Gday guys.
Ive been looking into doing something outside the box for a little while, and need to know a lot more about it before i go for it.
Im looking into running essentially 2 complete, separate exhaust systems. Ie: twin 2" setup with individual mufflers and cats for each.
The reason why i am looking into this is the sound. I dont want to hear people go on about it not being worthwhile cashwise. I dont care. My thinking was, it would a) allow me to do a lot more work to the motor without restriction and b) have a completely unique note.
The theory i am working on here when i am designing the exhaust is using the firing order along with the pipe size to create a certain sound.
I am looking at modifying my current extractors to have the twin pipes running 1,5,6 and 2,3,4 respectively in order to get some suckback as the firing order is 1,5,3,6,2,4.
What i would like people to fill me in on is:
1) What pipe diameter would be best to suit my application. I WILL be aiming around the 200+rwkw mark N/A
2) Will the setup (extractor design) based on firing order work?
3) Has anybody heard of anybody doing this sort of thing before?
4) Am i right in assuming there will be suckback in between firing due to my extractor design.

Im in need of some people who have a LOT more experience in this area than i do and all helpful suggestions are greatly appreciated.

Cheers!

Sam

My dad ran custom dual headers on a flat-head six in an old Plymouth "back in the day". He had a 4-into-1 and a 2-into-1 running into separate exhausts and said it sounded like a V8. Don't know the firing pulses for each branch. (It also had a spare windscreen washer full of oil directed into the carb, in case he needed to "lose" someone )

As I understand it, the main reason for a V8's "rumble" is the uneven pulses. A typical firing order will give exhaust pulses something like L-R-L-R-L-L-R-R on a normal 90-degree V8 with two-plane crank and 4-into-1 headers. If you cross over two pipes from each side (like a Ford GT40), you get true alternating left and right pulses, and it sounds completely different - more like a V12. Same if you have a flat-plane crank like TVR's own V8 (not the Rover-based engine) - though that is a 72-deg or 75-deg V, so it sounds different again.

Your suggested split of two branches will give short-med-long on one and short-long-med on the other (120-240-360deg and 120-360-240deg of crank rotation). It's anybody's guess how it would sound, but it would definitely be unique. It won't give you optimum performance, as the pressure/vacuum and gas motion at each exhaust port will vary all over the place - almost the opposite of a tuned exhaust. One thing's for sure: it won't be smooth. It might sound like a lumpy V8....or maybe like a flat-4 running on three cylinders Theoretical optimum for 3-into-1 headers on the Ford I6 is 1,2,3 and 4,5,6 - giving even 240-deg pulses down each side. Sounds like an interesting experiment, and could have pleasing results.

this type of thing was done in the oldschool cars..
i had a 6 cyl valiant that was set up this way, and it was not bad, but it was extremely raspy in the sound area.. which sounded great with the roar of the tripple carbies it had.. but im not sure how good it would sound on an e series??
i hope your not shooting for a v8 like sound from the duel exhaust? because it wont work... if anything it might sounds a bit like a v6 with a loud exhaust..

the eb xr6 had a half duel exhaust from the factory, split after the first muffler irc..

Cheers for the responses. That is exactly what i was looking for.
efxr6wagon, Are you saying that my choice of 1,5,6 and 2,3,4 wont give the optimum flow because of the position of the crank on said cylinders? But the question is: Can anyone possibly predict how the air will flow through the exhaust if they are run as i stated?
Troyman, Definately not looking for a v8 sound. I love the sound of my 6 at the moment. The lumpyness that you had in your valiant should be similar to what im looking at with running a 1521a. Do you have anymore information on how the valiants exhaust was setup? ie: layout, shared mufflers? x pipe?
I first thought that this sort of choice would make the car sound like a 3 cylinder, obviously, but i just dont know enough about flow mechanics to be able to help myself out.
Keep it coming please!

the valiant was a simple set up..
cyl 123 on one pipe and 456 on the other pipe. the extractors were split where the 2 would have become one.. being an older car there was no cats, just two straight pipes with 2 muflers, it was a true duel exhaust..

and the way the i6 fires it was a single pulse from each pipe..
eg l/r l/r l/r.. 1/5 3/6 2/4

Ahhhh, so it wasnt designed based on firing order. The sound was caused by the 'distance' between air pulses in each pipe. In regards to pipe diameter, do you see 2" being too large?

