Throughout the 1980s, US-made M-bodies were produced with a very restrictive exhaust system that consisted of a main catalytic converter after the y-pipe and one smaller "pre-cat" catalytic converter just below each inlet of the Y-pipe as it comes off the exhaust manifolds. As restrictive as these units were when they were new, they are also known to clog up and get worse with age. There have been reports of severely clogged pre-cats getting so hot that they glowed in the dark, with some even causing underhood fires.
The solution is to go down to your local parts store and order a Y-pipe for a 1982 to 1986 Canadian Plymouth Gran Fury (Caravelle) or Dodge Diplomat. This unit consists of only exhaust tubing, and does not have the restrictive pre-cats. It will fit your M-body like a glove. It is dimensionally the same, and is a direct replacement for the stock Y pipe. The part number to order is Maremont # 440047.
1978-79 Lebaron 360 Y-pipes will also work. They are roughly the same length as the stock y-pipe. There are two versions:
Maremont # 449934 is for 360s only, and has a straight outlet. The length is 42 3/4".
Maremont # 448793 is for 318s and 360s, and has a ball-type outlet. The length is 43 1/2". This also cross references to Walker 40481, which also fits 1979-1980 Volares with 318s and 360s. This unit may have to be slightly modified by cutting off the ball end.Photos:
- Stock pre-cat Y-pipe
- Catless Y-pipe (Maremont # 440047)
- Screen captures from the Maremont site showing the stock numbers mentioned above.