I'm installing single exhaust in a pickup, with the goals being simplicity, unobtrusive sound, and durability. There's two routes I could take --

#1, run both downpipes directly into the pictured muffler, and run a single exhaust out the back. The muffler has two 2.5" inlets, and one 3" outlet. It's designed to be mounted the other way, but Holley says it's reversible.

#2, run both downpipes into a Y-pipe, and almost immediately into a single-inlet/single-outlet muffler.

Which option would be the best overall? The dual-inlet muffler would be cheaper, since it eliminates the y-pipe. but would the flow be much worse?

 

They make those mufflers that have a Y pipe built in.

 

Cost for the 2 options is not much different. Y-pipe vs more pipes, more bends, more complex muffler, etc.

You can compare the 6.0 trucks (LQ4 & LQ9) to the 4.8 & 5.3 ones such as LM7. The 6.0s used the 2-inlet-muffler method while the smaller motors used a Y-pipe. In the stock trim of course the 6.0 method supported a little more power but in aftermarket setups they're not all that much different.

Part of your decision is probably determined by what fits best and where the muffler can / has to go, the choice of mufflers available, etc.

In my 2004 Avalanche 5.3 I have coated stainless long-tube headers w/ Y-pipe, a Vibrant stainless resonator, and Gibson stainless 3" single side-exit exhaust. Sounds quite good; low rumble kind of pitch, not loud at all (which I DO NOT want "loud"); durable obviously; hard to say about power.

 

I assume there's a muffler included in the Gibson exhaust? Is the resonator before the muffler? I thought the resonator was supposed to be the last thing in the chain, just before the exhaust outlet.

Are there any good resources about designing a custom exhaust? Such as a collection of rules-of-thumb and formulas for sizing and proportioning the different exhaust components?

 

Yes the Gibson system has a muffler. However the system as a whole was louder than I wanted, especially at idle; and kind of raspy besides. I realize some people like that but I don't. I wanted to tone it down.

Yes I put the resonator in front of the muffler. With my extremely limited fab capabilities, it was all I could do. It made a surprising amount of difference though.

No design resources I know of unfortunately. There's probably some out there somewhere but I've never come across any.