In theory, dual 2" should flow equivalent to a single 2.83" (2 x square root of 2, if you want the math). But duals have more surface area, more drag, so probably more equivalent to a single 2.5" in total flow. This size seems to be the consensus for mildly modded N/A 4-litres.

Exhaust theory is horrifically complex (to me anyway), and I quickly get out of my depth. It involves acoustic waves, resonance, harmonics, etc that are way beyond me. But the basic concept of a tuned exhaust as I understand it is to create a vacuum (or at least low pressure) at the back of the exhaust valve, so the combustion chamber is sucked empty as soon as the valve opens. Exhaust gas has mass (effectively, weight) and momentum, so a "lump" or pulse of high-pressure exhaust gas travels down the primary pipe when the exhaust valve opens. When the valve closes, the pulse keeps going under its own momentum, but nothing can enter the valve end of the tube, so the gas behind the high-pressure pulse is "stretched" and a vacuum or low-pressure area is created. As this pulse passes the first branch of a 3-into-1 header and continues into the exhaust pipe (or secondary), it creates a vacuum in the other two primary tubes. If either of these two exhaust valves open at this point, they open to a vacuum and the chamber is scavenged, so the engine doesn't consume horsepower pushing the exhaust gas out. This is normally what people mean when they talk about maintaining exhaust velocity.

But the exhaust gas can only be "stretched" so far - eventually, it loses momentum, stops and heads back toward the engine until the pressure is equalised. (Think of dropping a weight attached to the end of a spring). So, if the next exhaust valve opening is too late, it faces the high pressure wave coming back, and the engine has to work harder to force the exhaust gas out. On the other hand, if the next exhaust valve opens too early, the pulse has not entered the secondary tube, and there is no vacuum behind the valve. So, having even pulses is ideal - done right, it can create maximum exhaust scavenging. For this, the primaries should be equal lengths. The actual length and diameter of the primaries will determine the RPM at which scavenging is maximised. In real life, equal lengths are not often achievable, and the engine must operate efficiently across a wide range of RPMs, so a lot of compromises are made.

A 4-stroke engine has one exhaust pulse per cylinder every two RPMs - 720 degrees. So, for 3 cylinders going into one collector to produce even pulses, they need to be 240 degrees apart. Your proposed design has one exhaust pipe seeing 120-360-240 degrees and the other 120-240-360 degrees, so you might get scavenging on one of the three cylinders, but not on the other two. It will run unevenly, producing different power from each cylinder - effectively fighting against itself. But it might sound fantastic! I don't know anywhere near enough to predict.

I probably have some of this technically wrong, but you get the idea.

Thank you very much. That completely answers my questions and re-affirms some knowledge in my own mind. I will be looking at doing similar to what Troy has talked about and what you have suggested, 1,2,3 and 4,5,6. I dont think i have the balls to make up an exhaust not knowing what it will sound like or whether it will perform efficiently or not. However i could just make it and change the extractor layout if it sounds poo. Come to think of it. I dont think its worth having a car that has the power and ruin it by putting a restriction in the exhaust...

Doing an exhaust for 123 and 456 is easy. Just by any set of extractors, except Pacemaker 4480, and cut the last collector off. I'm interested to see how it turns out and to hear what it sounds like. Or if you want to be absolutely unique, cut the last collector off a set of 4480's and run three individual exhausts all to the centre off the rear bar. j/k of course.

Trying to fit two cats next to each other will be interesting.
How are you planning to run the tail pipes? one each side of the car, or dual on the same side?
If one each side, you might need to factor in that one side's pipe will be longer than the other as it has to cross the car to go out the other side.

