Here is a bit of stuff posted by Hammer Performance (aswracing) on the XLForum

There are a couple of pics referred to, but I couldn't get them.

" I think we can all agree that, while the exhaust valve is closed, it's pretty irrelevant what the exhaust system is doing.
What I'd like to know is when, during the period of time that it's open, it would be a good thing to have the exhaust system pushing back?
The exhaust valve opens roughly two thirds of the way down on the power stroke. It opens early like this because there's still some pressure in the cylinder, and this pressure will help get the spent exhaust gases moving. The rod angle is such that the pressure isn't really helping turn the crank anymore anyway.
The valve remains open as the the piston goes through bottom dead center. All you're trying to do here is empty the cylinder. You don't want anything pushing back.
By the time the piston starts coming up, the pressure has been relieved, and now the piston starts shoving the remaining spent exhaust out. Again, you don't want anything pushing back. You're pushing the gases out.
As the piston nears top dead center, the exhaust valve starts closing. But you really don't want it closed until all the exhaust is out.
Then the intake valve starts opening. You REALLY don't want the exhaust pushing back now. It'll push back right into the intake tract, shoving the charge back out the carb.
Now you've got both valves open, and the piston passes through top dead center. The piston starts down and starts pulling. But both valves are still open! Which port will it pull from?
You've got a cold, dense charge sitting still in the intake tract at near atmospheric pressure. You've got hot, high pressure sitting in the exhaust. Why doesn't it pull from the exhaust? Especially if the exhaust has "back pressure"?
Now the exhaust valve closes and the exhaust system becomes irrelevant again.
I didn't see anywhere in that whole exhaust cycle where it would've been helpful to have the exhaust system pushing back."

" Here's some more food for thought:
http://www.nrhsperformance.com/pictures/3lengths.gif
All three of these pulls were done on the same bike, with NO engine changes in between them. Same carbs, cams, etc.
The ONLY thing I changed was the exhaust system.
All 3 exhaust systems I made myself. All of them are OPEN drag pipes. No baffling whatsoever. No torque cones or anything like that.
The major difference between them is the length of the pipes. That's it.
The green is the shortest and the blue is the longest. If I had room to make the pipe even longer, I could've moved the torque peak even lower in rpm.
It's not the "back pressure", it's the tuned length of the pipe.
When the exhaust valve opens, it causes a pressure wave to come out of the cylinder and travel down the pipe. Remember, the exhaust valve opens while there's still pressure in the cylinder.
When the pressure wave gets to the end of the pipe, it reflects back as a negative pressure wave. It travels back to the cylinder and reflects back as a positive wave. These waves are travelling at the speed of sound.
The point is, the length of the pipe drives the timing of the waves.
As explained above, it's EXTREMELY critical that the exhaust not push back during overlap, when both valves are open. In fact, what you really want there is a negative pressure in the exhaust. Since both valves are open, a negative pressure on the exhaust actually pulls on the intake and gets the charge moving before the piston even starts pulling on it. It also keeps the piston from pulling on the exhaust as it starts going down.
The key point is that the waves travel at the speed of sound regardless of the rpm of the engine. So you may have the length of the pipe perfect, such that the negative wave arrives back at the chamber during overlap, but when you change the engine's rpm, the timing ain't so perfect anymore, because the wave's timing stayed the same.
At lower rpm, you need to make the wave travel take longer, so you need a longer pipe. Ata a higher rpm, you need a shorter pipe.
To make a short pipe work at low rpm, you have to diffuse the wave, make it broader so it covers a wider rpm range. Baffles and mufflers do this. They have the unfortunate side effect of causing pumping losses. If you could do it with the length of the pipe instead, and keep the pumping losses low, you'd make even more bottom end power. But it's not really practical to have 10 foot pipes on a street bike."

" http://www.nrhsperformance.com/pictures/3peaks.gif
Notice the 3 torque peaks. Those torque peaks correspond with the placement of steps in the pipe..
Each step "looks" like a small outlet to the exhaust gases as they move down the pipe, causing a negative pressure wave to travel back toward the engine. With careful placement of the steps, you can have the effect of widening the powerband.
Likewise, with careful placement of the steps, you can stack the small pressure waves from each step on top of each other, and make a larger, more concentrated wave. This stacking is possible because the wave actually travels up and down the pipe multiple times before arriving at the chamber during overlap.
And of course, you time that stacked up wave to stack on top of the wave from the outlet, and make a giant negative wave. And of course, design everything to place that wave at the rpm of interest. For the race bikes, that means a high rpm.
Exhaust augmented intake flow is a key key component to making power."

This pic relates to your article as well 408.

Digest that quote.

The length issue is apparent when folk buy a two brothers 2 into 1 AND a cam which has figures suitable for getting good 1/4 mile times, then come in the door sooking about poor low end and throttle response. The pipes are nicely made and sound good ( if you like it that loud) , on a STOCK engined  M8 or TC they get higher dyno figures than some others, but people persist in reading the numbers on the dyno graph , instead of reading the graph.  If you want to go drag racing, or really do ride your Harley above 4500rpm all the time, then buy stuff that is marketed in that way. 

Agree with you totally on the the shorter pipes, although they have their place. 

 The pressure waves are a different thing to the actual gas flow, they are zipping around in the pipe regardless but harnessed right can help the last of the cylinder scavenging and help get the intake charge started, that was my understanding of it anyway, interestingly even as these waves travel at the speed of sound in the exhaust that can be 500MPS due to the hi heat conditions in the pipe, I found formulas for calculating it all but man they are out of my depth, more useful for NASCAR or similar running at a constant RPM, the stepped headers still help us though so I'm getting some...... eventually 😁

Thanks for checking in and keeping it real Lushy, appreciate your time👍

Also very much appreciate your time and input here Lushy. On a car forum that I used to participate in (it's dead now unfortunately) there were a few very switched on professionals who used to very generously give their time and info to us lesser mortals.