whats the forumula to figure out air comp cfm..

I know a lot about math and not much about compressors, so this calculation is based entirely on the figures you have provided. If something about the nature of compressors changes the method of calculation, I wouldn't know about it.

The formula for the volume of a cylinder is the area of the cross section times the height. That figure times 4 (four cylinder engine) times 700 (rpm) should give the cubic inches per minute. A cubic foot is 1728 cu in (12x12x12) so divide the total cu in by 1728.

Formula for area of a circle is pi times the radius squared. pi is 3.1429 (I use 22/7 because I can remember it and it's close enough.) The radius, 1.25 squared (times itself) is 1.5625

1.5625 x 3.1429 x 3 x 4 x 700 = 41,251

41,251 divided by 1,728 = 23.87 cfm

I don't mean to insult you by adding information you may already know (like squared means times itself.) I occasionally tutor high school students in math, and I've found that it's best not to assume anything about what they know.

All the best, Stan

The throughput of any reciprocating compressor can be estimated by multiplying the displaced cfm by a volumetric efficiency (EFF). The displaced cfm is the bore area x the stroke x the rpm. For a single stage compressor, the bore area is the sum of the areas of all the pistons. For a multi-stage compressor, the bore area is the area of the first stage piston only. The volumetric efficiency depends on the design of the compressor, the compression ratio (depends on the operating pressure at the discharge), the piston clearance volume and frictional and other losses in the machine.

In your example:
The pistons are all the same size, so it is a single stage compressor.
Displ CFM = (2.5x2.5x0.7854x4x3x700)/1728 = 23.9 CFM
Delivered CFM = 23.9xEFF
The exact EFF cannot be determined without manufacturer's test data but is likely to be such that your example would provide a CFM rate of between 15 and 18 at a usefull discharge pressure of 100 psig.

Rod