Pauls:
As someone said, 'Flattery will get you anything.' Thanks for your kind remarks. I'm just a little bored in my current job; maybe 'bored' is too strong a word, but it's just not as much fun as working on Corvette exhausts, running tests at the Arizona and Milford Proving Grounds. So I don't mind talking/writing about the little I know about exhaust.
About mufflers: First, with basically a stock 305 engine with the cats removed, you may be into the point of diminishing returns for exhaust mods. From this point it might involve massive transfusions of cash to get small power increases, and you might be unhappy with the overall sound you end up with. Does your 305 use the same muffler as a 350? If not, changing to the 350 muffler might be your cheapest and overall most satisfactory way to make another improvement. I'm sure you can get one cheap that someone took off to put on a Flowmaster
To answer your questions about mufflers will require a short discussion of what goes on inside one: there are two primary types of passive silencers for cars and trucks. The first, the one we use most of in this country, and what is found on Corvettes and Camaros, is the 'cancellation'-type silencer; the other, found mostly in Europe on luxury cars, is the 'absorption'-type. The cancellation silencer for passenger cars and light trucks works usually by providing multiple flow paths, and will have multiple internal chambers, three or four in production mufflers, often just two on a high-performance muffler.
What happens is that these chambers and paths provide opportunities for sound wave cancellation, since by taking different paths, sound waves of a given frequency will arrive at the same point out of phase with each other, and cancel out. Unfortunately, there's just not enough space under a car to cancel ALL frequencies, so the muffler AND its position under the car is selected to cancel out the most significant frequencies, the low frequencies that are reinforced or amplified by the exhaust system pipe lengths. Beyond this, perforations and small absorption silencers will often be incorporated into a muffler to provide some reduction in the high-frequencies.
What this all means to you is that the Camaro muffler is handicapped in its' internal layout by the need to have one of the tailpipes exiting the same end as the inlet - you can't get as many 'passes' as you'd like to inside that muffler. An aftermarket muffler maker is going to be faced with the same problems, and probably won't have either the sophisticated tools nor the inclination to spend a lot of time solving those problems. He *might* reduce the backpressure by simply giving you a large mostly-empty can, but my guess is it'll sound pretty awful. I recall working on a Camaro, and it was difficult for us to come up with any major improvement over the production system, which we considered to be unsatisfactory in its' overall performance. The Camaro seemed to be designed without any regard for an exhaust system - except for that huge catalyst hump under the passenger's feet!
If they could be fit into the car. a pair of well-designed low-restriction mufflers would work better - sound better and have lower restriction. My guess is the Flowmaster 3-chamber design would be OK, 3 chambers being better than two. The only Walker mufflers I've used were a pair of Dynomax (same as Ultraflow, except not stainless steel) jobs on a Mustang GT (don't ask!). That car then sounded more like a 15-year-old truck than a sports car; it certainly rumbled, but it would set off car alarms as you cruised through a parking lot! That's not to say they would sound the same on your car, of course, but they are VERY 'open.'
As far as muffler location goes, you don't want the muffler ALL the way at the rear of the car (like a Corvette!) since you'd like to a) have a little acoustic pressure to work with in the attempt at cancellation; and b) since you want to 'break-up' the long pipes in the system into shorter ones of unequal lengths to avoid the (organ-pipe type) pipe resonances that plague, oh, the Corvette, for example! The only advantage, backpressure wise, to locating the muffler further toward the rear of the car is that the exhaust gases passing through it are at a lower temperature, and therefore have a lower flow rate, since they have a greater density. Flow resistance effects are dependent on volumetric flow, not mass flow - i.e., they depend on cubic feet/minute, not on pounds/minute - so lower temperature gases passing over a sharp edge or through an orifice will have a lesser pressure drop than high-temperature gases. This, BTW, is a great argument for NOT wrapping your headers with insulation, unless they're getting so hot they set stuff on fire under the hood!
Let's see now: 'H'-pipe diameter: How about putting in a larger-diameter 'H' in the location shown in your photo, maybe 2" or even larger, but with a flange connection in the middle? Would there be room for that? Then make several stainless steel restrictor plates or shims (.030"?) to go in the flange joint. If a straight-through 2" was too large, you could put in, say, a 1.75" restrictor, or smaller. Worst case, you could go all the way down to 1", or you could go back up to full size for a run down the strip. I'm going to try to attach a sketch,don't know if it'll work.
Remember to design the joint with the shim in place, and to always run with one, even if it's the same as pipe size.
With regard to backpressure, like anything else an 'H'-pipe would only make an improvement at WOT - that's the only time you have any significant amount of backpressure, because that's when the exhaust flow is up, and the exhaust gas temperature is up.
And speaking of 'H'-pipes, if you were to go to dual mufflers, I'd strongly recommend devising a way to have an 'H'-pipe in that system, too.
Have I missed anything? If so, try me again.
Regards,
R
