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Old 01-22-2010, 09:54 AM   #1 (permalink)
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Homemade X pipe

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.

Balance
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.
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Last edited by dazecars; 01-25-2010 at 07:00 PM.
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Old 01-22-2010, 08:45 PM   #2 (permalink)
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Re: Homemade X pipe

Great writeup - only suggestion is that I'd spend a few bucks more on mandrel bent. The crimps will disrupt the flow a bit.
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Old 01-23-2010, 06:35 AM   #3 (permalink)
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Re: Homemade X pipe

Nice write up and pointing out the tricks for saving time.
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Old 01-23-2010, 09:35 AM   #4 (permalink)
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Re: Homemade X pipe

Nice write-up, but the crush type bend and wrinkled tubing are going to do a LOT more harm than good. If you don't want it to lose power, you'll need to re-do it with mandrel bent tubing.

Read this...
WarpSpeed Performance Mandrel

ALL muffler shops use pressure bending that crushes the pipe. It leaves bends that look like this.



Once you lose all that cross section to the crush bends AND cut part of it out where you weld the pipes together.... a TREMENDOUS restriction is created.

X-pipes don't do a whole lot except change the sound, when they are done right with mandrel bends. Do one wrong, and you may as well stick a potato on one of the tailpipes.

Nice job of writing it up, though.
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Old 01-23-2010, 10:29 AM   #5 (permalink)
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Re: Homemade X pipe

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Originally Posted by n2omike View Post
X-pipes don't do a whole lot except change the sound, when they are done right with mandrel bends. Do one wrong, and you may as well stick a potato on one of the tailpipes.
I always like criticism, because it helps me to improve, but I don't know how you figure that. Dino tests have been done comparing dual exhaust with out a crossover to H pipes and X pipes, and the over all improvement with an H pipe is about 10 % and with an X pipe is about 15% All the facts and figures aside, the proof is in application. Every performance race car such as those found in nascar is running an X pipe, and I know for a fact that if it was not an improvement they wouldn't be running it.

As far as the crimped elbows, for my application the ribs are a good thing IMHO. They create just enough back pressure to still maintain some of the low-end gains seen with an H pipe. yes it will not perform as well as a smooth bend X pipe at full RPM but thats OK by me, my car is not a race car. One thing I didn't put in the article, that I should have mentioned is that the X pipe is made from 2.5" elbows and the rest of my exhaust is 2.25". Because of that there is almost no loss of volume at the cross-section of the bend where the two pieces are welded together.

I concede that mandrel bent bends would have been better for total HP but in this particular case I think the difference is negligible, and surely nothing like having a potato in my exhaust

Once again I appreciate your comments.
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Last edited by dazecars; 01-23-2010 at 10:39 AM.
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Old 01-23-2010, 11:01 AM   #6 (permalink)
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Re: Homemade X pipe

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Originally Posted by dazecars View Post
As far as the crimped elbows, for my application the ribs are a good thing IMHO. They create just enough back pressure to still maintain some of the low-end gains seen with an H pipe. yes it will not perform as well as a smooth bend X pipe at full RPM but thats OK by me, my car is not a race car. One thing I didn't put in the article, that I should have mentioned is that the X pipe is made from 2.5" elbows and the rest of my exhaust is 2.25". Because of that there is almost no loss of volume at the cross-section of the bend where the two pieces are welded together.
First I think you did a nice job and your tips for a nice fit are clever as always.

The only comment I have isn't really about the x pipe. Its the backpressure comment.

Backpressure is 100% always bad. Always The low end torque thing is often confused with primary header pipe (and to a much lesser extent overall exhaust) sizing. It is common to think that a smaller exhaust is helping torque because of backpressure, but it couldnt be farther from the truth.

When the exhaust valve opens, backpressure is always harmful. It backs up cylinder evacuation and limits the cylinder fill in two ways.

One: it keeps the exhaust from scavenging and pulling MORE air and fuel in, and

Two: it can leave residual gas in the chamber which dilutes the air fuel mix with something that doesn't burn.

However, SMALLER primary header pipes make the exhaust charge move faster, which makes cylinder fill and evacuation more efficient at a lower rpm.

However, once you get to the point where there is any backpressure, or by design the airflow can't keep its speed up, you lose power, whether its idle or 5000 rpm.

