Topic: Question about 4-2 exhaust to a 4-1 exhaust 1973 CB750K

[jorwesflow]

i searched a lot of queries, but didn't find the info i was looking for. so here's my question. i'm helping my buddy who just picked up a 73 CB750K—his first bike. the PO had some sort of aftermarket 4-2 exhaust on there, but my friend wants to go to a 4-1 exhaust.

Can someone explain the physics of back pressure when going to a 4-1 exhaust, and how that affects the mixture and further the main jet size?

I own a 1976 CB550F (4-1) and the stock mains are 98's, but in the 1976 CB550K (4-4) the stock mains are 100's or 105's... (i can't remember) so i am led to believe that when you go from a 4-2 or 4-4 exhaust to a 4-1 exhaust, you should decrease your jet size as well. is this correct? or is it the opposite? and again, what are the physics behind this answer?

thanks!

[Coyote13]

There are a lot more differences between the F and K models than just the exhaust and jetting.  However, the difference going from 4-2 to 4-1 should not be a huge one, unless you're going from very baffled to very unbaffled, or vice versa.  If that is the case, you'll need to jet up or down to compensate for the loss or gain in backpressure, respectively.

[Retro Rocket]

If you fit a performance 4 into 1 like the Yoshi replica's getting around now then you will need to go up in jet sizes, more flow = more fuel.
Lesser performance pipes like the Mac are less likely to need rejetting.

[jorwesflow]

If you fit a performance 4 into 1 like the Yoshi replica's getting around now then you will need to go up in jet sizes, more flow = more fuel.
Lesser performance pipes like the Mac are less likely to need rejetting.

I guess I'm confused as to how reducing the number of exhaust pipes creates more airflow. I am trying to understand the physics of how that's possible. I would think that a 4-1 exhaust would let less air out (as compared to a 4-4) and thus less air would be drawn in. but apparently this isn't so. can someone explain?

[Retro Rocket]

If you fit a performance 4 into 1 like the Yoshi replica's getting around now then you will need to go up in jet sizes, more flow = more fuel.
Lesser performance pipes like the Mac are less likely to need rejetting.

I guess I'm confused as to how reducing the number of exhaust pipes creates more airflow. I am trying to understand the physics of how that's possible. I would think that a 4-1 exhaust would let less air out (as compared to a 4-4) and thus less air would be drawn in. but apparently this isn't so. can someone explain?

The baffles are far more restrictive in the stock pipes to meet noise and pollution standards, most 4 into 1 pipes are far freer flowing therefore requiring bigger jets to run properly and producing a slight rise in HP....   Most 4 cylinder race bikes use 4into1 pipes for that very reason..  The early Honda race bikes used an unbaffled and less restrictive set of 4 into 4 pipes, as technology improved they went to using 4 into 1's, thats where Pops Yoshimura came into the scene..... If i am correct, he was the first one to run 4 into 1 's on the early Honda's and did so on the Suzuki's later on in his career.....At the time he was the best tuner on the planet....