I have a 91 w/strait 6, it looks like it has two cats on it. A smaller one that is a two pipe in one out and then a large second one. Are these really both cats. Needing to add some flow, it would be a lot easier if I only had to work on the second one.

 

They are both cats, 2 different types. If you got a replacement you could get an all in one.

 

Make sure the exhaust port is a noticeably larger diameter than the two entry port diameters. The cat will provide sufficient back pressure for proper valve function so you don't need any muffler w/flow restriction built in. Flowmaster glasspac = noticeably respectable, but not overt.

 

Is back pressure really a issue on these older motors. Thought you could run strait pipes or no cats on these without a issue? I am not planning on going that far, but would like to know.

 

Could be just a vicious rumor. I was told that by my muffler shop guy Iv'e used for 20 some years. Now that I think about it, I used to get pulled behind boats w/straight dry stacks & running blowers but they shut down after a 1/4 mile. As per Alldata though back pressure begin to interfere with valve function at around 4 psi.