So, the deal here is that I dont think your x-pipe would cause any backpressure or limit to flow at low rpm. The area of your X pipe probably is plenty, sure looks it to me.

Also, even though the crimp bend reduces area, a wrinkle in the short side of the turn may actually let the exhaust make a turn faster. At least up to the point where cross section of the tightest point is too low for the volume of exhaust passing through it. Think of it like a bulldozer dragging one track, the fast side turns toward the slow side.

So I don't disagree that your x-pipe is a good thing EXACTLY as it is set up. I just want to try to dispel the myth of backpressure helping torque.

There is no case in a 4 stroke engine that having a resistance to exhaust flow helps combustion, it's just a common misconception based on primary pipe sizing (primarily) and it's effect on cylinder evacuation and fill.

Hope this helps
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Last edited by My427stang; 01-23-2010 at 11:06 AM.
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Old 01-23-2010, 11:10 AM   #7 (permalink)
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Re: Homemade X pipe

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Originally Posted by dazecars View Post
Dino tests have been done comparing dual exhaust with out a crossover to H pipes and X pipes, and the over all improvement with an H pipe is about 10 % and with an X pipe is about 15%
Those are drastically inflated numbers.
Using those numbers, adding an H-hipe to a 500hp engine would increase its output to 550hp.
Adding an X-Pipe would crank it up to 575hp.
That does NOT happen.

An H or X pipe done RIGHT may add a little -something- somewhere in the powerband, mostly in low compression engines, but it's not much, and that's about it. Most any gains seen from these devices are at the lower engine speeds, and can be mostly picked up with simple collector extensions.

Any gains from H or X pipes are small. People add them because they read about others doing it, and because it sounds different. The claims are grossly inflated. Done RIGHT, there may be a -little- something there. Done WRONG, they are no more than a restriction. Wrinkles in pipe create a serious disruption to flow. Reduced diameter with the crushed bends does as well. As for the wrinkles... Remember 'glass pack' mufflers that you could 'see through'? Those flow very badly because of their louvers that extend into the flow path. Those wrinkles do pretty much the same thing.

How do you plan to smoothly adapt the 2.5" X-pipe to the 2.25" exhaust system? If that part isn't done right, it will further reduce flow.

This looks like a fun modification for somebody to duplicate, but don't expect a lot more power, and expect a decrease if it isn't done right with mandrel bends.

X-pipes make clutch/transmission service pretty much impossible unless you make the tailpipes removable. Anybody planning one needs to keep that in mind.

I'm not trying to be an ass, I just don't want mis-information spreading, that can cause people to go the wrong direction with in their 'upgrades'.

Nice write-up... Just be careful with the claims, and use mandrel bent pipe if you want any chance of a performance improvement anywhere in the powerband.
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Old 01-23-2010, 12:34 PM   #8 (permalink)
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Re: Homemade X pipe

My427stang, thanks for the information, I found it to be very informative and constructive. I do have one question when int comes to "back pressure" (this is not an argument just looking for info to correct what I always thought to be true) I have heard of people running straight pipes and loosing low end torque. The answer was always the lack of back pressure. If back pressure is always a bad thing, do you know why straight pipes would have less low end torque over a mufflered exhaust???
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Old 01-23-2010, 02:13 PM   #9 (permalink)
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Re: Homemade X pipe

In that case it is probably a function of collector length.

Often even an open headered car will gain on the bottom and mid when you add an 18 inch header extension. So by dropping the headers, you lose the benefit of the volume of that exhaust pulling on the primaries.

Basically, the collector pulls on the primary pipes which pull on the chamber to make a better cylinder fill. Shorter "collector" when you drop the exhaust, less pull on the primaries, so less pull on the cylinder.

Ultimately I bet if you added an extension to the headers on your example, but no mufflers, the torque would come back.

Now there is another issue buried in there, less likely applying to your question, but, at some point, an extension can be too long and based on exhaust harmonics, can actually hurt. (I am no expert on this by the way)

So if you were comparing a full straight pipe exhaust all the way to the bumper against the same exhaust with an efficient muffler in the proper place, you may find an increase in power if the muffler acted as a "termination box".

