Any time an aftermarket exhaust is bolted on and the intake remains stock, no jetting modifications are needed. No extra air in.... no extra gas is needed.
I'll have to disagree with that. In fact, so does Honda.
Here is an example:
Cb550K 74-76 w/ 4 into 4 exhaust. Slide needle in 4th position from top and #100 Main jet.
CB550F 75-77 w/ 4 into 1 exhaust. The muffler is quieter muffler than the 4 into 4 and it runs at higher pressures than the 4 into 4.
Slide needle in clipped in second position from top and the Main jet is #98.
Both machines have the same engine, cam, and induction system.
Why?
Higher pressure exhaust reduces the scavenging of the cylinder slightly. With some of the previous burn's hydrocarbons remaining in the chamber, more oxygen and less fuel are needed for the next firing cycle. This makes the engine more efficient in the mid to low RPM ranges normally used on the street. Almost certainly a trade off for some peak power available at or near red line and max power demands.
Another Honda support, is the 77-78 K models. They have a 4 into 4 pipes, but the outlet size of each is reduced diameter from those of prior models. Again, no changes to the induction system ahead of the carbs or engine, cam, valves, etc. Only the exhaust changed (and it's operating pressures), and the carbs changed to PD type. #90 mains, and an idle circuit to fine tune the idle mixture for optimum burn. This was no doubt an attempt to make the engine a lean burn engine that the EPA was looking for. The point is, they raised the exhaust pressure, toward the goal of having it use less fuel for the oxygen delivered to the chamber.
The upshot is that if you change only the exhaust pipe pressure reflected back to the engine, that can indeed change the fuel delivery requirements.
How is the Motowerks pipe pressure different than what Honda jetted to accommodate? I don't know. And they may not either, if they only focused on how the pipe looks from the outside. That engineering detail is left up to the buyer of the exhaust.
