Lonnie:

Not 90 degrees, no. A five cylinder engine has the cylinders 72 degrees apart, and the firing intervals are 144 degrees. The five 144 degree cycles total 720 degrees of crank rotation, and there is your two crank turns for a complete engine cycle.

The seven has the cylinders at 51.43- degrees, and the nine has 40 degree cylinder spacing. In all cases the firing interval is twice the cylinder spacing.

There is a craftsman's web page on building your own radial engine, and another on the development of the P&W R-2800 engine, but click [link=http://travel.howstuffworks.com/radial-engine.htm]here[/link] for a simple explanation. Want more, come back and I'll direct you to another site.

Bill.