How to figure cfm exhaust from small engine?

I’m trying to figure the cubic feet per minute of exhaust gas from a 420 cc generator at 3600 RPM. If my thought process is right, it should be 420CC x 1CF/28317CC x 3600RPM/2 ~ 27 CFM for a 4-stroke engine and about 54 CFM for a 2-stroke engine.

Now I realize that the internal volume of exhaust gas is still under a little pressure inside the cylinder. How much... I don’t know. So, I’ll figure it expands another 20% as it exits the exhaust pipe for around 32 CFM/4-stroke and 64 CFM/2-stroke.

The reason I ask is that I have placed my generator inside a small shed with a 1300 CFM fan drawing air through the shed from one side and bottom, and out the other side. I’m having trouble getting my clamp to hold my tailpipe extension on that extends through the wall to the outside. If my math is right, the fan should draw plenty fresh air into the shed for complete combustion, and ventilate all the exhaust with plenty fresh air to spare without the need for the exhaust extension to the outside.

Anybody care to weigh in on this subject? Am I missing anything?