Shackleford

iTrader: (12)

I told him I have a couple of hi-flows (3" Magnaflows) I need put on, and he told me now days the factory cats are better and that Magnaflow and whoever else is full of ****. lol. Is he right? It's actually a reputable shop, too.

My car also has 128K miles on it. I was thinking maybe it's time to replace them anyway.

Black01M6SS

iTrader: (3)

I doubt the stock ones are better for performance but we had to take a set of new Magnaflows off of a car about a month ago. The cats took to long to heat up and start working. That would cause a catalyst inefficency code to be set.

Shackleford

iTrader: (12)

I had a 3" Magnaflow put on my old LT1. The stock one was clogged up.

IROC403

iTrader: (2)

that could be true if the stock ones were 3" dia. but there not.if you have manifolds still then your outlet is small and better cats would not help.

Fifedogg

iTrader: (15)

I find that hard to believe

Shackleford

iTrader: (12)

Why???

RPM WS6

iTrader: (3)

"Better" is a realitive term. When it comes to longevity and filtering, that guy is absolutely correct; in that respect the stock cats are superior to anything the aftermarket puts out. Now, that's not to say that stock cats flow better (they don't) or are better for performance (they aren't). But when it comes to life span, and actually doing their job (filtering exhaust), a modern factory cat is better than the aftermarket stuff.

Shackleford

iTrader: (12)

So, should I even replace my factory cats with the Magnaflow 3" high flows?

FirebirdPhil

iTrader: (8)

Throwing in an aftermarket cat won't give you much more than 5 HP if you're lucky. The factory ones are made much better now than what they were in the 70's, 80's and early 90's. Like RPM WS6 said, the factory ones filter way better, but the aftermarket ones have superior flow.

RPM WS6

iTrader: (3)

That depends. What's the reason you're wanting to replace them? Getting codes for a bad cat? Or just looking for a HP increase?

Shackleford

iTrader: (12)

Well, I'd like a little horsepower increase. But if they're going to go out in a not-so-very-long time, then I would think twice about putting them on. My front oxygen sensors are going out, too, so the consensus was in another thread. How long would the aftermarkets last?

RPM WS6

iTrader: (3)

I wouldn't necessarily say that they'll definitely fail in a short time, but it's possibile. I've seen a lot people here have Magnaflows go out in short order, but those are usually cammed cars that dump way more fuel into the exhaust which tends to burn up any cat quicker.

If your engine is stock, and running well, I'm sure they'll last for some time. But don't expect anywhere near as long as your 100k+ mile stockers have already gone.

Other thing to consider is price. I think you said that you've already got the Magnaflows? If so, you'd probably lose some money selling them, then pay a WHOLE lot more to get stock cats from GM.

Moral of the story: The GM cats will last much longer, but cost a great deal more. So if you keep the car long enough, the cost will probably equal out. As for power, if you're sticking with stock manifolds and a stock Y-pipe, then you'll probably not see any significant or noticeable power gain with the aftermarket cats.

What I would do: Assuming your current cats are still fine, I'd just replace the O2 sensors and be done with it. No need to replace factory cats for a stock manifold car unless they're bad.

Shackleford

iTrader: (12)

Well, I only paid $110 shipped for these on here. I do plan on keeping the car for a long time. I don't think my cats are bad either.

Black01M6SS

iTrader: (3)

That is basically what I was trying to say. I'm a Ford Auto tech. The cars I've seen aftermarket cats on them were not performance cars. The people had replaced their bad factory cats with aftermarket ones because the aftermarket ones are much cheeper. Factory cats for Fords usually run around 700$ each.

Just a couple months ago I diagnosed an SUV that had bad cats. The owner declined the 1500$ estimate for replacement factory cats. About a month later he was back with the check engine light on again with the same problem. The new Magnaflow cats he had were not doing a good enough job filtering the exhaust at idle and it was causing the catalyst inefficency codes to be set. The cats would work after they got hot but if the rear o2 sensors are still switching like the front after set amount of time it well set the code. That was the case with this vehicle and the man ended up having the Magnaflows replaced with factory cats like I suggested the first time. It is likely the muffler shop is trying to avoid this same situation.

JScamaro

iTrader: (6)

Muffler shop guys don't know crap about our cars. Take what they say with a grain of salt.

Shackleford

iTrader: (12)

It seems several guys here are corroborating his statements.

RPM WS6

iTrader: (3)

Then I'd resell the cats and just leave the stockers alone. I doubt you'd even see a MPG gain with the aftermarket cats (much less a performance gain, considering the stock manifolds & Y) unless the stock units were bad.

Damn, just imagine if someone told him that for the mere price of a couple O2 sims, his problem would have gone away.

While I agree that you shouldn't take just anything you hear from a muffler shop as gold, in this case I agree completely with the muffler guy.

99345hp

I read an article years ago, and they had flow results from many different cats. The stock f-body cats outflowed all stock cats they tested by far. They flowed equally as well as the aftermarket ones they tested. I cant remember which ones they tested or any of the specifics, but i remember them saying the stock f-body cats were the best out there.

Johnnystock

iTrader: (6)

man, this info is new to me^ never thought the stock were better.

On the other hand, I can tell the OP, that my SLP dual dual are in since 3 years(+all years of the before owners) and no problem so far even if I'm H/C/I and has been running pig rich before my tune(2months ago).

FlashLCD33

iTrader: (8)

I'd keep them, and when you get longtubes/y pipe have them put on. I wouldn't replace stock right now but might as well start fresh when you do more mods. Then, sell your stock cats and get $140 for them.