time for a new muffler

[Bugeye]

ok, i started having problems with my truck not being able to idle at 650 rpms after I fixed an exhaust leak at the manifold, didnt think nothing of it, thought i had a vac leak, well i did and i fixed it, still couldnt get the truck to idle, so today at school i said fuck it, drilled a hole just before the muffler and stuck a back pressure tester up in to it, started the truck...holy shit it had over 3psi of back pressure, not good. I know its not the cat because i just replaced it at the start of the year. so i take out a ball peen hammer and start tapping on the muffler, from the tail pipe moving up towards the cat, i hear the hollow sound and right where the muffler clamps to the exhaust pipe all i hear is "thud":lol::lol: so i left the hole in the pipe to see how it drive, it drives a hell of a lot better now, so tomorrow im going to the muffler shop and having the flowmaster put on, but while im there i want to change the dia of the pipe from the cat back. what size should i make that pipe and be able to keep the required amount of back pressure?.....OH AND THIS IS THE FACTORY MUFFLER I AM REPLACING :lol:

Edited November 18, 2008 by Bugeye

[Pacific coast Datsun]

Id say 1 3/4 but no bigger than 2 inches.

[Nissan_Boy85]

Best part of that story, "so today at school...". I am guilty of modifications being made in the school parking lot too. Ok, not so much as modifications and replacing parts. Ok parts needed to get home...like a starter.

[Bugeye]

Best part of that story, "so today at school...". I am guilty of modifications being made in the school parking lot too. Ok, not so much as modifications and replacing parts. Ok parts needed to get home...like a starter.

 

i didnt do it in the parking lot, i did it on the lift in the shop....at my college :lol:

[datsunfish]

I would say that if you are going "cat-back" then the diameter shouldnt matter as much since the cat gives plenty of backpressure and a flowmaster is essentially free flowing. I would personally go with 2 1/4" or 2 1/2" just for some tone. Oh.and a nice chrome tip. What size is the current pipe at the cat?

[datzenmike]

The exhaust pipe ideally should be as large as the manifold outlet or a bit more. The Z24 is 2" so the L20B is 1 3/4 or 1 7/8? 2" on an L20B would be plenty. Going from 1 3/4" to 2" is an increase of over 30%, how much do you need???

 

Again, ideally the muffler should be at the very back to give the exhaust gasses enough distance to cool down as much as possible without any restriction. As the gasses cool they shrink and need less room and their pressure drops. Catalytic converters are an unavoidable but necessary evil because they have to run hot to work and so are up near the manifold. If you look inside one you'll see that the honeycomb design is not that restrictive as it has cross sectional diameter that is 2 to 3 times larger than the pipes in and out. If you are creative get an even larger one from a V8. Use any 2" inlet muffler. Mufflers without tail pipes always sound louger and deeper so that's fine. Put a down elbow on the outlet.

[Nissan_Boy85]

i didnt do it in the parking lot, i did it on the lift in the shop....at my college :lol:

 

Psh...using a lift...I do it too at my college. Hell thats how I found out i need to bribe one of the welding guys next door to re-weld my exhaust hanger back on.