Listen to the video. It does a pretty good explanation of the difference between X and H pipes. But, like he mentions, back pressure is key with engines, especially with truck engines. A freer flowing exhaust is great for 5,000 RPM power, but it can kill our 2,000 RPM power. Where do you drive your truck at? 5,000 RPM or 2,000 RPM? You may find that if you are worried about mileage, the "super" free flowing systems will drive your mileage down because your motor is making less power in the lower RPMs, so, you are having to put your foot into the gas to make the same power. In a lot of cases, you are wanting more power to get up to speed faster, so, you are putting your foot into it even more, compounding the issue. This is especially noticeable during in town driving.
Now, am I saying that Ford has created the perfect exhaust system and there is no need to improve it to make it more efficient. By no means. The factory system is truely designed around minimizing noise as the primary consideration with trying to get as much efficiency out of the motor as a secondary thing.
So, with this being said (and as is talked about in the video), placement of components and what you use is a function of what your vehicle needs. The exhaust system for a truck like what I have is completely different than what someone may put in to a Lightning for strip use. Question is, how do you use your truck? The components that you pick are going to have some effect on the exhaust note you get out, but for the most part, the last 3 feet or so of your exhaust is going to depict whether you get the low rumble or some other sound. The muffler is going to also play some part in this as the noise you hear is a function of how much the pulses are minimized and turned into a continual flow of air. In theory, if you can take the pulses of air coming out of the motor and remove the pulses to make it all one constant stream of air (all of it moving at the same speed), you would have a very free flowing exhaust that makes no noise. The problem is, to get rid of the pulses, you have to provide some restriction to the air to force the pulse into the valley between the pulses to smooth things out.
To get a bit more into the X pipe way of working. Pick up a soda bottle and blow across the top of the bottle. I bet you heard it make a noise. What is happening is as you blow across the top of the bottle, your breathe is pulling some of the air out of the bottle (scavaging) and then once the vacuum inside of the bottle reaches a certain point, it will suck in a bunch of air to get back to the pressure it was at before you started blowing. It is doing this many, many times a second. Kinda like if you do this with a 20 ounce bottle and a 2 liter, you will notice that the 2 liter has a lower sound to it. It is equalizing the pressure fewer times a second. The X-pipe is doing this by taking the exhaust from one side of the motor and "blowing" across the header for the other side of the motor, drawing a vacuum on that side. Very simple idea.
I am sure you are asking why am I bringing pop bottles into this? Simple, the diameter of your exhaust pipes, the length of tubing between the motor and where the X-pipe is affects how big your "bottle" is. If you have large diameter piping and a lot of distance between the X-pipe and the motor, it is going to act like a big bottle and provide great scavaging at low frequencies (or, in the case of the motor, low RPM). But, because it has a specific point it wants to work at, when you get up in high RPM, it is not going to be as effective. The big trick is figuring out how to size the volume of pipe (both diameter and length) such that the X-pipe provides the maximum amount of scavaging at where you want the most power. The idea is, you want the pipe to be at the biggest vacuum on the side that is going to be opening the exhaust valve at that split second before the exhaust valve opens so you have the biggest differential pressure between the cylinder and the exhaust system. This will cause the cylinder to clear out the fastest possible, allowing you to put more fresh air/fuel in for the next time.
Here is where people get into trouble. It is great that you are making an exhaust system that cleans out the cylinders great and can move the exhaust quickly down the exhaust pipe. But, what ends up happening is the fuel does not burn instantaneously. It takes a finite amount of time for all the fuel to burn. In a lot of cases, the cylinder is still in a full flame condition as the exhaust valve is opening. So, you have fuel being burned inside the exhaust header and pipe. If you get this flame down to the O2 sensors, it is going to cause them to see the motor as not running efficiently, throwing codes.
Is there some magical exhaust setup. By no means. People that are way smarter than me get paid big bucks to try and maximize power and not cause the computer to have fits too. There is no simple "if I do this, will I cause the computer to have problems" answer. Some times you have to do something and see what happens.
For those that are looking at Nascar exhaust systems, you will note that the X-pipe is mounted quite a ways from the motor with very large diameter piping. This has to do with harmonics. I don't want to get into this as you will find it will make your head hurt (imagine high and low pressures inside the same pipe with vacuum pulses travelling towards the motor and pressure pulses leaving the motor, their interactions, etc, etc, etc). There is some science there and it does work. But, it can also do as much good as it can create bad too.
