When it comes to the exhaust on a two cylinder head, type engine, such as a V8 or V6 there are three basic options: single exhaust with a Y to connect the exhaust bank from each head, dual exhaust with no cross over, and dual exhaust with some sort of crossover between the two different exhaust banks. The main reason for these different types of exhaust is cost and ease of installation with the first two options being the cheapest and easiest to install, however the third option is the best option. Dual exhaust with a crossover allows the engine to run more efficiently which results in more power, fewer emissions and better MPG. With all the benefits, it makes sense to have some sort of crossover on your exhaust system.
The first question you may have is why is the engine made more efficient by adding a cross over between exhaust banks?? The answer is simple, “to balance the system”. In order to understand why there is a lack of balance we need to look at firing order. As an example, on the basic non HO Ford 302 the firing order is 1-5-4-2-6-3-7-8. The firing order reduces exhaust balance during the two times when cylinders under the same head detonate one after the other: 4 then 2 on the passenger’s side and 7 then 8 on the driver’s side.
The problem with back-to-back detonation is that when either the number 2 or the number 8 cylinders fire there is already pressure in the exhaust bank from the detonation that occurred right before it. In contrast the other exhaust bank is temporarily unpressurized from not having any exhaust gasses from two consecutive detonations.
This unbalanced pressure between exhaust banks can even heard on a dual exhaust with no crossover, as it will have an uneven rhythm to it. The extra pressure created by back-to-back detonations on the same side robs power from the engine, as more effort is required for the pistons in cylinders 2 and 8 to push the exhaust out of the cylinder. Also the increased pressure on 2 and will cause these cylinders to where slightly faster than the other cylinders resulting in uneven power production on higher mileage engines. A cross over eliminates these issues as it allows the pressure to be equalized or nearly equalized between the two exhaust banks regardless of firing order.
With the benefits of a crossover clear, the first thing you need to decide is, H pipe or X pipe? An X pipe will produce more over all torque and horsepower than an H pipe however the H pipe will perform better on the lower end were back pressure is more important. As with any performance upgrade going on your car, take a hard look at the complete system as well as how you drive; also take a close look at the car’s gear ratios created by the rear end and transmission. Gear ratios will determine what kind of RPMs the engine will be seeing during normal driving and this has direct bearing on weather over all torque and horsepower are more important than low-end torque and horsepower. With all this information in mind pick the crossover best suited for your application. For my application I wanted an X pipe due to a fairly high lift cam, with free breathing induction, a low rear end ratio and a well-balanced engine tat runs strong at higher RPMs
Building the X pipe
You can buy an X pipe form just about any performance parts house, but as usual I wanted to build my own. This is the third one of these I have made and it is a really easy project. If you have a couple of hours, and can cut grind and weld, then you can build an X pipe!!!
To build the X pipe you need to start with two off the shelf elbows. I purchased the pair from my local auto parts store for $10.00. Once you have the elbows the first task is to cut the crowns off of them. As you will soon see the first elbow is always the hardest to cut and shape. To aide in cutting you need to mark the pipe to get a rough idea of where to cut. Lay the elbow on the table on its side and freehanded a “straight line” on to the crown of the elbow. The reason I say “straight line” with quotes is there is nothing straight at all about the actual lines due to the shape of the crown on the elbow. However, if you do it right the line will appear straight when looking down on the elbow as it sits on the table.
Once you have one side marked flip the elbow over and make a similar line on the other side. The final result when you turn the pipe up on its tips so that you are looking directly at the crown of the elbow should be two lines that come together to form a football shape.
Don’t worry about the lines being perfect, but small freehand adjustments should be made to get a roughly symmetrical shape that is centered on the crown of the elbow. The lines can then be followed with a cutting wheel to remove the section. From there quite a bit of grinding will need to be done to enlarge the opening and shape it so that it is approximately flat, symmetrical, and centered. To check flatness place the elbow with the freshly cut opening on a flat surface.
This will give you an idea of where metal needs to be removed from, so that you can begin grinding. As you grind continue to check the opening on a flat surface until you have it fairly close.
Once the elbow opening is roughly ground flat put it in a vice and take a hand file to it.
This will allow you to fine tune the opening and get a nice shape. As you file, the opening should get more flat and true.
One trick to reduce the amount of filing is use the corner where the freshly filed surface meets the edge of the tube as a guide line for the openings shape and grind out to the corner with a die grinder.
This will reduce the material that you need to remove with the file. When you get the opening flattened out, check it again on a flat surface and if it is flat enough, make one more pass with the die-grinder to finish enlarging the opening and to remove any burs on the inside of the elbow.
With the opening fully enlarged you can then set it on the table next to the other elbow and actually fit the second elbow in to the opening of the first elbow.
By putting the two pieces together you can use a Sharpie to mark the second elbow so you know exactly where to cut.
Then place the cut and uncut elbow side by side, tips down, on the table and freehand a few small changes in to the marks on the uncut elbow.
The first time I made an X pipe I did not do it this way. I freehanded both openings and cut both openings with no reference to each other. Then when I went to fit the two pieces together I had a lot of grinding and shaping to do to get the parts to mate up the way I wanted them to. It took me three times longer and the fit was not nearly as good when finished.
With the elbow marked cut the section out, as before, with a cut off tool but to insure you have enough material to work with make the cuts just inside the lines. The reason for leaving this extra material is so that you will be able to true up the cut and make it a perfect fit using the same grinding and filing techniques as described when cutting the opening in to the first elbow. As I was making mine, the opening on the second elbow was close enough to where it needed to be that I did not have to use the bench grinder at all.
I simply put the elbow in the vise and filed it down with the hand file and die grinder. As I said before the second elbow is way easier to make than the first one and the second elbow only took me about 10 minutes to cut and shape, as opposed to the hour I spent on the first elbow’s opening. Setting the elbows side by side confirmed that the fit was good.
With the both halves fitting together nicely I fired up the welder and tacked both pieces together. From there it was as simple as running a couple of beads of weld to finish the pipe up.
The final result is a solid X pipe with a true flow through design.
As you can see the “football” shaped openings appear round when looking through the X pipe. It is clear why this is such an effective, free flowing, design.