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ID.4, 997.1
15,439 Posts
Discussion Starter · #1 ·
I have some data logged on my own car, '03 MINI Cooper S (15% smaller supercharger drive pulley, Alta intake, Magnaflow cat-back) as well as my friend's car, '02 MINI Cooper S (17% smaller supercharger drive pulley, stock intake, Supertrapp cat-back). I have been trying to figure out the airflow of these engines, especially the effect of putting smaller and smaller pullies on them (I have a 19% pulley ready to go on). These engines use speed-density rather than MAF, so I don't have any direct way of measuring airflow.
I'm able to log time, rpm, pressure upstream of the supercharger, manifold pressure, and manifold temperature. Here are the results from 2nd gear pulls to redline in each vehicle:

So, using the formulas in Corky Bell's "Supercharged", I used:
airflow = cid x rpm x 0.5 Ev / 1728
The engine is a 1598 cc (98 cu. in.) 4-cylinder with 4 valves per cylinder operated by a chain-driven single camshaft via rocker arms. As a guesstimate for volumetric efficiency, I used 90%. Another assumption was the temperature of the air inside the air filter. Based on testing from another MINI owner with an Alta intake, I added 8 C to the ambient measurement of 2 C. Here is the spreadsheet I came up with:
Here is a plot of airflow:

I'd appreciate any comments/suggestions/etc. Is this a valid way to estimate airflow? How far off is my volumetric efficiency guess? Thanks

813 Posts
Re: Airflow Calculation and Volumetric Efficiency ([email protected])

Well, I'm no expert on the topic but I can count...

I think your spot on here, I doublechecked your numbers and came up pretty close (like within 3-5 cfm)
the basic formula of airflow = cid x rpm x 0.5 Ev / 1728
Multiplied by the pressure ratio at that specific RPM should give you an accurate measure of airflow. I didn't even factor in air temps.
I didn't realize SC's build that much boost! I mean that from 2000 to redline the boost increases that much, the only SC system I've ever actually worked on was on a '78 Nova and it held 10psi pretty steady across the range...

Eh! i'm just babbling...
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