Catalytic Converter

[dst2680]

The converter on my '75 deVille is the original pellet type, and I understand it is quite restrictive. I would like to lose the converter entirely, however I know it's illegal. I've been looking at high flow "universal" converters, but then when I check the compatibility it says it doesnt fit my car. What are some of your experiences with replacing these, and do you have any recommendations for less restrictive alternatives? Does it really make a difference in terms of power? Thank you!

[R Schroeder]

I ordered my whole system from NAPA , for my 78 DeVille.
New style CAT, and regular pipes. Try them.
The CAT was an exact fit.

Roy

[dst2680]

Roy, I have tried NAPA and many other places. Exhaust parts for the 75 are really difficult to find apparently. Thanks for the suggestion though!

[TJ Hopland]

I think what is unique to the 75-78 Eldo is the cat bolts to a cast T manifold so its got a flange.   Most apps in the era the crossover pipe came down there and the cat slid over that.     Probably not enough people out there with 75-78's to make it worth the trouble to make one.   

If it was me I would go to a custom shop and have them weld up a nice smooth Y pipe to dump into an aftermarket universal cat.   That cast T thing can't flow very well.   The 71-74 was a little more of a Y.     

["Cadillac Kid" Greg Surfas 15364]

If you read close enough in the service manual for 75's and 76's you will see them talking about a "test pipe" (somewhere it had a tool part number) to be installed in place of the cat to test the system when a bad cat is suspected.  This was a flanged pipe that directly replaced the bolt in cat on these cars for "temporary" use.  I have seen several of this era cars with these "temporary" pipes still installed. Still testing I guess.
Greg Surfas

[Barry]

My question has always been , does complete removal of the cat add any performance or additional exhaust noise. I like mine to be as quiet as possible. B. Norman

[TJ Hopland]

On an otherwise stock and typically driven Cadillac I would not expect a huge difference in performance assuming your existing cat is not plugged up.   With higher RPM engines there seems to be a larger difference in the sound but I have not noticed as much change in the larger slower turning engines. 

If all is well now I would not mess with it.   

[66 Eldo]

The original AC brand Cats on the late 70s GM vehicles were serviceable. There is a metal fill plug which can be removed, the pellets emptied and new ones installed to "recharge" it. Many of these cats were short-lived due to rich air/fuel mixes from the non computer controlled carburetors. Some of us may remember "rotten egg" odors from the exhaust on early catalytic converter vehicles. This was from too rich of a fuel mix. Ford was notorious for this.

So...from what I have heard, you can empty the pellets in your cat to reduce back pressure. I can't remember if the plug is on the bottom and is visible. I think it is so just take it out and start the car to blow them out. Careful, they will be very hot and could start a fire if dry brush is nearby. Also, this violates any emission requirements and laws.

As a side note ant to follow-up on TJs comment. The 75s and '76s had a notable exhaust hiss under wide open throttle. My friend's dad had a new '75  and he drove it hard through our neighborhood.

Barry, usually removal of the catalytic converter will make the exhaust note louder but will improve performance and fuel economy. An aftermarker high-flow cat will keep it legal but may not be as quiet as the original cat. Looking at the list of cars you own, your '76s will have the BIG and HEAVY AC cats I mentioned above if they haven't been changed. If you don't need the cats for state emissions but want to improve fuel mileage or performance AND want it quiet, maybe a bullet style resonator, glass pack or small muffler (all straight through type) may keep it as quiet as you like but add performance. I'm not a muffler specialist so ask one about this approach.