Remove cat converter

[Captmike]

Trying to get better exhust sound. May replace it with new performance type. Anyone have info on that, also may just remove it first see what it sounds like??

[2seater]

Quote

Trying to get better exhust sound. May replace it with new performance type. Anyone have info on that' date=' also may just remove it first see what it sounds like??

 

Many moons ago I removed my converter, and the exhaust sound deepened, and it was that way for years, but the smell became intolerable. Fast forward several years, and I installed a Walker universal replacement cat, 2.5" inlet and outlet. I also went to a straight through Dynomax Ultra-flow muffler, also 2.5" with a single 2.75" outlet in stock location. I really like the resulting sound, but the one caveat is; both of my engines are turbocharged, more or less, and that in itself does knock the sharp edges off. For purposes you are looking at, I can give you some rough flow rates from my flowbench tests of various configurations. Not exact figures because I lost some of the data, but the relationship is correct:

 

Stock cat. = 250 cfm

Gutted cat. = 350 cfm

Walker cat. = 375 cfm

Straight pipe = 425 cfm 

 

My theory on the gutted cat. being relatively poor flow is the turbulence caused by the expansion and re-contraction of the flow in a container not designed for it. The other item that is important is removing the restriction in the rear manifold outlet. This is the "before" pic of the latest manifold that I completely reconfigured. They are all like this with a small hole that looks torched out the manifold log.

[Captmike]

 

Many moons ago I removed my converter, and the exhaust sound deepened, and it was that way for years, but the smell became intolerable. Fast forward several years, and I installed a Walker universal replacement cat, 2.5" inlet and outlet. I also went to a straight through Dynomax Ultra-flow muffler, also 2.5" with a single 2.75" outlet in stock location. I really like the resulting sound, but the one caveat is; both of my engines are turbocharged, more or less, and that in itself does knock the sharp edges off. For purposes you are looking at, I can give you some rough flow rates from my flowbench tests of various configurations. Not exact figures because I lost some of the data, but the relationship is correct:

 

Stock cat. = 250 cfm

Gutted cat. = 350 cfm

Walker cat. = 375 cfm

Straight pipe = 425 cfm 

 

My theory on the gutted cat. being relatively poor flow is the turbulence caused by the expansion and re-contraction of the flow in a container not designed for it. The other item that is important is removing the restriction in the rear manifold outlet. This is the "before" pic of the latest manifold that I completely reconfigured. They are all like this with a small hole that looks torched out the manifold log.

 

So my best bet is a cat test straight  pipe....425 cfm...where would smell. come from? I assume u used a straight pipe "for years"?. So there was no additional rumble under car with straight pipe? I am not changing stock muffler...thank you good info

[Ronnie]

...

Stock cat. = 250 cfm

Gutted cat. = 350 cfm

Walker cat. = 375 cfm

Straight pipe = 425 cfm 

 

My theory on the gutted cat. being relatively poor flow is the turbulence caused by the expansion and re-contraction of the flow in a container not designed for it. ...

 

Looks like a gutted converter has better flow than a stock one. How do you think gutting the converter would effect the sound if you keep the stock muffler?

 

The reason I ask is I think my converter has loose parts inside by the way the exhaust sounds. It has a tinny, rattling sound when listening from underneath or driving alongside a wall while accelerating. Gutting the cat would be the cheapest and easiest way to deal with that. I wouldn't mind a little extra sound but I wouldn't want it to be too noticeable. I don't think members of my antique car club would like to be cruising around with a car that sounded like it had one of those tomato juice can mufflers on it. :)

[2seater]

 

So my best bet is a cat test straight  pipe....425 cfm...where would smell. come from? I assume u used a straight pipe "for years"?. So there was no additional rumble under car with straight pipe? I am not changing stock muffler...thank you good info

 

It does make the exhaust somewhat louder, even with the stock muffler, which is what I had at first. Not obnoxious to me, and there is no additional sound from below the car, only out the tail. The rich smell is pretty much just what it seems like. These engines run pretty clean but maybe a little rich which helps keep the cat. hot so it works. With no cat. anything unburned comes right through.

 

According to conventional wisdom regarding a zero loss exhaust system, if there is such a thing, is you need 2.2cfm @ 28"w.c. per horsepower. You can go from there.  Frankly, I like my replacement cat. even though I am continuously fooling with my engines with performance in mind, turbocharging, more compression etc.. The vast majority of the time the engine is operating well below redline, in low stress mode, so I would recommend a modern cat. which flow better than the old tech stuff and are quite cheap.

[2seater]

 

Looks like a gutted converter has better flow than a stock one. How do you think gutting the converter would effect the sound if you keep the stock muffler?

 

The reason I ask is I think my converter has loose parts inside by the way the exhaust sounds. It has a tinny, rattling sound when listening from underneath or driving alongside a wall while accelerating. Gutting the cat would be the cheapest and easiest way to deal with that. I wouldn't mind a little extra sound but I wouldn't want it to be too noticeable. I don't think members of my antique car club would like to be cruising around with a car that sounded like it had one of those tomato juice can mufflers on it. :)

 

The rattling could be one of the heat shields under the car. Some are just pop-riveted to brackets. Gutting the cat. will add a little volume but the stock muffler is pretty large and reasonably efficient, so I would believe it wouldn't be objectionable. I don't know if the hollow can of a gutted cat. would have any sort of resonant or drumming sound or not? 