Secondary pipes lengths after the cat isn't really an issue when running dual exhaust system (one on either side of the car). For the E-series cars (probably AU too), the fuel tank is in the way to run a dual system...it can be done if it is a dedicated gas system with the tank being in the boot (for a more legitimate/safer reason). It can also be done by having a Y-piece after the diff clearance diverting one pipe along the back of the bumper, a few inches from the fuel tank and then a 90 degree bend out...however this is rather dangerous especially during a rear end accident. One can remove the fuel tank, get it cut and welded for more clearance, heat wrap the exhaust and have a secondary heat shield/absorber on the tank to remedy this, but you will lose a fair bit of fuel capacity. Legalities wise I'm not sure of.

Thumbs up to efxr6wagon...awesome information .

Thanks again everybody!
I am now definately leaning towards the line of the 1,2,3 4,5,6 setup as to maintain as perfect flow as possible. It will be twin 2" as to gain better clearance both under the car and over the diff. I will be purchasing a second venom 200cpsi 2.5" cat and am going to run them slightly staggered, ie: one slightly further down than the other, to fit them easier (they have a 4" body so theyre not exactly small). Im thinking of running 2 2"dia. 12" long hotdogs side by side in the factory muffler space. Although i may need to go to the 16" to keep noise down a bit.

MAD: The tailpipes will run down the same side as per stock as i think its enough effort already just trying to sort out a custom setup like this let alone trying to figure out pipe placement on the passenger side. I am, however, thinking of dropping the twins out sideways through the right of the rear bumper, just for something different. Might look alright too. The thing i am most worried about is the over the diff section. I think i will need to put a flange either side of the diff just to make it easier with removal and installation.

Phongus: The e-series tanks are plastic

efxr6wagon: I think you need to do a little bit of a write-up for the tech docos with some flow mechanics information in it so we can get rid of all these exhaust topics that are all the same!! (pretty much) More seriously, thank you very much for your information. Without your help i would have been welding in the dark

Cheers!!

Sam

I forgot! How would i go about setting up the o2 sensors with this exhaust? I have 2 sensors in my extractors at the moment (1 narrowband for the ecu and 1 wideband for the wbo2 controller setup)

My Coby headers (6-2-1 style) have the O2 sensor measuring only one set of three cylinders. I guess that's good enough if the both sets of three cylinders are running basically the same AFR.

Still can be cut an welded...just plastic type

Those mufflers will be very loud. I have a 2.5" system with a Hooker Aerochamber and a 12" glasspack and it is loud.

As far as design of your headers go, this is the most critical aspect of your system.

efxr6wagon has already described this somewhat, but I will try and give a 3 point breakdown of theory behind it.

1. Exhaust gas velocity: hot air has less mass and is easier for the engine to push out. A smaller diameter pipe will keep hotter air travelling faster, but there is efficiency will drop off once the velocity is too high (i.e. using headers designed for low to mid range where you are trying to make power at 5000rpm). Note that hot gas also occupies a greater volume, but as far as the relationship between mass vs volume is concerned here the performance benefit is not linear.

2. Pulse tuning: A positive pressure pulse travels to the end of your primary, at which point continuing away whilst sending a negative pressure pulse back up the primary towards your exhaust valve (reflection). A pressure wave will rebound off a closed valve, or reflect off an open valve/open end of primary. These pressure waves continue getting weaker with each reflection, and generally are only tuned to the 3rd pressure wave. Ideally you want negative pressure pulses to be at the exhaust valve when it opens as this will suck more gas out of a cylinder and, depending on your valve overlap, more fresh air/fuel mixture into the cylinder.

3. Following on from the last point, you want your collectors to be scavenging or fellating all the pipes in unison. I.e. less positive pressure waves arriving at the same time at each collector. Obviously this becomes quite complex as you go along the header. This is easiest achieved if all pipes are the same length.

Notes:
- pulses travel at the speed of sound, which will change depending on temperature
- for point 2 & 3 cam/valve timing needs to be considered
- for point 3 firing order needs to be considered
- rpm will also determine when valves are opening, obviously affecting when you want pulses to arrive

Can't write anymore at the moment as at work. **edit: cbf typing more but essentially you are wasting your time pissing in the dark unless you are prepared to do the maths.

Last edited by skidder on Thu Jul 21, 2011 9:51 pm, edited 2 times in total.