dual in, single out? (04 4x4 SSV 5.4L 2V)

looking for input regarding an exhaust setup for my 2004 SSV 4x4 XLT Expedition with the 5.4L 2V. I have a tuner on it already, and I have the FORSCAN adapter and SW but I've yet to use it. I'm running the 87 octane tune on it - it is barely better than the OEM police tune it had, and mileage is awful. 12mpg is a good day. I put 34" Nitto Grapplers on it, but otherwise stock (well, stock for an SSV, anyway). The plugs were replaced years ago.
it has the OEM exhaust manifolds on it, with dual cats on each side running back into a single muffler and then on out. I want to improve low end torque. NOT HP, NOT top speed, NOT drag times... JUST low end torque. I hope everyone understands that.
In looking at duals for it, I realized I will have to run pipes side by side at some point - there's just not enough room for a pipe to run down each side by itself, especially if I add mufflers, etc. Stacks might get around that, but stacks on a 4 door SUV have their own mounting issues.
Yes, I am considering CAI, an improved intake, and a larger throttle body. I've tried the GOTTS mod on an F150. It was a complete waste of time, so I'm not doing that again. I am using a (dry) K&N filter.

So, this is what I'm considering at the moment: shorty headers, 2.25" or 2.5" pipes (whatever keeps exhaust velocity up), low restriction cats, dual glass packs, and instead of an H pipe, combining the 2.5" pipes into one 3.5" pipe. One problem will be that the pipes won't be equal length due to the crossover from one side to the other. I don't know what the problems from that will be. I assume that combining the duals into one (DISO!) will gain the same advantage as an X or H pipe between duals. I assume.
Whatever the final diameter is, it needs to be twice the area (NOT the diameter) of the pipes coming back from the headers. Hence 3.5" final from the 2.5" duals, or 3.25" final from 2.25" duals... the idea is to match the final pipe's flow capacity to the dual's combined flowrate, to keep up exhaust velocity. The exhaust will dump somewhere out the side, not straight out the back.
Here is my question: What is your experience with a dual in, single out, exhaust?

thanks for your time.
jtfg

PS I have absolutely no interest in the "True Dual" exhaust argument. I could not care less. Please, Please, don't bother.
PPS the SSV is the Special Service Vehicle: Fordspeak for police car; this one was actually a fire department command car.

dual in single out is known as swept side exhaust...gibson makes them and someone else prefab...but most on here have that due to being told the IRS prevents true duals...you have to go smaller tubing to get duals AND keep the spare...im riding 26s...so i dropped the spare and have turned out duals...before all of this i had swept side exhaust with flowmaster 40 on it...good response and sound just wasnt aggressive enough...loud enough or the look i wanted

thanks! that's the kind of info I needed. I am dropping the spare; it's going to a bumper mount, probably on the front.eventually, I'll be installing an aux fuel tank in that location. if i did the math right, a tank the same volume as the spare, considering the spare as a square, (height x width x length) using the spare's diameter (34") as width and length, should hold 42 gallons.
I'll be checking out gibson.

joethefordguy said:

thanks! that's the kind of info I needed. I am dropping the spare; it's going to a bumper mount, probably on the front.eventually, I'll be installing an aux fuel tank in that location. if i did the math right, a tank the same volume as the spare, considering the spare as a square, (height x width x length) using the spare's diameter (34") as width and length, should hold 42 gallons.
I'll be checking out gibson.

Click to expand...

Might be cheaper at exhaust shop... check with them

Sent from my SM-G981U using Tapatalk

I've looked at the Gibson offering. I'm not paying over 500 bucks for what amounts to a replacement pipe and a better muffler.

EDIT
i just did some math, which i either did wrong, or not at all. either way, a 3.5" pipe does not double the area of a 2.5" pipe. that would require a 5" pipe.
BUT... I don't need to match the area of the 2.5" pipe duals, i need to match the flow velocity.
which i don't know how to do, but I strongly suspect does not require twice the area.

I know you're looking to improve torque only, but your idea of a 3.5" single out is WAYY overkill. I've been in a couple of LS1 forums way back in the day, and it was generally accepted that a single 3" from the Y-pipe back would support around 400 whp along with similar torque numbers. Typically, a single 3" moves roughly the same volume of air as dual 2.5". As for the notion of a 5" pipe? A single 5" pipe will support at least 1650 lb-ft of torque - that's what the last dump truck I drove in Tennessee had, fed by a 450-hp 15L Cummins ISX.

