My 88' has a 3.9L and I replaced the cat with a pipe 6 month's ago, instantly picked up more engine response. The muffler ended up blowing a small hole in its edge after the tailpipe fell off...this worked fine until it cracked off the mounts. The top is factory, the middle is when I removed the cat and used a larger slip pipe and the bottom is what I just did today:



After the muffler broke off I drove the truck for a week, swear I got better gas mileage with no muffler and no cat...was loud as chit though! Problem was I lost all my acceleration and my power-band was broken up all over the place (no muffler back pressure) seems like the removal of the cat combined with the factory muffler was working great. I ended up buying a Thrush Welded dual 2.25" offset output muffler with a centered 2.25" inlet, the stock I believe is 2" I have to say this new muffler is louder then the stock with a hole, great tone, more acceleration then no muffler....but I swear not as much as response as the stock with a hole and no cat!



I stripped it down to the pipe that comes after the "Y" pipe that crosses over the rear tranny cross member (before the cat) as you can see there is a step down reduction lip here I am guessing 2.25" down to 2.00" is it crucial to have a smaller pipe between the cat and muffler for back pressure sort of like how the expansion chamber on a 2-stroke engine works? I took a grinder to this lip and removed it down to the wider diameter (removed about an inch of length off the end). I then took 3ft of 2.25" OD pipe and slipped a 2.25" ID connector sleeve over the end by about 2" and then hammered the other end of the sleeve over this rusted pipe it was a tight fit and I was only able to get it about 1.5" over the end of this pipe so I think the headers are 2.25" but with a clamp on both sides of the 2.25" ID connector sleeve there are no leaks:



If your still following me I slipped the Thrush muffler over the 2.25" OD pipe because the inlet is 2.25" ID so there is zero reduction bottle-necking between the muffler and "transmission crossover pipe" that connects to the headers/Y pipe. This muffler sounds great when idling I still need some turn downs or tail pipes but it's loud and if I put my hand over either one of the outlets the tone goes way down. All I can really think of here is the muffler is not restrictive enough: http://www.jegs.com/i/Dynomax/289/17...oductId=763864

 

Perhaps the larger diameter pipe messed up the exhaust velocity, that is the scavenging effect created by the momentary vacuum created by the exhaust gasses leaving the small diameter pipe, which helps during the combustion stroke during the split second both intake and exhaust valves are a little open because it pulls in more air. It's not really back pressure you want, but the scavenging. Back pressure is generally speaking a bad thing from my understanding. I don't really know in your case, though, perhaps a more well-versed member of the forum could tie in. I also don't know if your truck is throttle bodied (like my 88) or carb'd, but I know on a lot of the jeep guys run into problems after exhaust changes because they don't reset their computers and the computer doesn't relearn the pressures and subtle changes in the way things flow and so it runs like crap. Food for thought, anyway.

 

this is not exactly what you are talking about. but this happened to me. i had a 1963 300 J 500 in max short crossram, 727 trans 2.73 gear. came out of 2 nd at 125 MPH. i had two glasspacks on each side, 2.5 pipes it was VERY noisey. my muffler guy said he will reduce the the last 2-3 in by 20-30 % dia. but no change in performance. that reduced the noise a lot. it might also give some back pressure for you. , but not harm performance.

 

I just installed 90 degree turns today and it quieted right down however it made the tone even deeper, so deep it physically vibrates my cab now but the droning pretty much went away:



Blake my truck is TBI I reset the computer and its a little bit more powerful now I think it had to adjust itself, there is zero restriction with this new system.

Moe I was thinking about trying two reducers on the muffler output to bring them down to 2" or even 1.75" just to see what it does and sounds like....I did remove a 1/4" step down after the Y pipe.

 

an other trick. is cut the end at a 45 degree angle. what you did is what being a car guy is all about, try this or that. you are on the right track. one time i made a muffler. that i could pull the baffle out of. on 64 dart with a slant 6. i put 318 2 bbl carb on it long before chrysler did. it was like adding a second engine.

 

The thrush muffler won't have a bunch of restriction in it, but it could be in the wrong place and is making your exhaust system act as though it did have a restriction like you'll get with a plugged cat. I would take the muffler off and see how the truck reacts - if it's better, I'd position the muffler just behind the cab and run 2 pipes back to the tail & see how that works.

 

my friend and I did that on his ram. worked really well.

ive also had good luck with dumpin the exhaust in front of the axle right after the muffler.

neither trucks have cats and both are v8s not sure if its make a difference though

 

The 45 end cuts were next to try, and removing the muffler makes me loose all power completely. I reset the computer and its not that bad now where it is, never thought about moving it I did not realize where the split was (distance from engine) made a difference.

 

you might need more restriction. you might have to run some more pipe after the muffler.

try putting a like a flap over the end of the exhaust(something that opens and closes) and see if you get some of the power back.

 

As much as everyone says back pressure of any sort does not help, for the sake of trying it myself I was going to try reducers on the output of the muffler maybe 2.25-to-2" just to see what it does but I will admit after a computer restart it got much better.