Need expert exhaust design help. -
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post #1 of 4 Old 02-03-2013, 04:52 PM Thread Starter
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1981 CJ5 
Join Date: Oct 2012
Location: New Orleans
Posts: 25
Need expert exhaust design help.

I have a 81 cj5 with a CSB 350. Almost everything on the vehicle is not stock, and I find when I ask a mechanic or shop for help, all they want to do is type in the year, make and model and be told what it should use. For what I am looking for, a stock cj5 exhaust will not work.

What I am looking to do is make this thing as quiet and silent as possible for use in the woods, yet maintain as much performance power as possible. I am well aware that reducing air flow will reduce performance and that I am looking for a unicorn. However, I need a solution that has a balance, leaning more toward the quiet if I have to sacrifice any power.

I am also designing it to be a decent climber/mudder to play with. I have no intention of traveling to any rock climbing trails, most of what I get into is steep hill that are VERY muddy.

Using a dual exhaust system with stock mufflers designed for the engine (from a 1973 camaro) is too loud due to the short distance from the exhaust manifold to the rear of the vehicle.

What I have researched to make it quiet:
1. Use a X cross over for heat dissipation
2. Mount Muffler as far to rear as possible
3. Always use the largest muffler you can fit underneath (plenty of room under, SOA, changing lift to OME 2.5 (FROM SuPERLIFT 4in with 2in shackle lift)
4. Tail pipe diameter 2 1/4 or 2 1/2
5. Use the largest resonators you can fit
6. If you have distance to travel use long tapered cone reducers.
7. Most performance mufflers are much LOUDER than stock
8. A quiet muffler will be packed with steel wool and glass to reduce sound waves.
9. reducing air flow= reducing engine performance

The issues I see with project:
1. The limited distance between the exhaust manifold and the rear of the vehicle.
2. The dropped transfer case could limit the possibility of a X Cross over.

I have found at least one manufacturer (SOLO Performance) which makes some products under their STEALTH line which is primarily steel wool packed mufflers, resonators, etc. A bit pricey (stainless steel), but the first I have found online.

My first design is the following:
From each manifold come out in 2.5in pipe and immediately Y, (4 pipes in total). From each pipe, put a resonator in front of the stealth muffler. So there would be FOUR resonators and FOUR mufflers, pretty much coming straight off the manifold and less than a foot before entering a 24in resonator, and then directly into a 11x15 muffler.

My question is, would the air flow be greater, equal or less than if I did the design with one pipe, resonator, muffler off of each manifold? Since the air is immediately doubled (Y) into 2 paths I would think this would cut the backflow in half. Is that right?

Would the noise be greater, equal or less than if I did the design with one pipe, resonator, muffler off of each manifold? Since there are 4 mufflers, I would suspect that the sound reduction will double that of 2, however, the backflow should double. or will it?

I know the COST WILL be double but I know I am looking for something very unique.

Thanks for any help and direction, I hope I dont sound like a *******.

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post #2 of 4 Old 02-03-2013, 05:24 PM
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1983 CJ7 
Join Date: May 2009
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Are you going to run headers? If so, inside frame or outside? Do you happen to know the cam specs and compression ratio?


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post #3 of 4 Old 02-03-2013, 05:28 PM
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- The "X" pipe isn't just for heat dissipation, it will help with exhaust system balance and cylinder scavenging.
- There's no reason you can't mount the muffler transversely, which will allow somewhat longer effective tube length as well.
- Before you decide on a tube size, what RPM band do you expect to spend the most time in? Going with too large of a tube is just as bad as going with too small a tube...
- How quiet do you want to go? Bear in mind that reducing noise will involve creating some internal turbulence, which will impede exhaust flow (refer back to the previous point - some flexibility can be found, if you wiggle it right.)
- By "resonator," do you mean a "resonant chamber" or a "Helmholtz resonator tube?" Different animals.
- Most performance mufflers are louder than OEM because they're reducing the baffling and turbulence that makes the OEM muffler quiet. There's a very good reason that Top Fuel engines just use "stub stacks" - open primaries about 30" long...

Chew on your goals, and we can kick it around some more. You may impel me to dig out some of the books I have on the subject...

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post #4 of 4 Old 02-03-2013, 08:36 PM Thread Starter
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1981 CJ5 
Join Date: Oct 2012
Location: New Orleans
Posts: 25
I am not going to run headers. I will remain with the cast iron manifolds. Low rpms are normal, but during the winter and in the thick of the bad weather, the roads are Really muddy, 8-12 inches thick going up 35-50degree hills for 400-500 yards, so running higher rpms is normal, I would guess 3000-4000, I did not have a tac installed this year, it is in the plans. Put it this way, 40+% of the time in the woods, ALL 4 wheels are spinning and slinging mud 15-20 feet in the air.

Last compressions taken last fall where:
190. 187
170. 189
200. 184
200. 190

Resonators I am looking at are the "quiet" resonator from also looking at their stealth series of muffler.

I was thinking the transverse type of mounting would be limited by the amount of space between the transfer case and the gas tank. Also was thinking right angles would cause more air restriction than adding the dual muffler/resonator on each manifold.

Goal is be silent or very quiet, but only give up 20-30 HP

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