I just stumbled over this one. I'm a newbie here. I have worked on quite a few types of engines. Cars and bikes both 2 and 4 stroke. Expansion chambers to 4 into 1. First I'd say the Mopar figures for pipe sizes can be very inaccurate, the camshaft and headwork can very easily tilt those numbers into uselessness. They're totally useless on bikes. My view they are generally too small. One of my specialties was American Motors V-8, not many people know those but those that do know that they were extremely underrated engines. I have run several different sizes of them from street type engines all the way up to 9 second 1/4 milers at 140 mph. Also have some VERY BIG big block Chevy experience with normally aspirated engines from 500 to over 700 inches that ran over 200 mph. You can start with exhaust formulas to get started but everything after that is trial and error. The formula (there are like 50 of them) will NEVER be dead on. For instance on the AMC cars we found that the heads breathed so well that we were using carbs totally in the realm of impossibility if you went by like the Holley carb suggestion. 800 cfm carbs on 304 inch motors along with 1 7/8 headers, needed them bigger. That on engines with only 8.5/1 compression and hydraulic cams, motors would spin to 7500 rpm all day long. Same with exhaust, we used much bigger primary OD than normal and engines ran like hell, yet had plenty of mid range too. Cars are easier to do that than bikes are as cars have narrower rpm ranges than bikes. When was the last time you saw a car on the street running at 10,000 rpm? Car torque peak almost always locked in between 3 and 4 thousand rpm, there's just not a lot of movement there. Bikes though, can move peak around much more than that. My view is that straights are worthless, they have too narrow a powerband because reverse pulse is set up based on the open end of the pipe. There's not enough working reflection area. I have seen people take off 4 into 1 and add straights to kill power because they were no good at setting car up to bite. 4 into 4? A waste of time, it's a stock exhaust and will stay that way. 4 into 1 much better because each cylinder helps out the others to make a pipe that can work well in mid range AND on the other end. 4 into 2 or 4-2-1 not that useful to me, they help bottom and mid, but you will lose some top with them. Any 4 to 2 combo needs to match the pipes so that the event spacings are even at 180 degrees on 4 cylinder, that means the 2 middle cylinders pair and the 2 outside pair, you don't see many pipes like that. Pairing 1-2 and 3-4 is worthless, the events are lopsided and lose max effect. I notice no one here has mentioned a megaphone type 4 into 1, that system makes the most power. The meg spreads out the sonic wave to make the maximum use of it. Makes for a broad sonic wave that covers more than just a few hundred rpm. The hottest ones can even have a reverse cone on the end to prevent over extraction when using a really aggressive angle in the diverging cone. Diverging cone generally 7 to 11 degrees, you know it's right by the noise quotient. When right those sound waves get to ear shattering level just like a 2 stroke chamber. I happen to have an old beat up CB550F with the original header on it, I cobbled up a meg to fit with a 1 3/4 core, it picked up some mid range when I did it. With a meg, you can also pick a certain type baffle core (say like Kerker type with many small holes), yank the glass out and have a system that is fairly quiet when around the cops but sounds great when hotdogging it. That type core will even let the megaphone work similar to open exhaust, it will pick up a very noticeable amount of torque even with the baffle in since the sound wave can still reflect down the cone like uncorked. As far as needing some back pressure, it's an old wives tale to me. It probably helps on single pipes, but on collected pipes like header I have seen virtually every engine I ever worked on make more power the less pressure you had in system.
Hope I didn't talk your ear off.
