I plan on buying a muffler and having a shop bend up the pipe to have the exhaust redone sometime in the near future, as the current exhaust is starting to rust apart. I think for cost's sake, i'll just stay with a single exhaust, but should I got with 2.5" or 3"? I want the sound to retain the deepness that it has as of now. I plan on using either a borla pro xs, or a dynomax ultra flo muffler.

Any vids with efi 300's with exhausts using straight through mufflers (not cheap glasspacks and such with louvers and unnecessary stuff inside), post them here!

 

For a single exhaust a 3" diameter is optimal. For duals run 2.5".

 

3" even for a 300? that's what i'm running now but i'm just making sure that's not too much for a single exhaust...

 

I wouldn't go any larger than 3", but you won't hurt anything by running it either. Personally, I am going to go with true duals at 2.25" diameter.

You have to remember that this engine is 300ci, nearly the same size as a SB v8. It produces plenty of exhaust gas.

 

I just finished converting my '82 300 to EFI exhaust manifolds. I bought factory down-pipes from the manifolds to near the converter. The JY cut the cat off. I made a collector to bring the two pipes together into a 3" stainless steel pipe. I bought a Magnaflow 3" in/3" out cat. After that is a 28" piece of straight 3" stainless steel pipe to a magnaflow muffler (13742). I bought a 3" stainless steel tailpipe. It flows really well and high RPM performance is noticeably better. I don't feel any loss in low end torque. However, this system is a bit loud for my taste. It has a very low, deep sound, but just too loud for me. I just finished cutting the tailpipe behind the rear tire and added a second small 3" in/3" out muffler there. It sounds much better now.

If I were to do it all over again, I would probably go with 2.5" single, just to make it a little quieter compared to what I have now.

Edit: I want to add that the added performance at higher RPM is probably due mainly to 1) switching from the carb log style exhaust manifold to the two EFI manifolds, and 2) my old catalytic converter was about 75% plugged. I think the 3" exhaust probably has minimal benefit over a 2.5" exhaust.

 

2.5 is more than enough performance wise on a single out. 2.0-2.25 is plenty for dual out. Anything else is manipulating the noise factor.

 

Thanks for all of your thoughts and suggestions guys. For now I'm just going to get this:

Dynomax Performance 17221 - Dynomax Ultra Flo Welded Mufflers - Overview - SummitRacing.com

and just keep the remaining pipe that is after the muffler (about 2.5') because that still has some life left in it. It's more the muffler on there now is falling apart due to the salt in the winters.

 

2.5" is definitely plenty. I think if you get much bigger than that, you start to reduce the flow speed through the pipe, which can actually hinder performance.

 

I probably would have went with 2.5" but I wanted to still use the remaining tailpipe for now which is 3".

 

I promise it is fine. You won't notice any difference between 2.5" and 3" diameter pipe anyways, performance wise.

 

I'm going through and replacing my entire system right now using 2.5" from Manifolds back. I think my Dynomax Turbo muffle actually has a 3" tail pipe but the rest of the system is a 2.5". From everything I've been told by the guys over on FordSix, Here and on FSB, once you get up to and beyond 3", all you're doing is loosing torque due to a lack of needed back pressure and then performance goes down after that.

 

4-cycle engines do not require back pressure to operate correctly. It is not a "lack of needed back pressure" that causes poor performance with a too-large exhaust. It is a lack of velocity through the exhaust. If the exhaust is too large in diameter, the flow rate through the exhaust is too low. The slower moving exhaust gasses cool, which makes the gasses more dense. This impedes flow. A system that is too large will actually flow LESS at low RPM than a smaller exhaust system.

 

I've never heard it explained like that before. Makes sense though.

 

Three yrs ago my 96 F150 had a pretty nasty exhaust system that needed work. It also threw a CEL that pointed at the converters. So I cut the exhaust out in front of the first converter. Bought new converters, turbo muffler and 2.5 inch pipe. Welded it all together and it exits in front of the right rear tire.

I noticed right away that the truck had more power (maybe just because the converters were not clogged). The CEL stayed off, and it made a little more noise. Other than the crappy rusted out turbo muffler I replaced this spring I have been happy with the results.