I've spent a lot of weekends going over pipe combinations that work on a dynanometer from chaps like Vizard, Kevin Barlett, Peter Brock, racer D i c k Johnston, and info from the later Ford exhast systems. Another invaluable source is the Lukey technical 'advertorial' in Australian Street Machine. Although it's a straight advertisment trying to beat up sales, the very best article was a Commodore 3.3 owner who wanted the badest (Aussie for worst) exhast note around. The recomemded set-up was exactly the same as Jack used in his Mustang! No wonder Jack hates it!
Without fail, these guys discuss the balance between
back pressure,
size,
and engine note.
Its like a good, quick, cheap building job.
It never exsits in real life, you can have any of two elements, but not all three. What gives you low back pressure has the biggest size, and the worst engine note. What gives you the best note, may be too small, and have too much back pressure. That's why people use balance pipes. They allow systems which are not the ideal work better. A system on a 351C, M,W or 5.0 after 1981 is totally different in operation to a 260-289-302 or early Chev small block. Fireing orders differed. An odd fire Buick needs a different spec to an even fire GM 3800. A six in line doesn't even use the X-pipe the way a V8 does. Mark P discussed how the X-pipe on a V8 operates like a Y -pipe at low speed. An I-6 can't operate on a preferential system, becasue there are totally even firing pulses, while the V8 fireing pulses have a reverse scavange at certain engine speeds.
My advice is not to bust your head over X or H tubes. It's only ever important when you want to ellicit a beautiful engine note while cruising around town. It is more important when you have a very low restriction exhast (deap and boomy, the pipe moderates the noise) or a very resticted set-up (it ensures the reverse pressure waves from the mufflers don't travel back up the exhast).
If you go too big with a single exhast, sixes vibrate, resonate and drone between the off idle and at cruise (the 800 to 2500 rpm area ).
You need to find combinations which work, and avoid those that don't.
Dollar for dollar, a stock size V8 exhast with a good dual out header system is going to make you smile. An X pipe will help remove a drone. I'd sugest on a six, it will allow you to live with a dual set-up which is too big .
I spend most of my time listing to engine notes, and see the hottest sixes on my 4 mile hike to and from work each day. As I decend down 400 feet of my hill to work, I get to see cars slowing from 70 mph to 30 mph in one lane, and speeding from 30 mph to 80 mph or more in the other. I keep a note of the sweetest and worst sounds, and if I see the car around, I do a quick check of the set-up. Generally, the worst ones are the later model XR6's and any Commodore with a Holden, Nissan, or Buick six which has a big single exhast fitted. The best are Lexus IS's with the six, early BMW 323 with the dual exhast, later 328's, M3's, early manual XR6's, and the old sivler 351 Cleveland XF Falcon with dual exhasts which come out under the axle. I see it every day. It's got a balance tube, and sounds like a big Norton Commando or Ducati 851 when given a razzle up. Without a balance tube, all Clevlenads sounds very lazy. The bigger the dual pipe size, the worse it sounds. Add the x-tube or H tube, and ithey sound magic. DeTomaso's with 5.8's or 351's of any denomiation just stuck to smaller quad pipes, and no balance tube. The Italians know ehxasts better than anyone.
The issue is that any in-line six with a big diameter single sounds bad off idle to about 2500 rpm. If you use an X or H pipe, you can run a small diameter dual exhast and gain some accoustics that are sadly lacking with a big single.
In NZ and Australia, the six cylinder Commodore V6's with GM 3800's, and the XR6's with 4.0 sixes, sound dreadfull when an aftermarket 'big gun' single exhast is added, and you can't do any low level profilin' with a big gun exhast. You are noticed blocks away.
The X-pipe is like wsaIII's. That fits with no trans crossmember mods. It's elaborate, but pipe is cheap, and crossmember mods are not always simple.
The H pipe is fitted using a dual out header which then goes to a stock Mustang V8 or Falcon Sprint dual set-up. It has little influence on the pipe accoustics.
The last option is a variant on wsaIII's excellent set-up. Normally, the trans crossmember gets close to hitting any dual out header pipe. To gain space for the X pipe, you just use the common aftermarket Fox V8 transmission adaptor. This basically allows a dual out header to run to a side by side dual exhast like any other set-up. The create the x-pipe, it just needs to touch at the bends with a oval window whic allows the gasses to move without creating a rumble.
If you check OffenokieComets pictures of his 2V 200 Mercury Comet exhast, you'll realise that wsaIII's set up can be done by merly cutting a window into the basic side by side pipe set-up he used. It doesn't have to be elaborate.
Lesson One is from the Aussies. A six cylinder exhast with a dual out header improves low end torque and reduces resonance. Holden had to do this first in 1966 X2 Holden sixes with there cast dual out header. Chrylser Valiant 265 I6's with the Triple Weber Six Pack had a very load exhast, but using the smaller 318 and 340 dual exhast pipes past the header made it tollerably quiet enouigh for the street. In 1985, there last Holden L6 202 engines had a divided dual header to build higher gas velocity before joining the pipes together. It used a the 304 single exhast which was restrictive on the 164 hp V8's, but fine on the 142 hp L6. I might add that none of these cars used cross-over pipes, but the noise with big singles was dreadfull.
Lesson Two was Jack Collins and other F-series guys from the Big Six forum. The f150's with big singles and the Aussie X-flow in what is now Brian's ( mutangsix2 ) car were a disapointment at lower cruise rpms. Big single pipes with big mufflers are about as sexy as a fart in the bathroom. Go over the posts, and see there benchmarks.
Lesson Three. The first guys to run dual exhasts sixes here have found it removes the low speed rumble, and makes it more V-8 like.
Lesson Four was wsaIII's set-up. It breaks the curse of the low end six cylinder engine note when you have bigger pipes. His set-up is quite a large set-up. If you run duals without an X-tube, you'll be fine untill you go too big.
Too big is when the total area of the total set-up past the collector (called the tail pipe) is above the a certain level. It' s based on square inches of tail pipe verses cubic inches or engine. As you go up to higher horsepower levles, you run into a higher ration of exhast size to cubic inches. Horsepower governs the choice ideal pipe size formula. This is why Italian cars run multiple pipe set-ups. Its becasue they've understood the principal of ideal gas speed. There are not any accustic levels by formula.
The thing to realise is that the exhast is just a large saxaphone. You find out the bad notes, and stay away from them if they don't impress anyone. If you like big vibrating singles, then it'll rock the town.
Following the ricer (big singles with large bore mufflers), or the V-8 crowd ('huge' duals with cross-overs and over sized exhasts or 3" singles) isn't going to help much to the six cylinder guy.
