1990CONV

I have a 1990 6 speed Convertible that I bought about 24 years ago. It has 31K on it and is flawless. Over the years I have added upgrades from light engine mods to exhaust. It currently has an oversized exhaust system with no cats. The center cat is still in place but it is gutted. The mufflers are Walker full stainless steel models that are no longer available. I like the mufflers because they have dual square exhaust tips and tuck up very tight under the body. I also have removed the emission air pump. I have no desire to reinstall the air pump so lets leave that topic alone. I have no problem getting the car inspected. The car is very loud. I would like to quiet it down a bit. My thought is to reinstall a new center cat. My concern is that there will be no upfront primary cats and no air being injected via the air pump. Will that center cat get too hot due to the amount of raw exhaust gasses? I suspect the cat would tone the car down a little. Has anyone ever tried this setup? Thanks

MatthewMiller

I don't know the answers to your questions about installing a center cat. However, how about fitting a large center muffler instead? If you have a single exhaust at that point, then a straight-through muffler with perforated pipe and filling wouldn't restrict flow and would definitely quiet the car somewhat. If you have dual exhaust right now, you could also try a muffled x-pipe. I think multiple companies make these now.

65Z01

I thought the primary purpose of the air injection was to warm up the cat quicker.

1990CONV

I have read conflicting write ups on the design needs for cat air injection. I wish someone with firm knowledge could add chime in.

MatthewMiller

It's there to help burn any unburned fuel that gets past the exhaust valves, because that fuel will ruin a cat pretty quickly. The need is typically highest at startup. On OBD2 cars it is also used by the ECM to periodically check the functioning (efficiency) of the cats, but that doesn't happen on an OBD1 car like yours. I guess yours was a belt-driven air pump, right? On my OBD2 car it's electric and runs on demand (or it did, when it was on the car...).

84 4+3

The air pump helps burn the extra fuel in openloop at startup so the cat doesn't become clogged. If I'm remembering my reaction engineering correctly, increasing flow in the catalyst increases reaction and thus heat. If you have more air it operates more efficiently to a point, there is a stoich ratio for every reaction where you hit max conversion for flow and operating temperature. A good cat doesn't need an air pump and a good cat won't hurt power in any appreciable way. It won't quiet it down much but it will deepen and round out the tone making it seem quieter... itll calm down the bite. Many guys delete the air pump on my truck because they are an expensive failure point. The SAIS system. GM also removed them later down the line so you'll probably have now problems with a modern type catalyst loading.

Tomorrow I'll ask my buddy who works with this way more than I do for a second opinion. I think you'll be okay as long as warm up time isn't excessive.

aklim

I have tried a gutted cat and it was noisy. I had a Random Tech cat that I have used for over 10 years. I just replaced it with a Magnaflow one because RT is out of business and my welds rusted and the pipe came apart. My AIR system fell off about the time I had the CAT so....

RetroGuy

If you aren't required to have a cat in your state, you might try the Magnaflow X-pipe resonator (non restrictive), see Magnaflow's site. It will be a tight fit, but doable for anyone/place with welding & pipe bending knowledge.

billschroeder5842

IHBD is correct. A center "something" (resonator, muffer, functional cat) will quiet the car down considerably.

If it were me, I'd go with a Hi Perf cat. The ones today are very low restrictive and it keeps you legal (for possible resale?)

steven mack

A pair of Walker quiet Flow mufflers will help with the noise and drone quit a bit.I put them on my last Vette and the difference was amazing.

C6_Racer_X

A resonator or "glass pack" type muffler in the location of the center cat might be a better choice.

1990CONV

Thanks to all. It appears I hava two fairly good choices. One is leave the front two small cats off and replace the center one with a more modern high flow cat. This will deaden the sound slightly. The second choice is to repalce the gutted cat with a similar sized straight thru small muffler. If I go this way I would like fabricate it out of stainless. The problem here is the expanded end with the flange. I would have top mimic that. That might be a challenge unless some has a good solution.

C6_Racer_X

Several solutions. Take the existing cat with the flange to an exhaust shop (or welding shop), along with your stainless steel resonator/muffler of choice. Have them cut the pipe with the flange joint off the existing cat weld it to the new muffler for you. That's the best solution, although that pipe piece with the joint might not be stainless (I'm not sure).

You could also cut off the mating part of the flanged joint and just use a butt joint or lap joint with a proper clamp. The only "proper clamp" for exhaust pipes is the wider, two bolt band clamps like the Walker Mega Clamps or Dynomax Performance clamps. I'm not absolutely brand loyal, but those brands are the correct shape/type of clamp.

If you end up with clamped joints, the wide (two bolt) band clamps. They are available for both "lap joints"/slip joints (where one pipe is slighlty expanded and slips over the other) and "butt joints" where there are two pieces, the same outer diameter, that just touch. Summit has the "dynomax performance" stainless ones from Walker (here https://www.summitracing.com/search?...7%2B4294842029 ). There are also the Walker "mega clamps" in aluminized in both styles. I usually use Summit to find the part numbers to shop on other sites and at the FLAPS because they have the easiest search features for those parts. Sometimes they have the best price, too, but I keep them honest.