In this case, it is essentially fooling the upstream exhaust that it has exited the pipe and dampening negative harmonics that slowed flow.

However, I can't talk too intelligently on where and how that happens and I assume it depends on a lot on rpm/volume/pipe size/flow/resonant harmonic variables, etc.

However, those same harmonics are what you are attempting to dampen with the X-pipe, and that's why such a little device helps

BTW Mike is right on the amount of gain, based on those harmonics, there will be peak areas that have more of a gain than others. Usually low rpm throttle response type of gains, but they aren't big numbers, more part throttle drivability and sound in my experience.

However, they are enough that for a street car I run an X or H on everything I own.
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Old 01-24-2010, 09:50 AM   #10 (permalink)
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Re: Homemade X pipe

Quote:
Originally Posted by dazecars View Post
I do have one question when int comes to "back pressure" (this is not an argument just looking for info to correct what I always thought to be true) I have heard of people running straight pipes and loosing low end torque.
I've raced several 4-stroke motorcycles over the years, they did back up with straight pipes until the air/fuel mixture was corrected, after which there were noticable gains in the 1/4. V-twin motorcycles always need more jet with straight pipes. Lastly I ran a set of "stepped" open headers/straight pipes which worked very well, but required even more jet.

With that being said I don't buy into the small vs big tail pipe myth.


In non-race applications I always install a H or an X pipe just to make to make the exhaust sound better. I have a set of Flowmasters (never again) on my tow rig, I could not stand them until I added the H pipe.
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Old 01-24-2010, 10:51 AM   #11 (permalink)
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Re: Homemade X pipe

This shows some some ingenuity. I would like too say that the size of the hole in the "crossover" and the distance downstream from the exhaust port is the secret to "gains". Many engine tuners spend a lot of time fine tuning this to gain maybe 15-20 ft-lbs of torque off the turn at a track. It is not a "one size fits all" kind of thing. Experiment and you might just find a little something.
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Old 01-24-2010, 10:34 PM   #12 (permalink)
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Re: Homemade X pipe

Great article. You took the time to detail the steps of doing a good job at building your own instead of buying ready-made. Any of the complainers can freely buy mandrel bent pipe and write their own article.....your hit the important points and that's what counts.

As for X v. H, on a street car I don't buy the hype. Run either one and your gains are about the same. Running a cross-over pipe of some type is all that matters until you hit the track. At that point, you need a dyno to gain the 'claimed' benefits of a X-pipe. I use an X-pipe now only because it did quell the noise a little more than an H-pipe on my side exit, bullet muffled exhaust....at least my 42yr old ears are happy with it.
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Old 01-25-2010, 04:52 AM   #13 (permalink)
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Re: Homemade X pipe

Cool idea, thanks for the time and meticulous article layout. great contribution to FM and to everyone interested in building their own X pipes.
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Old 01-25-2010, 05:40 AM   #14 (permalink)
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Re: Homemade X pipe

Quote:
Originally Posted by dazecars View Post
My427stang, thanks for the information, I found it to be very informative and constructive. I do have one question when int comes to "back pressure" (this is not an argument just looking for info to correct what I always thought to be true) I have heard of people running straight pipes and loosing low end torque. The answer was always the lack of back pressure. If back pressure is always a bad thing, do you know why straight pipes would have less low end torque over a mufflered exhaust???
It's because they did not re-tune their carb.

If you have a restrictive exhaust system, you'll end up leaning out the carb, to get the engine to run properly. Take away the restriction, and you'll REALLY lean out the engine (possibly losing power) until you correct the a/f ratio.

If you have a motor that REALLY needs a certain amount of backpressure to run properly, then you probably have a really poorly designed camshaft.

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Old 01-25-2010, 01:37 PM   #15 (permalink)
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Re: Homemade X pipe

I have made homemade x-pipes before. Actually, the best way to make the center openings is to use an upright belt sander with an aggressive belt. You just run each pipe on the sander until the belt grinds away enough that you start making a hole. Then it's just a matter of how big you want your holes to be by keep sanding the pipes. Then compare the two pipes to see that both openings are equal. It makes a perfectly flush surface to weld the two pieces together. Before welding it up, I usually deburr the openings. I would say with a belt sander and the time it takes to weld it up, it took me about a 1/2 hour.
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