Alright, what is up with all these exhaust myths. I've heard everything from "There's no room." to "It'll blow your engine!" What?!? Hopefully we can get to the bottom of these and break through to the real issues or we can just disprove them. (Kinda like that show "Mythbusters" on the Discovery Channel, haha.)

I have a '97 Ram with the great old 318 (5.2L) under the hood. I have a custom built true-dual exhaust system. It fits, it sounds great (I think it is one of the best sounding systems out there, and I know it's one of the loudest... =P), and it works wonderfully.

I ran into no problems what so ever with the fabrication of it. I didn't have access to a pipe bender at the time, so I used a peice of flex-pipe to go over the rear axle which I have had to replace recently due to rust and wear (flex-pipes aren't very durable...) but I now have a bender and so a solid pipe will soon be fitted. Here's what I did:

I cut the Y right before the cat and removed about 4-6"s of pipe that ran under the transmission the rewelded the pipe together so that there were two seperate pipe outlets, not just one. (Yes, this system requires dual cats if you wanna be legal...) To save money, I cut a circular cap and welded that to the second entrance to the existing cat so I didn't need to get a replacement. The two pipes were then run parallel to each other with about 3" in between. I stayed with the stock pipe size, 2 1/2" I believe... The old muffler was trash and needed to be replaced anyway, that is why I went ahead and modified the whole system. I replaced it with two 24" Thrush Glasspacks that were staggered to leave plenty of access and clearance room. (One lets out right at the back of the cab, the other begins there.) The pipes are both run on the passenger's side of the truck, as the fuel tank inhibits placement on the driver's side. The stock tail pipe was cut off, but the hump over the axle was left as is. The flex pipe was used for the driver's side pipe and runs over the top of the axle and behind the spare tire where it it again a solid pipe that runs to the rear of the truck. I have a 10,000lb Draw-Tite reciever that has holes pre-drilled in it for exhaust hangers, so I used them to hang the two 18" long, 3" diameter chrome resonator tips that protrude about 3/8" past the lower lip of the rear bumper. And there you have it; true-duals...

Now, about these myths:

"There isn't room" - Yes there is, you just have to stay on the passenger side until you get to the rear.

"Those pickups were not made to run on two O2 sensors." - Maybe not, I don't know on this one, but I didn't even add a second one. Most of today's emmission regulators and monitors do more harm than good, so I just left it with one 02 sensor that stayed in the pipe (which is now used by the passenger side bank) where the factory put it. It has only given my trouble once when a rusty bolt that I had over tighted broke free and allowed the echaust to come out of the manifold before it got to the 02 sensor and it gave me a "fuel-lean mixture" fault code, nothing happened to the engine though.

"It won't pass emmissions tests." - Yes, it will. No more emmissions are being pumped from the engine if you add a second pipe, infact it should be lower as the efficiency of the engine is improved by the increased exhaust flow. I know my truck runs better now. With the system I have, since I used the same size pipe, but lowered the total airflow from that of 8 cylinders per pipe, to 4, the exhaust efficiency was improved 100%, not considering the improved flow of the glasspacks versus that of the factory muffler. I estimate a total improvement of around 115% (and you can tell it when you punch it, hehe).

"It will blow you engine!" - It will what?!? Well mine hasn't so far, but that's not to say that eventually it won't although I don't see how. Anyone got any ideas where this could have come from? What evidence is there? I'm puzzled on this one, but as I said, my engine runs great.

I can't remember exactly, but I beleive everything needed for this build can be bought for about $150 at your local auto parts store. The glasspacks are about $40 each and the tips are about $20, plus the pipe, fasteners and hangers which aren't very expensive. I am very happy with it, and everyone I know is impressed with it. I hope to make a kit to sell on EBay that is just bolt-on, no mods with an added "boost" in the system, but that's still in it's infancy. If I get anywhere on it, I will let everyone on here know first. Good luck!

 

How about this myth: There's no such thing as too much pipe.
Exhaust systems are designed with certain things in mind, such as exhaust gas scavenging. Too small of a pipe and your high-rpm power is hurt because the gasses cannot be pushed through the pipe efficiently. Too large of a pipe and the gasses slow down and the spent gasses don't get purged completely from the cylinder, making less room for clean fuel/air mixture, meaning there's less fuel to ignite and less power made. A large pipe has greater surface area, which radiates more heat, which in turn cools the gasses off, which makes them more dense and slower moving, not too mention the added volume of the pipe which also slows down the flow. This actually causes a loss of low end grunt.
I'm not saying that your setup is too much, because if it's dual 2.25 or 2.5 then it's probably about right, but I'm going to suggest that people will lose low-end if they try upgrading to 3" from the manifolds/headers back.

The one thing I did not follow was about welding a second inlet onto the existing cat. I'm confused. Could you post a picture?

 

I somewhat agree with what you said, but I doubt that that much a difference would be made running over a 3" pipe. The average temp inside of a standard exhaust pipe is over 600*F, even if it were lowered some, I don't think it would make that much of a difference. However, I am working on a device that will create an almost 100% efficient exhaust system. If it can get my hands on the right tools, I'll be making them shortly. They will be the extra "boost" in the kit I mentioned...

