Hey, R1jake, thanks for your interest in this.
In a 4-stroke engine, the crank rotates twice (720 degrees) to go through a complete "suck-squish-bang-blow" cycle, with the piston being at top dead centre (TDC) twice, once at the top of the compression stroke and once at the top of the exhaust stroke.
So, a single-cylinder engine would fire every 720 degrees of crankshaft rotation.
A two-cylinder engine would fire every 360 degrees.
A 4-banger would fire every 180 degrees (so a flat [horizontally-opposed, or "boxer" configuration is ideal).
A 6-cylinder engine fires every 120 degrees.
An 8-cylinder engine fires every 90 degrees.
One crazy exception was the 90-degree V6 engines that GM developed in the '70s. They used a V8 block, but cut the block (and crankshaft) down to six cylinders). Unmodified from there, the firing order between cylinders would not have been an even 120 degrees, but a mix of cylinders firing 90 and 180 degrees apart. (GM deleted the #3 and #6 cylinder throws from the crankshaft - the 3rd pair from the front.) But to compensate, they offset the crank pins by 30 degrees to achieve even firing spacing.
Check out GM's 4.3 liter/262 CID V6, for example - it has the same bore (4.00") and stroke (3.48") as the legendary 5.7 litre/350 CID small-block V8. The V6 is literally 3/4 of the V8 engine.
Likewise, the 3.8 is literally 3/4 of a 305, with the same stroke as a 350 but a smaller bore. I think, but am not positive, that Dodge did the same thing in cutting down a 5.2 (318) V8 to create the 3.9 V6 used in Dakota trucks back in the day.
I just found this article, which explains it much better than I can. I wish I'd read it years and years ago.
https://www.caranddriver.com/features/a15126436/the-physics-of-engine-cylinder-bank-angles-feature/
Related, a few years I wondered why the most common firing order for V8 engines was 1-8-4-3-6-5-7-2. (Before you Ford folks complain, Ford's firing order is the same, once you translate Ford's cylinder-numbering protocol into the same as one as GM and Chrysler use. I like the GM and Chrysler method because they observe the old rule of folding the two Vees up to make an inline engine, and then numbering the cylinders 1 to 8 from front to back.)
Anyway, it bugged me that the firing order was uneven - that is, that it didn't nicely alternate side-to-side.
Drawing it out helped me understand why this was, and why there are only a couple of possible firing orders with a cross-plane V8.
