Seems to me like four tube header designs - where they are looking for max power, tuned rpm bands, etc. - have some theoretical and practical formulas that "work". The motorcycle, F-1 and NASCAR engine designers and tuners have been tweaking this stuff for years. Usually these vehicles don't have mufflers or "tailpipes" as such. 

Guys that wanted torque band improvements in street cars have tended toward small tube "Tri-Y" designs and I've seen some dyno proofs that show that approach works too. But after the exhaust comes out of the header - low back pressure rules - how much noise can you tolerate for how low a pressure.

The most interesting example I can remember was a big block Cuda with some funny little caps over the "boom tubes" sticking out the back. The caps had holes about 1-1/8 ...1-1/4 diameter, clipped on with a single bolt - and made the nasty big block much more tolerable for the drive-in circuit (and soon disappeared when the money came out). There is a street exhaust system sold for my daily driver Camaro (by SLP) that actually gets tuned for sound by inserting washers in the tailpipe joint - its just a matter of what you are willing to trade ... noise or backpressure.

The exhaust crossovers and "X" pipes are both devices that have been invented to aid the mufflers and quell noise while keeping down the backpressure. The "X" pipe seems to work well on my big block Olds - but I don't have any frame of reference - didn't try anything else on the way. There is a young man that hangs out at a shop I frequent that put one (an "X") on his late model Mustang - and hates it because of a particular reasonant frequency on the freeway. My hearing isn't that sensitive and it didn't sound that bad to me.

 
 Steve Metzger       Tucson, Arizona

Re; header size and exhaust pipe size; when I read down on that forum the same question was asked and the answer was that the same formula applies to headers and exhaust pipes.



The mismatched muffler was something that I first became aware of after I bought my 86 Mustang GT. Ford put different sized mufflers on the dual exhaust system to avoid the droning. Apparently mufflers resonate, and if you have matched mufflers, they'll resonate with each other, hence the drone. I have one muffler slightly smaller than the other. No drone at all.



Regarding 'backpressure' and performance; I don't think that many exhaust systems present much backpressure. It's not at all like the intake where you have atmospheric pressure, about 14 psi, to push air into the engine. I think that the residual pressure in the cylinders pushing the exhaust gas out is rather high, hundreds of psi. I think that's why exhaust ports are smaller than intake ports; it doesn't require much to get the exhaust gasses out because they're under so much pressure. Rather, I think the intent is to 'tune' the exhaust system so that the inertia of the exhaust blow-down helps evacuate the cylinder. If your pipes are too big, the timing of the exhaust pulse will be off and you'll end up with residual exhaust gas. It doesn't hurt the engine. In fact it works as EGR when you put pipes on the engine that are too big. But if you really want more power, you have to find the right size. Too big is less performance.

Best regards,



Paul Menten

Meridian, Idaho

X pipes work. With 2½ inch pipes all the way to the rear of the red car and an X pipe in front of the mufflers, the car runs faster than it does with the caps off ahead of the crossover. A cross over or an X pipe will help ANY V motor.  It gives each pulse twice the area of muffler to absorb the pulse, Also will take the edge off the noise.  Smaller pipes are quieter, and will not help torque.  They have very little effect on a motor unless they're too small. you guys can figure areas until your chrome tips turn blue, but every time I try larger than the so-called formulas, and I've read a bunch of them, the et goes down and the mph gets higher, so does the noise.  The only place you can really tell a difference is in the primary header size or in the collector size, length.

     46, aim the pipes back away from the cab, helps a ton. pc, it's more the length of the primaries that affects the "Timing "of the negative wave, and you lose that when the rpm changes. size affects gas velocity, has very little to do with the wave timing.

Frank/Rebop

Bristol, In ( by Elkhart) 

Seems to me that the limitation to making the tailpipes bigger  - (aside from getting them to fit in the space allowed) is when the car starts slowing down from all of pipe on board.

Using a 90° turn-down at the outlet helps - and I once saw a couple of big washers welded onto the side of the muffler cans to stiffen the tin and ease some of the "ting ting ting" stuff..................

 
 Steve Metzger       Tucson, Arizona

I've heard the theory all most 50 years that engines "need" back pressure.  My simpleton way of looking at things says the dragsters have much more cross sectional area per cubic inch.  If they could get more troque or HP by restricting the exhaust flow yu would see one inch or smaller stacks.

In my opinion, bigger is better as long as it fits.  That's why we ran dumps in the 60's.  Honest officer I don't know why those things are behind my front wheels, the car was like that when I bought it.

TC

You gotta have the right tools and know how to use 'em.

TC - Austin, Texas

Regardless of the firing order on an American V8, there will inherently be two cylinders on each bank that fires consecutively with each other.  As a result there is a high pressure wave created in each tailpipe that can be reduced by installing a crossover or X-pipe that’s the same size as the exhaust system.  When installing one of these in the exhaust, it’s typically recommended that it's done as close as possible to the engine for the most advantage.  The further downstream it’s placed, the less effective it becomes.

 

Some of the older 180° header designs actually took two tubes from each bank and criss-crossed them under the engine so that a true 180° firing took place within the collectors to eliminate the same intermittent high pressure pulse within the collector.  Headers built on this particular design can actually use a collector size that’s much smaller as a result.  Most vehicle chassis layouts do not accommodate this so you don’t hear much about it today but this particular style of header would not see as much advantage from the use of an X-pipe by lieu of not having to balance out a non-existent intermittent higher pressure wave.

 

Just all food for thought and an excellent thread.

Lorena, Texas (South of Waco)

Concerning the benefit of dual exhausts with X-pipes:

I remember when Harley introduced the interconnected EVO exhaust, the factory warned that installing straight pipes with no X connection would cancel out most of the expected power gain. I assumed back then that the connection was really for noise control, and they were just trying to discourage exhaust modification and avoid the bad PR associated with loud pipes.

But now, since the power benefit of the X is proven, I wonder if any data is available re: the effect on back-pressure of the Y-block's intake manifold heat riser passage? I DO know that blocking the gasket holes will change the exhaust tone, but now I wonder if there is any power lost just by eliminating the interconnection (disregarding the obvious gain from lowering the intake/charge temp)?

6 VOLTS/POS. GRD. NW INDIANA

Daniel/ Ted,

          You got it, Harley did too. Ever notice the XR's ran a 2 into one into 2 setup?

     The flattrack bikes after about 1970, (Aluminum XR's) made more torque that way, came of the corners harder. Guys tried to duplicate the older drag pipe style and managed to slow the bikes down. Went back to the factory setup immediately. The only form of racing that does not use some type of collector style exhaust to promote cylinder ehaust extraction by the engines firing order is the fuel cars. With 6000 hp and close to that in torque, I guess they feel they don't need a lot more low end! 

 The X pipe or crossover steup has it's greatest benefit on the bottom end thru midrange, with mufflers. It reduces the peak pressure of the pulses by splitting them between two mufflers. Works. 

Frank/Rebop

Bristol, In ( by Elkhart) 

I really appreciate all the input .But all I wanted to know was should i change my exhaust system from stock to a larger one when i install my headers. 55birdman

55Birdman  Hickory NC

55,

            See, you heard more than you really wanted to know. This is the problem of dealing with a bunch of old "know it alls" with too much time on their hands. Author included. Sorry, im sure you'll be totally happy with the 2" and glass packs, should sound neater tha H...!

Frank/Rebop

Bristol, In ( by Elkhart)