I didn't weld a second inlet, I welded a cap onto the second inlet so that the system is closed. The orignal setup had two pipes flowing into the cat and one out. Instead of having to buy a single inlet / single outlet cat, I just capped the unused inlet, creating a single inlet / single outlet. Hope that clears it up.

I'll try to get a pic or at least a diagram up soon.

 

Well in the land of Subaru 2.5 liters, I can tell you for a fact that there is a definate loss of power when anything over 2.5" is used, and since half a 318 is 2.6 liters in size, it's a fair comparison.

And for the cat, are you running a cat on one side but not the other?

 

You cant go too big in diameter cause you will lose some backpressure thus loosing Torque.. Torque is what gives you that "grunt"off the line!
This is why most people go with "X' or "Y" pipe set up. To keep some backpressure this gives you HP and torgue.. Not just HP.
Unless your running 2K HP then who cares..

 

It's not backpressure that you want. It's velocity. A lot of people get the two confused. You need a reasonable diameter pipe to keep the gasses hot and moving, because when they move fast they create a suction of sorts that helps to scavenge the spent gasses from the cylinders at low RPMs. This is especially important with a lumpier cam where there's more overlap on the intake and exhaust valves.

 

Here ya go.. Went diggin in my LS1TECH site.. I had em confused.. Hope this helps.

You have probably heard words like: back pressure, scavenging, tuned length, merged collector, rotational firing order, compatible combination and many others that meant something, but how they relate to a header may be a little vague. This article should give you a basic understanding of how a header works, what the terminology means, and how it plays a part in the header's performance gains.

The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance. Just the opposite is true. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.

The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s. If too small a diameter is used the engine will pull hard at low R.P.M.s but at some point in the higher R.P.M.s the tube will not be able to flow as much air as the engine is pumping out, and the engine will "sign off" early, not reaching its potential peak R.P.M. This situation would require going one size larger in tube diameter.

The second consideration is the proper tube length. The length directly controls the power band in the R.P.M. range. Longer tube lengths pull the torque down to a lower R.P.M. range. Shorter tubes move the power band up into a higher R.P.M. range. Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26" long. Engines that are torquers and Red Line at 5,500 R.P.M.s would need a tube length of 36". This is what is meant by the term "Tuned Length". The tube length is tuned to make the engine operate at a desired R.P.M. range.

The third consideration is the collector outlet diameter and extension length. This is where major differences occur between four cylinder engines and V-8 engines. The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. (Example: 1 3/4" primary tubes need a 2" collector outlet diameter.) The rule of thumb here is two tube sizes. This keeps the velocity fast to increase scavenging, especially at lower R.P.M.s. Going to a larger outlet diameter will hurt the midrange and low R.P.M. torque.

The amount of straight in the collector extension can move the engines torque up or down in the R.P.M. range. Longer extension length will pull the torque down into the midrange.

Engines that "Red Line" at 10,000 R.P.M. would only need 2" of straight between the collector and the megaphone. This is just enough length to straighten out the air flow before it enters the megaphone. This creates an orifice action that enhances exhaust velocity.

In the case of V-8 firing order, the five pulses fire alternately back and forth from left to right collector, giving the ideal 180 degree firing cycle. Then it fires two in succession into the left collector, then two in succession into the right collector. If the proper collector outlet diameter is being used (two sizes larger than primaries) the two pulses in succession load up the collector with more air than it can flow. This results in a very strong midrange torque, but causes the engine to "sign off" early, not reaching its potential peek R.P.M. The improper firing order on a V-8 engine results in the need to use large diameter collectors so the engine will perform well at high R.P.M.s. Unfortunately the large diameter collectors cause a tremendous drop in air velocity, resulting in less scavenging through the entire R.P.M. range.

Often cams are used with extended valve timing to help the exhaust cycling. This results in valve timing overlap (Intake and Exhaust valves both open at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this happens, exhaust actually backs up into the cylinder causing intake air to be pushed back out the intake. This reversion causes "Standoff" (fuel blowing out of the Intake) at low R.P.M.s. This whole improper cycling has resulted in a number of "Cure Alls" to help stop this reversion and standoff.

The plentum intake was created to stop the fuel "Standoff". Then came "Anti Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the header, extended collector lengths and even plentums in the exhaust tubes.

In this chain of events beginning with improper firing order, a series of cures has developed, each one causing a new problem.

The optimum cure to this whole problem is to correct the exhaust firing cycle. The two cylinders that fire in succession into each collector have to be separated. This can be done partially by a "Tri-Y" header, where the four primary tubes from each bank merge into two secondary tubes (separating the two pulses firing in succession) and finally collect into a single collector. This type of header helps, but the two pulses are still coming back together at the collector.

The second optimum cure is to cross the two center tubes from each bank, across the engine running them into the collector on the opposite side. This makes the firing cycle in each collector 180 degrees apart, the same as a four cylinder engine. Once this firing order is achieved, the small collector outlet diameter can be used and the "High Velocity Scavenging" at low R.P.M.s cures the reversion problems and eliminates the need for extreme cam duration.