Not sure what you're wanting exhaust velocity for either, honestly. It's easier to move a lot of air slowly than to move the same amount of air quickly. To that effect, I'll never understand why Ford ran dual 2.5" pipes into a single 2.25" tailpipe - I just scrapped the stock exhaust on my '97 from the Y-pipe back and made my own dual 2.5" glasspack system, and my gas mileage went up a little. Just shy of 18mpg from a 5700lb SUV with over 300,000 miles ain't that bad if you ask me.

And I couldn't agree more about Gibson's exhaust - $500 for not even $10 worth of metal. Unless you plan on forced induction or nitrous, a single 3" from the Y-pipe back will more than suffice. If you're handy with a welder and a hacksaw, you can cut the single end big enough to form the end of a 3" pipe onto and weld it in place. That'll eliminate any bottleneck in the transition to the single pipe.

Eric, thanks!
I'm no exhaust expert. everything I'm writing here I'm getting from research. the point of exhaust velocity is to assist the engine in emptying the cylinders effectively. slowing the flow down with a big pipe interferes with that. Or, so I've read.
I've already figured out that a 5" pipe is grossly counter productive. I was even beginning to be uncomfortable with the 3.5" pipe. I think you've just helped put a nail in that coffin. More and more, it's beginning to look like a set of shorties, performance cats and a good hiflow muffler will give me all the improvement I'll ever get short of a very expensive dyno day with custom bending on every trial. You probably already know, this is a police package. It has two cats on each side before the Y comes together.
Here's a question would a good glass pack /cherry bomb style muffler work in this application? the motor's going to stay basically stock. I may put in a bigger throttle body... an improved intake is not unlikely, but I haven't found one that seems to be recommended. I don't see touching the cam, though cleaning up the heads is not out of the question. I have yet to see any CAI that would yield low end torque, so that's out. I've done the gotts mod on an F150. don't need to repeat that experiment. I'm using a good K&N air filter so I think I'm good there.

another possibility might be to cut the pipe something like 6" or so AFTER the Y has come together, (this would essentially form an X crossover) then break back out to set of two pipes from that point, same size as what went into the Y connection; those duals going back to high performance mufflers for each pipe. the whole mess would still have to stay on the same side of the truck, so the mufflers would have to be staggered to fit.
I keep going back and forth on this one. Whatever value there is in a crossover, it seems to me I get that as soon as the pipes come together. If I keep from disrupting flow from that point back, then there's no value in using two pipes from there back. If i get the right muffler, then one pipe from the Y pipe should be fine. What do you think?

I would kill for 18 mpg. on a good day, downhill, with the wind at my back; a semi in front to break wind, and a Toyota ePrius pushing, I can't get better than 12. If I actually start the motor, even that's not doable.

this is every tank since I bought it. the jump in octane in the middle is when i loaded a high octane performance tune. My next step is to look for a vacuum leak.
the last 93 octane tank, at the bottom, is because they were out of regular.

joethefordguy said:

another possibility might be to cut the pipe something like 6" or so AFTER the Y has come together, (this would essentially form an X crossover) then break back out to set of two pipes from that point, same size as what went into the Y connection; those duals going back to high performance mufflers for each pipe.

Click to expand...

This is exactly what I just did on mine! Where the Y comes together, I cut it to where I could form the end of a 3" to weld onto it, and left that piece about four inches long - enough to comfortably hang the other piece with a band clamp. The whole 3" section runs just long enough to clear the transmission crossmember and drop down like the OEM did for muffler clearance, and then it Ys back out into a pair of 36" glasspacks and then over the axles. I ran mine out on both sides, but it wasn't fun doing it - plus I'm gonna need to rework it later for more fuel tank clearance.

In hindsight, I think 30" glasspacks would've been easier and better sounding. But I'm happy with it. One thing to help too if you go this route, make sure the inside of the mufflers are a perforated core - not louvered like mine ended up being(the one thing I would change above all else on mine). Perforated core glasspacks flow a good bit better than louvered cores.

As far as exhaust velocity, stepped tube headers are made for that very thing - get the most effect from the exhaust pulse, and dump it into a larger pipe. Two things kill low-end power/torque with headers - excess scavenging which sucks some of the fuel charge ou ur with the exhaust, and reversion - that momentary vacuum after the exhaust pulse which sucks exhaust back into the chamber. A stepped-tube header, or a shorty into a bigger pipe would combat both of them.

Eric, i may have learned more from your post than in all the time I've been on this forum.
Thank you!