The dispacement is 121.7 cubic inches. So the answer in terms CFM used at, say 3,000RPM) would be:

3,000 (revolutions) x (121.7 cubic inches displacement) / 1,728 cubic inches@cubic foot) / 2 (cyles/revolution) = 105 CFM

If the engine comparment were about 2.5' wide, 6' deep, and 3'.5' tall, then it's "gross" volume would be about 53 Cubic Feet. (of course the engine takes, say 1/4 of that out).

So roughly each minute you're going to be changing the air about about 2.5 times or so with the engine running at (for us) very high RPM's.

The air comes from some indeterminant sources (whatever is easiest) from a combination of i) the supply vents directly from the outside, ii) holes or pathways to the bilge, iii) lockers, iv) main cabin, etc.

You can see that if you don't shut the engine down very quickly, the concentration of fire suppression gas goes down very quickly.