This sounds so easy, you are probably asking why wasn't this done from the start?

If you have ever seen a set of 180 degree headers you would understand.

On today's cars, with space virtually nonexistent, crossing four tubes either under the oil pan or around the front or rear of the engine presents major problems. On racing applications where it is possible, there is still the problem of keeping the tube length down to a reasonable 32" long. If that's not enough challenge, then try to arrange the tubes into each collector so they fire in a "Rotational Firing" pattern. Then you have, what has been called "A Bundle of Snakes".

Arranging the tubes to fire rotationally adds to the scavenging capabilities. The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more suction on that tube than it would on a tube on the opposite side of the collector.

The next problem is "Turbulence" in the collector. When four round tubes are grouped together in a square pattern, so a collector can be attached, you notice a gapping hole in the center of the four tubes. The standard method in manufacturing headers is to cap this hole off with a square plate. This plate in the center of the four tubes creates dead air space, or turbulence, disrupting the high velocity in the collector. This problem is solved by using a "Merge Collector". This collector is formed from four tubes, cut at approximately an 8 degree angle on two sides. When the tubes are all fitted together they form a collector with a "Pyramid" in the center. This has eliminated the need for the square plate and has taken up some of the volume inside the collector, speeding up the air velocity.

Other methods of curing this problem are: fabricating a pyramid out of sheet metal and welding it over the hole between the tubes, or squaring the tubes on two sides so they fit together forming a "+" weld in the center eliminating the hole all together.

You can see that there are a great many factors that go into making a good header. When the header, intake system, and cam timing are all designed to operate to their maximum in the same R.P.M. range, then you have a "Compatible Combination". This combination can be tuned to deliver maximum power at any desired R.P.M. range.

These are some of the "Basics" you need to know about building a good high performance header. There are many other adjustments that can be made to fine tune a header, but this should give you a basic understanding of how all the components work together.

 

Ok, for the theoretical aspects of this-

#1- 2-1/4" to 2-1/2" makes a big change, going to 3" makes a HUGE change. When you double the diameter of a pipe, you quadrouple it's volume. So 1/4" and 1/2" changes make a huge difference in flow and temperature characteristics.

#2- The larger the pipe the more outside surface area you have, the more area you have for heat to dissipate. The exhaust gasses near the surfact of the pipe will cool more rapidly than the gasses near the center, which will pull the center gasses to the outside and cause the gasses to "tumble", creating turbulance inside the pipe, instead of flowing laminarly like they should. Remember, the same 4:1 ratio applies ot surface area as well as volume when increasing size.

#3- If I read this right... you capped off one of the inputs on your cat, then ran a straight pipe on the other bank of the engine... If you did this, you've got MUCH different inharent pressures on the different banks of your engine. This is very much bad... It basically unbalances the engine, cause one side will work easier than the other, and the two halves of the motor will be fighting eath other... Even if you have a single in/out cat on the new side, it'll be closer but far from good.

#4- You pieced your system together from bits and pieces? And used FLEX PIPE? The stock crossmember is so horribly restrictive it's insane. And flex pipe? I'm not sure what to say... Your credibility is shot.

My head hurts reading this thread...

 

Yeah, flex pipe, shame on me, I know... As I said, I used it 'cuase I didn't have access to a pipebender... I do now, so the flex is coming out... The crossmember is the same size as the other pipe, so I left it. It's 2 1/2" like everything else. I don't have headers - yet anyway - and I don't have a crossflow or H-pipe system, but that's in the work too. The main reason was to show people that this could be done, not that my system was the perfect way to do it, although it works great for me.

The cat on mine is, umm, "cough, cough" very unrestictive, so I'm not worried about one bank working too much harder than the other... What little more resistance is present is made up for in the slightly longer length of the other side, which apparently slows down the air - I don't see how though...

To me, if the air is being pushed into a closed system (assuming there are no leaks) then it has to either leave the pipe, or compress itself... I think that unless the pipe is jammed, it will leave the pipe as it is the easiest of the two tasks. I don't think it has anything to do with heat. I can prove that the velocity at a given RPM will remain constant, but won't for now.

 

Well, you're not completely right on that. Temperature does make a big difference, mostly in turbulance, but also in density of the fluid (in this case exhaust gasses) as stated above. If you have one side longer than the other, you're going to have more drag on that side, cause it's got more volume for turbulance and drag. And velocity may be constant, but you want to be looking at how much work it takes to create that velocity. And there are MANY variables to consider.

If you want to research this for your own information, I'm basically stating all this based on the Conservation of Energy laws. Energy can neither be created or destroyed, it just changes forms. You have energy being input into the system from the creation of the heat, and the force pushing the exhaust out. That energy has to go somewhere. Conversly, if you have a system that dissipates a lot of energy, it takes a large amount of input energy to maintain an ideal state. The two work together.

By the way, I'm an engineer. I've got a bit of a background in this stuff, from both the engineering and vehicle hobby aspects.