stupid question that I don't know answer
 

Are the 911 aircooled engins odd or even fireing engins?

Even.

thanks

Better yet, what is the definition of an even or odd firing engine?

My understanding is if ignition events are evenly spaced over crank rotation it is an "even fire" engine. The H6 Porsche is ignition every 120 degrees for 6 events over 2 crank rotations. Odd fire would mean different angles between ignition events.

Like a Harley Davidson V Twin, I would assume.

opposing cylinders fire: 1L, 6R, 2L, 4R, 3L, 5R (L=left bank R= right bank)

All flat 6's are even-firing (As far as I know). They are also inherently balanced. (like flat 12's, 60 degree V-12, 60 degree V-6, 90 degree V-8, straight 6, etc.)

The 90 degree GM V6's derived from a V8 were odd firing (Well there's the Harley Vee twin). They were 'rough' and were not used that long.

Even firing means that the time (degree) interval between firing is the same for all cylinders.

An uneven engine fired with longer intervals with every other fire.
(F short F long F short F long F short F... etc.)