BREAKING: new 4 in 4 exhaust

[Roper_imp]

I'd be interested in a 4 into 2 like on the early 90s 750 Nighthawks. Hmm, a search may be in order...

I saw a huge jump in torque and an extension of the power curve when I replaced the 2 into 2 stock exhaust on an '81 CB750F with a 4 into 1. As I understand it, there is a big advantage using a 4 into 1 over even straight pipes if it is tuned correctly. The exhaust pulses traveling down the header create a vacuum which scavenges more exhaust out of other cylinders.

I have heard this before, but I have a hard time visualizing it.

As exhaust travels through the headers, it remains in discrete pipes until the collector. So any vacuum created in the header behind the exhaust pulses wouldn't affect the other cylinders, whether it's 4-1 or 2-2.

In an I-4 engine, the outer two cylinders move together and the inner two cylinders move together.

With a 2-2 exhaust, when one cylinder is on its exhaust stroke, the other is on its power stroke. I suppose there could be a vacuum created in the collector to help "scavenge" exhaust from the other cylinder in the millisecond the valves are open. The exact same thing is happening on the other side.

With a 4-1 exhaust, the same thing is happening, but in pairs. Sure, there is double the exhaust volume which may create a larger vacuum, but it's also "scavenging" exhaust from double the cylinder volume at the same time. The ratios don't change.

So, I'm either misunderstanding something, or there's something else going on that I haven't taken into consideration.