CFM and Liters for a supercharger? *Edit*

[Radar]

**Question edited**
So I'm trying to choose my supercharger(twin screw) but I don't fully understand how you determine how much CFM it will flow at a certain boost?

I.E. 2L supercharger will obviously flow more CFM per PSI than a .87L supercharger, when they are geared to boost the same 8 PSI

So basically, instead of running 21psi to reach 450+ cfm using a smaller s/c,
you could only 10psi to reach 450+ cfm with a much lager s/c.
So yea how do you determine what the blower flows in cfm?

[RA35GT]

Hi,

The CFM is proportional to the boost pressure and the engine size and rpm. (Boost pressure is fixed and is proportional to pulley ratio)

What you need to do is calculate the CFM required for your engine size and max rpm and required boost pressure.

Let A = Cubic Inch Displacement
B = RPM
D = VE (Naturally Aspirated VE %)
C = Boost in PSI
CFM = ((A/3456)*B*[(14.7+C)/14.7])*0.01*D

I.e. with a 103CID Engine revving to 7000rpm at 8psi requires 273CFM of air. (Estimated 85% VE)

Once you know the maximum air flow required for your engine, you can then choose a supercharger that is capable of delivering this airflow at the required pressure.

435cfm @ 8psi per rev doesn't make sense. Superchargers are usually rated in terms of volume per rev and not a volumetric flowrate per rev.

I have attached a spreadsheet i knocked up, so you just have to punch in some numbers.

(P.S. The spreadsheet is not exact but close enough).

Rudi

[Radar]

Still way confused... *Smoke too much*
.0298x7000x1.54x0.01x85 273cfm 8psi
.0298x7000x2.42x0.01x85 429cfm 21psi
Why cant I run a larger supercharger, so I don't need as much PSI to flow the same CFM? I think that's the way I'm trying to word this.
How do you work out what size supercharger to use *
I've 100%lost my self...

[Norbie]

The manifold pressure generated by the supercharger is a function of engine CFM and supercharger CFM. So for example if the supercharger delivers twice as much CFM as the engine you'll end up with about 14-15psi boost (ie twice atmospheric pressure). This is a gross oversimplification, but you get the idea.

So once you know how big your engine is and how much boost you want to run, it's pretty easy to figure out how much CFM the supercharger needs to deliver and then you can look at flow maps to determine what size blower you need.

[Radar]

I understand!!!!
Thanks heaps lads, I'd totally confused my self!
Rudi, cheers heaps for the spread sheet, it finally helped me get it!
Much appreciated.

Basically
1.7L Engine 85%VE - - 1.3L S/C 90%VE 1.12 pulley pumping 15psi
I would produce an actual 12 PSI of boost on my motor
Sorry about all that.