You ever get this finished. I'm looking to do something similar. Speed engineering long tube headers with dual 2.5" pipe to an X-pipe into the Flo-pro 2504 muffler I already have. It's dual 3" in and dual 2.5" out and is only about 10-15% louder than stock.

 

I have all the parts sitting right here, but I screwed up my truetrac install, so I'm going to pull out the axle. After I get the axle back I am going to add the sway bar and caltracs then install the pipes since those will have the most adjustability. Once that's done (hopefully in another week or two), I'll then get the exhaust on.

My preliminary measurements - It looked like where I want to cut the cat's collector off, I SHOULD have a ton of wiggle room to make it all fit. I have an x pipe, 4' straight pipes, and 5' straight pipes, and a couple S pipes that are for the tail, but I could use as intermediate pipes to fix an angle if needed. I might have to use one flex pipe, and I can always cut and twist the "second cat pipe" that will come from the driver's side. The only tight space is at the trans crossmember, and that shouldn't be a problem at all. There is plenty of room to mount intermediate pipes and any size muffler.

I started out wanting to use an X-pipe right in place of the collector. I changed what muffler I wanted to use when I first had that plan, and the new muffler is an x pipe design. I plan to get the scavenging a little farther back. I know speed engineering has their own X-pipe that fits those headers. I would contact them, and see what center to center is on their x pipe down stream, and see if it measures equally to your muffler. I know on there system, they are running straight pipes from their x-pipe to their dual mufflers. Even if it doesn't fit, you SHOULD be able to get two straight tubes (they sell the tubes you would need). Even if it doesn't fit perfect to your muffler, you can cut one pipe down (can rent a tube cutter and a pipe expander if you aren't welding it at autozone rent a tool for about $80 refundable), and use a flex pipe to get a little flexibility in the tubing.

 

I had a rough time getting that truetrac installed, (I broke one of the factory shims, then had to have someone else pull the shim pack out). I had to pull it out, and then all the shops in town that would touch it wanted to charge me full price for a swap (~$1K), just to pull out the shim pack I wedged in, and to set the backlash. If you're near south Texas, I ended up using The Gear Guy in Spring Branch. Charged two hours, which even though WE knew he might be able to do it in 10 minutes (because I had the premeasurement and the broken shim), I thought that was fair. They seemed cool, and it was about a half hour drive for me, but I've had some bad experiences and I was happy with that.

So anyways, I have tried every lap band clamp I could find on Summit, ebay, and amazon. None of them were made well or fit right for my pipes. I have plans to eventually do Atomic's coilover, and I talked myself into doing the custom tailgate I have wanted for a while, so I just bought my first welder. I have to piece together the accessories, and learn how to use it, but I think with how I am doing this, welding is just the only option. I could have just taken it almost anywhere and had them do it, but where is the fun in that?

The only other way would be to see if the NNBS 6.2L will bolt up with little or no modification. I will keep an eye out if any one does it and update if I find out or even buy a cheap one just to try, and then probably sell it.

Based on an Engine Master's episode where they tested long tubes vs shorts on a 400cid motor, the difference IIRC was about 2hp. I really don't think the shortys are that bad. I have read a bunch that they are, and I have even said it; but that episode was the only time I've seen anyone show proof or any kind of testing, and it was almost nothing. (I will update with the episode when I find it). I really don't think long tubes would be that much better for this ~500hp range. I think the problem is the cats, specifically the collector. Long tubes get rid of 'em, and if you only do shortys you are leaving the main choke point.

I will hopefully get started running beads in the next week or two. When I feel they are good enough I'll get going on the exhaust. I'll measure the ID at that collector, and we can see.

 

Someone chime in here if I’m wrong but my thinking is that long tubes allow for more flow for a longer period of time before the choke point being the 3” collector. I think that is why people will go from a 3” collector to a 4” exhaust. Like stepped headers and such

 

The way I understand it; a single 3.5" / dual 2.5" should be good for almost 600hp, which should be just about everything short of: Nitrous, FI, or the BEST aftermarket heads on a 6.0. I think a longer runner helps in general, but it's not going to have nearly the impact the collector does.

I'm not thinking this exhaust will be perfect, but hoping for about 95% of best possible, while keeping it quiet and legal.

That just doesn't make sense to me, (but I know about enough to get me in trouble). It's like the weakest link in a chain, an exhaust is never going to outflow its most restrictive point. Its the same dynamics as water. If I have a garden hose, and then add a fire hose to the end, its going to flow the same. The only way going from 3" to 4" exhaust would make sense to me is if the muffler itself is restrictive, and you are only trying to match the flow rate. (If it has to be said, I'm only going off what I have learned second hand). I could definitely be wrong, and wasting what is a lot of money to me on all of this.

 

I never thought about a 3.5” exhaust from a 6.2 truck. I like the idea of quiet and more horsepower.

 

You won't really see much of a peak horsepower difference from properly sized short vs long tubes or even LS manifolds for that matter. If you run with tubing and collectors too big for 90% of the engines operation it typically increases torque above peak. Thats like 1-7/8 primary with 3.5" collector type stuff. With a properly sized LT, its the torque added under the curve that shows the biggest gains.


As for GM tubing size, not sure why GM does what they do half the time. My 245 HP vortec 350 came with 2.25" outlet manifolds and dual 3" OD to the muffler and a 2.75" tail. The head pipes in front of the cats necked up to 3" a few inches down from the manifolds. Some of the later versions of the same had 2" off the manifolds to the cats and 3" after the cats. I can understand going larger after the cats since the exhaust is hotter after the cats.

I ran the same 350 with a mild factory LT4 cam with stock manifolds and then later shorty headers. Then I pulled the shorties and added Thorley Tri-Ys with their matching 3" connecting pipes. I then swapped the cam for something with a tighter LSA to take advantage of the added scavenging from the tri-ys. Big jump in torque with the tri-ys and that was compared to the previous bump from the shorties. I now have a 383 in it and run a dual 3" inlet Walker QuietFlow muffler and matching Walker 4" tailpipe for an 01-06 3500 HD on it. It flow extremely well and is very tame sounding.

 

The 42hp I picked up at peak with 1 7/8 LTs vs stock manifolds begs to differ with your opinion

 

Exactly what I was saying. PEAK power gain. What did you gain at 1,500? What about 2,500 rpm? What did it do at 50% TPS at 2,000? I gurantee those 1-7/8 lost through most of that. Cammed engine going down the drag strip is one thing, those of us using a truck as a truck is a completely different thing. I do not drive around everywhere WOT at 6,000+ rpm.

 

I'll reach out to my tuner and you can compare the stock vs modded graphs for yourself

it gained everywhere. I've proved you wrong on multiple occasions, so I'll be happy to do it again. Cam and LTs were the single best mods I did to my truck. And yes, I tow with it- almost 2000mi a year, and yes, it tows great