I have a 1991 F150 with the 5.0. I recently put on a Flowmaster Super 44 muffler with a turndown and wasn't too terribly impressed. The truck currently has the two original cats and being that they're 18 years old I would imagine a replacement would not be a bad idea. Plus it would help the air flow and make the Flowmaster sound a bit better.

I have seen in other threads where guys said you need to take the new cat to a shop and have them put in a place for the O2 sensor after the cat. Those threads were all talking about newer model trucks so I am wondering if that also applies to my '91. But what confuses me is that Magnaflow offers a few cats that have places for the O2 sensor to plug in. For example, part numbers:
93516, 94105, 99135, 99235, 99335.

My plan is to take out both of the old cats and put in one of these Magnaflow's (and $ permitting a Bosch O2 sensor). Will one of these cats work with my current O2 sensor? Which cat would be best and would it go well with the Super 44 that is already on the truck?

 

that'll pretty much be the same setup on my 93. except i gutted out my front cat and put a high flow magnaflow where the rear one was. first i cut my rear cat out and put a piece of pipe there and that made a huge difference. but still wasnt loud enough. so i got a k&n cai and a magnaflow high flow and gutted out the front cat. my check engine light comes on every now and then but only when i'm sitting in traffic for long periods of time. my o2 sensor was before both cats.

 

On a 91, there is no after the cat sensor. That only applies to 96-up trucks.
On a 91, I don't know whether you have an O2 sensor for each bank or just a single O2 sensor. Whichever configuration you have now, that's what you need to keep.

 

I got under the truck today to take a look at the set-up. The only O2 sensor is before the first cat, which has a dual inlet and single outlet. Then there is a line that comes into the pipe after the first cat that looks like it is coming down from the smog pump. After that is the second cat, no O2 sensor, then the Flowmaster.

How do you go about gutting the first cat? And would that be better than just pulling it and getting a custom bent Y pipe put in it's place?

 

i cut the pipe right after the cat and took an air chisel to the insides and started it up a couple times to blow it out, its only a temporary deal right now until i get true duals. a y pipe will flow alot better than gutting out the cat.

 

Thank you, I appreciate the advice. I like the idea of getting a Y pipe and a magnaflow. The first cat has a curved pipe and the end of it, so I would probably need a Y pipe followed by a S curve and then the magnaflow.

Now I just need to figure out what is going on with that line coming from the smog pump. If it weren't for emissions I'd rip the whole thing out.

 

double post

 

the smog system doesnt rob any power. IMO it aint worth the trouble for no gains. just make sure you dont break the tube for the smog pump. you wont need a new pipe that goes over the cross member. just weld in the dual inlet end of the y pipe, and if you need to, extend the pipe so it reaches the outlet end of the y pipe.

 

I have the magnaflow too and love it, they are cheaper on ebay but I wanted a warranty (salt in winter kills an exhaust) so I got it here. magnaflow 94105, or you can try one of the vendors here, they are always running sales/specials. either way its a great system and very easy to install. good luck

 

Catco 9513 is a 2 x 2" inlet 1 x 2.5" outlet 3-way high cat that replaces both your front and rear cats. You will not need a Y-pipe, just connect to the existing downpipes.

I am sure Magnaflow makes an equivalent.

 

when i got my truck it had and still has the stock exhaust cut rite before the muffler and it sounds pretty good!! i would like it to be a bit louder though... not that this helps you question or anything but way cheaper than buying a new muffler