side as most don't like taking injectors past 85% even know in at 94%idc haha
pretty much this, you should tell us your turbo and if you have any other VE mods like ewg, op.
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Old 11-20-2012, 12:33 AM   #5
rtodnem
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Thanks for the input. The reason I am trying to calculate the air required is to size a turbo to match the injectors. Right now all I have is a stock '05 WRX. Going by the turbo list they are all flowing way more CFM than the number I came up with. Just trying to work out the numbers before buying parts. As far as fuel, the RC site doesn't give any options other than the Brake Specific Fuel consumption rate
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Old 11-20-2012, 06:47 AM   #6
Viral E
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Correct me if I'm wrong, but isn't the CFM you calculated for only one injector, and therefore only one cylinder? Shouldn't the CFM based on the equations you used actually total 164.45 x 4 = 657.8?

Just checked: 650 CFM is almost 47 lb/min at STP. ppayer38 definitely had the right answer from the get-go
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Old 11-20-2012, 10:55 AM   #7
rtodnem
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Bingo, Viral E, that's the answer I was looking for. Actually you were all correct. I just couldn't figure why the math was wrong. Thanks for your help.

Richard
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