After driving 200 miles, most of them offroad, on my way home on the highway my exhaust fell off at the cat. I suspect it was my fault, weak clamps and me putting off getting it welded for too long.. but anyways

It was before payday so I just threw on a Thrush turbo muffler and some straight pipe. I was shocked at how much more noisy it was, almost like the Muffler didn't make much of a difference without the Cat. I don't remember my 1978 cat-free or my 1980 cat-free being so loud, one had cherry bomb glasspacks and the other had cherry bomb turbos, but both had true-dual sub-2'' pipes.

So I'm split.. honestly, how much performance do I gain without the cat? My local area isn't going to do anything about being cat-free, one time a cop gave me a hard time but half the vehicles in this town are straight-piped or cat-free, so I think I can get away with it. Even though I hate noisy exhaust and noisy vehicles, I'm sort of enjoying the way the exhaust sounds.. heck, it's quieter than my tires on the highway anyways.

The main question is, I dunno if I do put the converter back on, if I should try to find a universal cat without the fresh-air line so I can get rid of my smog pump, whether I still need fresh-air into the heads if I bought one without the fresh air line, if I should go back with Magnaflow or if there is a better brand for around $100.
I sort of like keeping original equipment.. I still don't understand that if they can make a cat without fresh-air intake, why they even bother with making cats with fresh-air intake, which is making me sorta iffy about buying one without the fresh-air intake.

 

Performance gains resulting from the removal of the catalytic converter are a misnomer. (And I KNOW I'm gonna get arguments over that comment but its true and if you understand how they work you understand why its true). IF you see marked improvements in performance after removing the catalytic converter, it NEEDED replacing anyway! If you have ever seen even a piece of the substrate that is inside a functioning cat, its obvious why performance loss can happen quickly especially if the engine has ever spent any time running rich. Soot and crud will build up quickly in the tiny passages causing the cat to clog thereby reducing flow. HOWEVER, a clean, clear substrate is completely passive to the flow of exhaust... that's why its so much bigger than the pipe itself. If it were the same diameter of the exhaust pipe and filled with the honeycomb-type substrate that is in there, THEN yes, there would be a flow reduction. This is why it is critical to make certain that when/if you DO replace a catalytic converter, it is sized properly to allow sufficient flow. Oversizing is possible but go too big and it won't heat up properly. Its physics.

Catalytic converters that require the fresh air intake utilize the fresh air to heat up further by allowing the crud that has started to heat up but not fully burn in the combustion cycle to continue to heat and expand. Older cats were designed in such a way that they needed the fresh air to achieve the necessary temperature to burn off the crud they are supposed to burn off. (Combustion trilogy = heat + fuel + air (oxygen)). Adding fresh air adds oxygen which is why the cat is DOWNSTREAM of the O2 sensor. (OBD-II put another O2 sensor downstream of the cat specifically to monitor the amount of oxygen consumed BY the cat so that the system could adjust things even better if there wasn't enough oxygen left AFTER the exhaust made it through the cat). Newer catalytic converters are more efficient at retaining heat and reaching the higher temperatures without the need for the fresh air intake. Which ever type cat you have/install, the same is true for BOTH types... they get significantly HOTTER than combustion cycle temperatures. This is the ONLY way they can burn off crud that doesn't burn off in the regular combustion cycle.

 

Ya know you might be on to something there, the stock cats often last late into the vehicles lifespan when the motor starts to consume more oil and various other fluids so I can see that producing more buildup in the cats. I have always seen noticable gains when swapping the stock cats for an aftermarket unit.. along with other exhaust changes, but like you said maybe it was time anyway.

Agreed there too, I see no reason at all not to at use aftermarket cats with a custom single or dual exhaust system, they really don't produce any flow restriction and really do clean up tailpipe emissions.

As for your questions about using a cat designed for air injection or one without, I have deleted the complete smog(air injection) system on my truck and have been using a non-air universal cat for many years with great results, the truck even passes the local tailpipe sniffer test.

 

I went with a single 3 way Magnaflow cat with no oxygen tube when i did my headers. The cats on my stock set-up were terribly gummed up.

I would go ahead and put a cat like the Magnaflow (stainless steel construction) on, just because you can pass inspections now, doesn't mean you won't move or they wont adopt cat inspections.

 

I don't think I was seriously considering keeping it off.. if the noise didn't make me hate driving my truck, the new smells would. I wish I could see side by side comparisons of some of these cats though, just to see if they do make some kind of difference from brand to brand. The Magnaflow parts feel really cheap to me, much cheaper than factory and I dunno, just sorta don't trust them.

So what do you guys think about fresh air into the rear of the heads? I know it doesn't take long to clog even a brand new cat if it doesn't warm up, or your truck runs too rich for it? I'm seeing some negative reviews for the tube-less universal Magnaflows where people complain that they clog and break up after 3000 miles.

Also considering a single in, dual out cat with dual 2.5'' pipes and dual mufflers, I bet I wouldn't be able to hear the Bronco at all even when I floor it. I'm the kinda guy that wants to hear his motor, not his exhaust. With 2.25'' , Magnaflow cat and thrush turbo, it was quiet.. but I had some deep reverb while sitting idle that I didn't like.

and grey, great point about the oversized pipe for the cat, never thought of it that way. I know I definitely feel like it has more power, but I also did when I ran without a muffler, I just always assumed it was a placebo effect because of the louder exhaust..
like those fools that throw on 3'' exhaust on their 1.8L Civics and idiots with 5'' smokestacks on gassers, or even the diesel people with pipes twice the diameter of what you'll see on a Semi truck. I swear I think exhaust is one of the most overrated aftermarket performance 'improvements' sold.