Making the Cat Scream
(Gutting a GTS Exhaust System)
By David Bell
Not too long ago I replaced my Pantera’s Big-Bore exhaust system (that was on the car when I bought it) with a hand-me-down GTS exhaust. I’m always looking for a little more power and my thought was that the big tri-Y GTS header tubes and 2-1/2” exhaust pipes would be less restrictive than the smaller diameter Big-Bore headers with their squished down 4 to 1 collectors and 1-3/4” pipes. Though not new by any means, the GTS exhaust system appeared to be in great shape once I’d cleaned everything up and shaken out the dirt-dobbers that had nested inside the pipes. My new-old GTS system was so solid that I went to the additional time and expense of having the headers bead-blasted and Jet-Hot coated so that they looked like new. And with the ceramic coating, I was pretty sure they’d last another 25 years or so.
After swapping everything out, I found that the GTS exhaust produced a much more civilized sound than the Big-Bores but still retained a healthy, throaty idle. The biggest improvement though was in the looks department – the Big-Bores were definitely visually challenged (actually they were downright ugly for my tastes). So, I was really happy with the exhaust system swap - that is until I stretched the car out at the next track event. On the TWS back straight I realized that I had lost a noticeable chunk of top-end oomph. I couldn’t do that screaming WOT run up to 6500 rpm like I could with the Big-Bores. Bummer!
Contrary to conventional GTS exhaust wisdom, my track experience led me to suspect that the extra baffling inside the GTS mufflers was sucking up high-rpm horsepower. Maybe the Big-Bore exhaust with those ugly glass-pack mufflers was more free-flowing than I’d thought. I began to wonder if maybe this exhaust system swap was not such a good idea after all.
But all was not lost. Gary Hall had mentioned to me once that the GTS mufflers that Hall Pantera sold were basically Ansa muffler cans that had been opened-up, stripped out inside, and then re-sealed. I’d heard the Hall GTS exhaust on POPKAL (Gary’s excellent blue ’72 Pre-L) at Texas World Speedway last year and was very impressed with the sound. So with some e-mail help and encouragement from Tony DiGiovanna, I dissected my GTS mufflers and yanked out all the insides.
After a series of full-throttle runs in the boonies, my seat-of-the-pants dynamometer indicated that the gutted GTS exhaust had regained at least some the lost high-rpm horsepower. And though the sound coming out is noticeably louder and more aggressive than unaltered GTS mufflers – to my ear it is still quieter than the ‘wall of loud’ that the Big-Bores blasted out. The wail of the gutted exhaust is most noticeable running wide-open but much more tame if you can force yourself to lay off the throttle. The annoying whistle-tweet bird noise that the stock GTS mufflers made when lifting off the gas had also disappeared. The bonus prize was that I got to keep the clean looks of the GTS exhaust system.
I’ll admit that this modification is not for everyone. But if you’re after a little more upper-range horsepower and not too worried about the extra Cleveland symphony orchestra volume that goes with it, this is how to do it. The main tools required are some heavy gloves, a die-grinder, a powered hacksaw, a mig welder, and a ~ 6” long piece of 2” X 2” angle iron (available at any hardware store).
1) Mark out a large rectangle on the top of each muffler can (a fine-point Sharpie works great here). Don’t be shy with the size of the opening as it needs to be large enough to get tools in and the baffles and other stuff out. But try and restrict the hole to the mostly flat top portion of the muffler. That way once the seams are re-welded, filed down and the mufflers mounted, everything will be hidden against the car’s underbody and essentially invisible.
2) Carefully cut along the marked lines on the muffler can with a die-grinder using a metal cutting disc. Thin cuts are much easier to weld back together than gapping holes and the die-grinder works much better for this job than a cutting torch (though a plasma cutter would definitely work best, most folks don’t have those handy). The GTS muffler sheet metal is very thick so take your time, wear safety glasses, and have lots of extra cutting discs (I think I used five or six for the two mufflers). The lid has to be in good enough shape to weld it back on, so try not to mangle it too much. 
3) There are two internal bulkheads inside each muffler that are positioned perpendicular to the exhaust flow direction. The bulkheads are held in place with tabs that extend through slots in the muffler shell that were then folded flat and welded. The tab welds are visible small lumps that will have to also be cut through with the die-grinder (I cut a couple of angled slits through each weld). When the top of the muffler can is free, lift it out and all the muffler internals will be exposed.
The overall exhaust flow is through the perforated center pipe to an exit at the twin muffler outlet tubes. Along the way the exhaust escapes though the center pipe perforations and then through more small holes in the two bulkheads as it makes its way out the back. All this banging around inside the muffler is what quiets the exhaust – but also absorbs some horsepower in the process. Time to take the baffles out.
4) Cut through both ends of the center perforated pipe using an electric or air powered hacksaw (I used a Saws-All with a metal cutting blade). Be careful because if the blade bottoms out on the muffler sheet metal then the saw will be ejected right back at you and probably bend the blade at the same time. Bi-metal blades are much better for this as they can take being bent and straightened multiple times. Then cut vertical slits into the two bulkheads on either side of the perforated pipe. You should now be able to lift out the center pipe out.
5) Next, cut out the remainder of the bulkhead to clear the exhaust path to the back of the muffler can. There is not much room to work inside the cans and it is a struggle to saw the two bulkheads into pieces and get them out - but it can be done. Minor yelling and throwing stuff may be required so send the kids into the house first and tell them to close all the doors and windows. The bulkheads can actually be removed more easily if you have access to a cutting torch, but it’s not too difficult to burn some extra holes in the muffle can with the torch so be careful.
6) Once all of the muffler insides have been removed, position a short piece of 2” X 2” angle iron vertically between the two outlet pipes and weld it in. The angle iron will split the gas flow toward the two outlets and also keep the hot exhaust from banging directly off the back end of the muffler.
I've heard more recently that the short perforated pipe sections that extend into the rear of the muffler cans (shown in the photo below) are major flow obstructions. Ideally, more streamlined gas flow out of the muffler can be obtained by removing the perforated pipe stubs flush with the back wall of the muffler. Unfortunately, they are very difficult to get to with a saw and probably require careful use of a cutting torch for removal. So in spite of the potential performance loss, the perforated pipe stubs remained in my muffler cans.
7) Finally, mig-weld the lid back on the top of the muffler can. Tack down a few spots first to hold the lid in place then weld solid beads along all the cut seams. The muffler metal is fairly thick so this is pretty easy welding, even for a raw amateur like me. If you don’t have access to a mig-welder, then take the mufflers to a muffler shop for welding. 
8) Grind or file the welds flat and then put some paint on to hide your crimes. I used a pewter colored, high-temperature engine paint (from Bill Hirch) that was left over from painting my engine and it has held up great on both the engine and mufflers.
9) Now re-install the mufflers and you’re good to go. 
Last, take the cat out for a drive and let her scream. Just be careful not to wake the neighbors.