Hey guys! Have a 2003 GMC 2500HD with the 8.1L and Allison 5-speed. Truck is only used for pulling and is completely stock with the exception of a K&N Gen. II mass air intake kit. Am looking to add a throttle body spacer and exhaust. I do not want the truck to be loud; just looking for increased airflow and hopefully a little better gas mileage. Am interested in Flowmaster 50 Series Big Block or 70 Series Big Block II mufflers. What king of a setup would you guys recommend for the muffler and back (i.e. muffler, pipe configuration and sizing)??? Thanks for your help!!!

I only have single exhust on my yukonxl 3" pipe from 70 Series dual inlet single 3" out,i was going to go with duals but i would have to remove my spare tire and shock controll valves....It has a nice rumble above 2300 rpm sounds real good not too loud for pulling on the long hwy drives.......
When i put on the k&n cold air kit and the 70 Series muffler and the superchips i got about 2 mpg better ...hope this helps...

get an aeroturbine muffler and use your stock 3.5" tailpipe.

First of all, can the throttle body spacer idea as they don't do a thing but suck you money out of your wallet. Your truck has true 3 inch duals all the way to the muffler, so true duals are a snap on this truck. Basically, you put two muffler's sandwiched together on edge and dual tail pipes and you have true duals. I have Dyno Max Super Turbos on mine with 3 inch internal pipeing. I can't remember what size the body is,but they are not loud at all, but rumble nicely at idle. With a set of echo tubes on the end, they wake up a little more. I put an H pipe on mine in front of the mufflers, and criss crossed the tail pipes to make both pipes equal in length from front to back to balance the system. There are some pics in my garage.

If you don't want a drone, I would stay with a more mellow muffler like what I got on mine. At 70 to 80 it is good.

I'd save your money and get a custom tuned PCM, spark plug mod and 180 thermostat. Exhaust upgrades should be lowest on the list.

Duromax04, howdy again. Did you ever figure if you got any appreciable increase in mileage with your exhaust system?

davefr: What is the spark plug mod? Why go with a low t'stat temp? My truck runs doggy until it is at total max temp. Am I missing something? My engine is totally stock right now. I have been working on getting my truck ready for accepting a truck camper and some hitching modifications.

"Doc"

the gibson setup is the best i'v ever used, true duals, one muffler 2 in and 2 out. very clean and eaisy install (pipes exit side by side)

I use Magnaflows in my shop, The dual in / dual out mufflers have a built in X-pipe in the center, Crisp sound, Performance and milage. They even have a lifetime warranty.
Your truck has 2 3/4 " headpipes, close enough to 3" , Don't mess with a balance tube, They create a weak spot in the system, The x-pipe system creates horsepower.
I personally don't see a reason for crisscrossed tail pipes? Once the exhaust gets into the header collector its scavenged together, eliminating the need for equal lenth pipe. You can't get equal length pipes front to rear anyways.
Flowmaster's have the sound, but you want sound, perf. and milage go MAGNAFLOW.
Ray,

I use Magnaflows in my shop, The dual in / dual out mufflers have a built in X-pipe in the center, Crisp sound, Performance and milage. They even have a lifetime warranty.
Your truck has 2 3/4 " headpipes, close enough to 3" , Don't mess with a balance tube, They create a weak spot in the system, The x-pipe system creates horsepower.
I personally don't see a reason for crisscrossed tail pipes? Once the exhaust gets into the header collector its scavenged together, eliminating the need for equal lenth pipe. You can't get equal length pipes front to rear anyways.
Flowmaster's have the sound, but you want sound, perf. and milage go MAGNAFLOW.
Ray,
Ray:
What is the MAGNAFLOW part number that you use on a K3500 8.1L, CC Longbed?

"Doc"

I would agree on Magnaflow. They have a nice, deep, throaty tone on gas engines. Thye dont have that annoying flowmaster crackle. I've run a Magnaflow system on my Ford, LT1 Caprice and 99 Tahoe. Deep and throaty and not really loud at all. The drone got to me on my Ford which promped me to go back to stock on that but the other two were nice sounding systems.

Doc,
part #12569 if you want 2 1/2 tailpipes or part #12599 if you want 3" tails.
Ray,

Ray:
What is the MAGNAFLOW part number that you use on a K3500 8.1L, CC Longbed?

"Doc"
Doc, the part number you are looking for is 12569, it is the universial big body.
Just had it put on my 2007 classic 3500 cc and it runs great, increased the HP, also reprogrammed the computer with hypertech

I have not seen any actual dyno charts that can prove that exhaust do much on these trucks, so I have to conclude that they don't. They make it sound better mostly. I don't drive mine that much, and I like to put my foot in it some, so I can't really tell you that my HP increased. It gets about 13 running 75 to 80 mph. If I would slow down, it would probably do better. As low of RPM as these run at highway speeds, dual exhaust really don't play that big of part in it. There is just not that much flowing at 1,900 RPM as there would be at 3,500 RPM like the old 3 speed days. A tune is the best non hard part upgrade for these engines I think. They are being pulled back so badly, that if you can get them tuned to run what they are capable of, you will be pleasantly surprised.

I am not sure what was meant by the exhaust collecting in the header pipe. The reason I criss cross the tail pipes is becuase the drivers side bank of cylinders have to cross over towards the passenger frame rail and then down that side, and if you put a regular two muffler setup and tails, you have to run the drivers side pipe back across the truck for it to exit the drivers side. If you measure that pipe vs the Pass side, you will find that it has far more pipe to push exhaust through than the Pass side. Therefore, if I criss cross the tails, I am putting the short/pass tailpipe on the longer/drivers main pipe and vice versa. This gives more of a balanced system. Couple that with a balance tube, and you have a smooth sound. I have done it on about 5 trucks now, and it works out nice. I use dual mufflers instead of one muffler with 2 in, 2out. I called Magna flows Tech line, and the guy on there told me that they didn't have a muffler for an 8.1 that wouldn't drone badly at highway speed. They said that in there testing, it was very annoying, so I went with a more mellow Super Turbo. With Echo tubes, they have a nice mellow sound, with no drone at highway speed. Even when pulling.

It gets about 13 running 75 to 80 mph. If I would slow down, it would probably do better.
Damn, I would kill to get that kind of mileage out of my truck. I am lucky to get 10.5 driving at 63 on cruise. City mileage really takes a toll on the wallet, at 8.25 MPG and if I use 4HI, it goes down to under 5. However when I tow a 10,000# 8.5 X 18' enclosed trailer at the same 63 MPH, I hold about 9.75 MPG and I usually don't use the tow haul mode ... unless I want to use more as.

This engine is much to ballsy for a daily driver. I would like to be able to turn it down until I hook up to a trailer and then turn it back up. I would also like to be able to set up the Allison to shift quicker in the city and change it back when I hook up to the trailer.

"Doc"

Damn, I would kill to get that kind of mileage out of my truck. I am lucky to get 10.5 driving at 63 on cruise. City mileage really takes a toll on the wallet, at 8.25 MPG and if I use 4HI, it goes down to under 5. .

I get the same 13-14 at 75-80. Slowing down doesnt seem to help much on my truck. Maybe its my 3.73 gearing.Its only running 2150 or so at 75 mph. City is where my truck kills me 8-10 mpg.:o:

Mine has a 6 speed Allison, which gives me a 10% more overdrive in the 5 speed version. My 2001 got 10 all the time, driven the same way, and same gears. So I guess the extra gear gets a little more mpg out of it.
This one is also a reg cab single rear wheel, so that might make a difference as well.

Best milage on mine was at 80mph. Any faster or slower milage droped.