Ok, the 87-92 Stangs came with 4 cats...2 on each side. I was wondering if you were to get an aftermarket X or H pipe and only get 1 cat on each side, would it still be able to pass emission testing? I have always wanted to know this.
yes it will sound louder and if you buy the aftermarket H or X from like BBK or some other company with only one converter on each side it should still pass unless you have other things horribly wrong with your engine or emission system.
It depends on where it's being tested and even who's doing the visual. There are some locations that are looking for a even replacement cats that mimick the factory set up -- that is, if they're looking for 2 cats on each side and they find only one, you've got a problem.
There are other locations that only care that the cats are there and that you pass the sniffer.
Beware - there are different cats for different jobs. If you're replacing 2 cats with one be sure it's a "3-way" cat you're getting - one that's set up to treat HC, CO and NOx. Both of the factory cats are needed to treat all 3 emitants - the first and second cats are different from each other.
I have not had a problem here in California. I believe that the rule says that you can only change cats when the car has a certain number of miles, and the cats are diagnosed as broken or inoperable. But I am unaware of any standard for what constitutes a "replacement" cat at that point. I don't know that a muffler shop is bound to install exact replacement convertors, or if any number of aftermarket ones of the same general type will do. The same goes using for 4 versus 2. I have a hard time believing that an authorized shop asked to replace worn out cats would go to the trouble of welding in 4 cats, if they could even do it. I think as long as the smog test guys see an actual cat in the pipe under the car, they proceed with the tailpipe sniff. I have been running a high flow 2.5" cat h-pipe with only 2 cats for 6 or seven years now, which is at least 3 smog test cycles, and have not had a single issue. Plus, I pass with flying colors, so it's difficult to say there is even a meaningful difference. Indeed, my GT40 head/E303 cam/MSD6A/Mac shorty/BBK h-pipe/Flowmaster '90 LX hatchback blows cleaner out the pipes than my identical but completely stock '90 LX convertible, which I attribute at least in part to the fact that the BBK pipe has only 15k miles on it, while the stocker has 135k miles on the original cats. I think you are actually doing the air a favor by upgrading rather than continuing to drive on worn out cats.
Good catch Sam - I missed the sig completely - how a car that's originally efi which has been converted to carb is gonna pass the visual let alone the sniffer -- well, I suspect some sort of bribe is the only way to a 'sticker'.