In the process of chasing down a weird exhaust sound I have deleted the resonator, then the secondary cats and am preparing to replace the primary cats in hopes of curing my problem (detailed in another thread I started regarding 'chambering'). I do not wish to spend $1500.00 to install new Mercedes cats, so I am going to have the place that deleted my secondary cats replace the primaries.

My problem is that I don't know exactly what to specify. Has anyone replaced their primary cats with aftermarket catalysts and had no problems? I assume the downstream O2 sensors will have to be installed in the pipe aft of the new cats to prevent a CEL; does that ensure a trouble-free swap or must the O2 sensors be located more precisely than just 'after the cat'? The shop does good fabrication work but do not work on European makes, so I need the computer to be happy. After reading the threads returned via searching, I still don't see a consensus on what cat is best. Can anyone recommend a size and/or brand? Magnaflow? Thanx a bunch, guys...

 

From what I've been reading you may be able to get away 300 cel primary cats and at the same time keep your O2 sensor from freaking out. Or, you could get a tuner to take care of any CEL in advance of replacing your primary cats...
Do a search for "hooleyboy", "AMS" and "cats" as those two sponsors frequently discuss this topic...

 

I was looking into random technology highflow cats, ones similar to those that Mike (Hooleyboy) was/is running on his car. I have read in the past that some people with aftermarket primaries had their cats go back relatively soon. I think it had to do something with a lot of heat and using the wrong substrate (not sure which one it is SUPPOSED to be for our car, but i think there is ceramic and metallic)

and you might be ok with 200 cell cats too.

 

Theres very little reason to delete the primaries, plus b/c its such a tight area its tough to get a good solid full bead weld in there.

If the "weird noise" you are describing is rasp, then that's because you deleted secondaries, anytime you do that you will get more rasp which is why I always recommend getting 300-cell metal core secondaries and leave primaries alone, then getting a shorty header. That's usually the best setup without completely cutting out the primaries. The primaries are actually less restrictive than the secondaries. Supposedly it was rumored that the primaries were metal core but they aren't (at least not on the older AMGs up to 2002), they are the same standard 400cell+ grid like ceramic substrate as your standard magnaflow cat or etc (and your stock secondaries for that matter)

The biggest restriction is the stock exhaust manifold, the primaries are tiny and choke off the engine. If you really want more power get a decent set of shorty headers and a set of high flow secondaries.

If you do high flow primaries, with no secondaries you probably won't like the sound as it may be too raspy and unrefined for your taste, just something to consider if you go down that route.

Hope that helps,
~AMS~

 

the sound? And have you checked cat efficiency? Doing deletes is going to fail you when you need to get smogged and most likely drop power as well.
Test before removing. Either with the thump or with an IR temp gun.
Magnaflow makes some nice 49 state units as well.

 

hmm. very interesting. what brand, cell count, and size high-flow secondary cats would you recommend if you do have a resonator delete and shorty headers on the C32? would 200-cell be too low?

 

secondary cat delete will not result in the C32 failing emissions testing, unless visual tests are done. They have also shown no drop in power. In fact, slightly better flow by replacing it with a less restrictive pipe. Gains from deleting the secondaries are really only noticeable (though still minimally) once the exhaust manifolds have been upgraded to tubular headers and resonator deleted

 

Magnaflow does offer a spun cat that has a metallic substate I've seen the used with relativity good results. There is more power to be had my chopping out the 2nd cats. (DONT CHOP OUT YOUR CLAMPS). With an aftermarket cat like a 5900 series spun magnaflow cat set up that replaces your primary cats you will get a little more cabbing noise down under the front floor boards

My general take on the 2nd cats is(I could be wrong)... I think they are restrictive because of the high cell count and they are far away from the engine. It makes me think are they just not getting hot enough to do the trick?

I'm so exhausted from exhaust talk. Its seems to be my life these days.

 

exhausted from exhaust talk? haha.....mike ur funny

 

200 cell Magnaflow cats on mine with no issues. Been running them for probably close to two years now.

 

No issues!

 

For a good set of 300-cell metal core cats ($180 shipped for the pair):

http://cgi.ebay.com/ebaymotors/High-...Q5fAccessories

 

so these can replace the secondaries on the C32? what would be the advantage of putting a higher flowing secondary cat there (such as this one) compared to just a straight pipe? is it just a sound thing or is this a performance thing too?

 

Hey Silverbullet how much drone do you have with just the magnaflow cats?

 

It really only drones in 1st gear between ~2k-3k rpm with moderate acceleration. It isn't much louder than stock cats as far as I can tell.

 

It has to just be for sound. Doesn't make sense to get any performance by adding a cat in place of straight pipe, in our application at least.

 

I am not deleting the primaries. I am replacing the original primaries because I think at least one of them is the source of a loud sound similar to an exhaust leak. The sound was present when I bought the car and all the original factory exhaust components were in place. It sounds like a bad exhaust leak when the engine is under load, but not at idle or steady-state cruising or deceleration. I have had the car on the lift at 5 different shops now (including 3 hours at the dealer) and it is definitely not a leak.

I deleted the resonator and the weird sound did not change. I deleted the secondary cats and the weird sound did not change. It comes from the area of the front cats but is not specific to any one spot. My next step is to replace the front cats. I don't want to pay $1500.00 to replace the front cats with new Mercedes units, so I am seeking advice on which aftermarket catalytic converters are best suited as a replacement. My main concern is eliminating the weird, loud, embarrassing sound coming from under the car and doing it without triggering a CEL. Any performance gains are a bonus but not my primary concern.

You are correct that deleting the secondaries added a raspy quality to the exhaust note. I will likely install a new resonator to quiet down the rasp after I find and eliminate the weird sound (sounds like a leak but isn't a leak).

Thank you very much for your insight here!

Do you recommend these as a replacement for the factory-original 400-cell ceramic-core primary cats? Again, thank you!

 

The sound is described (for better or worse) in my post above this one. I was unaware one could check the efficiency of a catalytic converter. Not one of the five exhaust shops I have visited mentioned it (including the dealer). Can you elaborate on the 'thump' method and the IR temp method? I have access to an IR camera...

 

Thanx for the advice on the 5900 series magnaflow. What is a 'cabbing' noise? The purpose of my whole exercise is to eliminate a peculiar sound; I do not wish to introduce a new sound. Thanx again!

 

You may want to check and make sure your headers are tight as well.

 

Sweet! Did you just relocate the downstream O2 sensor into the pipe aft of the new Magnaflow cats? Did you go with all new pipe or just replace the cats? Have you passed a smog test with this setup? Do they smog you down there in Mississippi?

 

You know what it is? Its a rather funny typo! lol "cabin noise"

My old Random Tech cats made a loud noise down by my feet. So they may not be the ones for you then.

 

Great! What setup are you running?

 

The exhaust manifolds are tight, both to the heads and to the downpipes. Nobody can find a leak even using an auto stethoscope with the car being powerbraked up on a lift. I stood under the car yesterday at the dealership with 2 mechanics and the service manager and we could not find the source of the sound. Everybody is stumped, and the only recommendations I can get are to replace the primary cats. It has become an easy recommendation to make since the resonator and secondary cats are gone.

 

Ha! You are 'exhausted' aren't you!

Thanx for the heads-up on the Random Tech cats; I am definitely trying to get ALL the sound to exit from the rear of the car...