 

Again, my suggestion would be a universal replacement cat. They are less than $100, and will require a little adapting, and you can do a little larger diameter, although the body is the same. If memory serves, the stock cat. has a ring shaped induction weld forward and rear of the cat. If a chain type exhaust cutter is used, just the outer pipe can be cut and pulled off the thick-walled stub on the inside. They flow plenty for any normal power level in these cars.

[Ronnie]

 

... Again, my suggestion would be a universal replacement cat. They are less than $100, and will require a little adapting, and you can do a little larger diameter, although the body is the same. If memory serves, the stock cat. has a ring shaped induction weld forward and rear of the cat. If a chain type exhaust cutter is used, just the outer pipe can be cut and pulled off the thick-walled stub on the inside. They flow plenty for any normal power level in these cars.

 

The universal replacement would probably be the way to go. Do you have a part number? I understand the induction welds but I can't get it straight in my head how you mean the old cat should be cut off with the chain cutter. Is there a smaller pipe attached to the cat that slides into pipe that goes to the muffler - and one that slides into the pipe that goes to the exhaust manifold? What kind of adapters would be needed to make the new cat fit the stock pipes? Thanks!

[2seater]

 

The universal replacement would probably be the way to go. Do you have a part number? I understand the induction welds but I can't get it straight in my head how you mean the old cat should be cut off with the chain cutter. Is there a smaller pipe attached to the cat that slides into pipe that goes to the muffler - and one that slides into the pipe that goes to the exhaust manifold? What kind of adapters would be needed to make the new cat fit the stock pipes? Thanks!

 

My car is put away for winter, far from home, so I can't confirm. I meant to say the pipe "could" be cut off with the exhaust wheel type cutter, mine is a chain type. It has been almost two decades since I did the job, but I am pretty certain the stock cat has an internal sub-pipe that doesn't show from the outside, which inserts into the flared part of the exhaust system both fore and aft of the cat. If just the outer pipe is cut just beyond the weld away from the cat, it will slide out. The nice thing is the cat can be reinstalled with muffler clamps if you change your mind.. I don't remember the exact dimensions of the flared part of the exhaust but it could be used to advantage if upsizing the inlet/outlet of the cat if desired. There isn't really an advantage upsizing the pipe unless there may be future plans for exhaust modification since the flow area through the cat is exactly the same. 

 

I used something like this one. They are shorter and lighter than stock.

 

https://www.summitracing.com/parts/wlk-15027?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-walker-exhaust&gclid=EAIaIQobChMIjcyisLPX1wIVmUwNCh2JcwKUEAQYAiABEgIPGvD_BwE

 

I "think" the inside dimension of the flared ends of the stock pipes is 2.25". When I first removed the cat, I used a straight section of 2.25"od. stainless steel tubing.  If I have this right, a short piece of tubing fore and aft of the cat will insert into the cat and into the stock exhaust pipes, clamp or weld. I would weld the stubs into the cat and clamp the other. The best clamps are the sheet metal ones that wrap around and don't crush the pipe. Speedway has stainless steel pipe for decent prices.

[Ronnie]

Thanks for all the great information! I will check it out. Installing the cat looks simple enough. I'd have to buy a chain pipe cutter tool.  I want to pull off the pipe from the manifold at the same time to grind out the restriction. All this will be on my winter to-do list when it's too cold to drive.

[2seater]

Thanks for all the great information! I will check it out. Installing the cat looks simple enough. I'd have to buy a chain pipe cutter tool.  I want to pull off the pipe from the manifold at the same time to grind out the restriction. All this will be on my winter to-do list when it's too cold to drive.

 

Yes, a very good time to remove the restriction. I have never ground one out from under the car, only on the bench. I have done it with a good hole saw or a die grinder. The hole saw should self guide and fit just inside the pipe, something like 2 1/16" but I don't know if there is enough room to get that in there under the car. A 1/4" die grinder with a 3/8" carbide  will make short work of opening it up but the pipe is a 400 series stainless steel and is pretty tough. Don't be afraid to remove the O2 sensor bung completely and take the whole thing out to the full diameter, all the welds are on the outside, so no danger of leaks.

[Ronnie]

I  hope the manifold opening is accessible from under the car.  I have a set of large burrs like these. Some of mine are worn or missing but one of them should do the job. I don't have a hole saw that size that would cut metal so I might not be able to get the bung that protrudes into the hole for the O2 sensor cut out. I think my hole saws are only for wood. I'm going to try to find a buddy that has a chain pipe cutter before I start. I hate to have to buy one that will probably not be used again for years. I think I know someone who has one. I usually have some nice days to drive all the way up until January so it will be after that before I take a look at it. Thanks for the tips.

[ski.dive]

Since my Reatta is CAT delete, [ yes I smell the exhaust= because it's CAT delete ]

 

does the O2 sensor by the rear spark plugs serve any purpose now??

[Ronnie]

does the O2 sensor by the rear spark plugs serve any purpose now??

 

Absolutely. The ECM depends on feedback from the O2 sensor to accurately adjust the fuel mixture to keep the engine running it's best.  If you have never installed a new O2 sensor (or know that someone else has recently) you should change it. I consider an O2 sensor to be a maintenance item. I always install a new one when I purchase a used car without knowing the last time the O2 sensor was replaced. JMHO

[2seater]

I agree with Ronnie completely. While maybe not a cure, it will make sure the exhaust is as clean as it should be. Probably help fuel mileage at the same time.