Exhaust / Cat issue

[Ken Mikos]

Recently purchased a 1985 911 Cab with 46K. The issue in question :
1. I need to slip the clutch when starting out as the car has no power when pulling out in low gear. Once its underway the car pulls strong.
2. The exhaust sounds like it is blocked. When driving at a steady 3k RPM the exhaust note changes (louder) sounding like it was unplugged and then goes back to the more muted tone
3. I get a very random backfire once in a while between shifts (unspent fuel from a clogged converter?)

I am leading to believe that the Catalytic Convert could be clogged or the Muffler has a baffle that has broken lose.

Where do I start
1. Remove the convert and look for an obstruction
2. Replace the muffler
3. Install a tester pipe

Any recommendations would be appreciated.
Thanks, Ken

[Bill Verburg]

Originally Posted by Ken Mikos

Recently purchased a 1985 911 Cab with 46K. The issue in question :
1. I need to slip the clutch when starting out as the car has no power when pulling out in low gear. Once its underway the car pulls strong.
2. The exhaust sounds like it is blocked. When driving at a steady 3k RPM the exhaust note changes (louder) sounding like it was unplugged and then goes back to the more muted tone
3. I get a very random backfire once in a while between shifts (unspent fuel from a clogged converter?)

I am leading to believe that the Catalytic Convert could be clogged or the Muffler has a baffle that has broken lose.

Where do I start
1. Remove the convert and look for an obstruction
2. Replace the muffler
3. Install a tester pipe

Any recommendations would be appreciated.
Thanks, Ken

This, remove the convert and look for an obstruction

I've seen multiple cat failures, the substrate will often fail mechanically, you usually hear rattling but it depends on the failure mode

[Ken Mikos]

Thanks for the reply, will do and post a reply

[Ken Mikos]

If the cat is obstructed, do I replace it with another cat or bypass ?

[darrin]

Originally Posted by Ken Mikos

If the cat is obstructed, do I replace it with another cat or bypass ?

yes

More specifically, You'd need to replace it with a cat if your car needs to be smog-legal/smog tested. If you don't NEED/want to be smog legal, you can replace it with a pipe or european pre-muffler OR can hollow out the cat to remove the obstruction and make it into a pipe.

I'm very happy with the active cat exhaust I purchased from M&K - check out the bottom of this page https://mkexhaust.com/911-2/ for options

[Avanti]

If you install a cat bypass the exhaust note will be a little louder.

[Lyle O]

Originally Posted by Avanti

If you install a cat bypass the exhaust note will be a little louder.

...and the car will run a lot better!

[Flat Six]

Ken, where are you located?

[Ken Mikos]

Thanks for the advise..... Leaning toward a cat bypass.

Flat Six, i am located east of Pittsburgh PA.

[josephvman]

It will also get rid of a lot of weight and heat in the engine bay/bumper/deck.

Originally Posted by Lyle O

...and the car will run a lot better!

[Arne2]

I noticed no power difference in my previous '84 between the catalyst and a pre-muffler. No difference in sound either.

But a huge difference in smells. With the catalyst removed the exhaust stink was bad, smelled the garage up something horrible. Probably has something to do with the chemical stew we have these days instead of real gas. But in any case, the catalyst cleared that all up, no more smelly garage.

[shawn908]

Originally Posted by Arne2

....a huge difference in smells. With the catalyst removed the exhaust stink was bad, smelled the garage up something horrible..

Interesting, second time in a month I've read a "no cat the exhaust stinks" comment. Just as an observation if you consider removing the cat; I had a "cat" on the 3.2 in my car and in the 5 yrs or so I've owned it never noticed the exhaust as WOW that stinks. I recently discovered the "cat" was nothing more than a hollowed out fat pipe. Exhaust has not smelled "bad". I wonder if related to brand/grade of fuel?

[darrin]

Clogged catalytic converter?

This thread has a few informative suggestions on how to confirm/rule out a clogged cat, including a) measuring its temp (after engine's warmed up) at the front and back -- if front significantly hotter than back, good sign cat's clogged or b) drilling a hole into the very front part of the cat and measuring the level of back pressure with the engine running (problem if over 5psi) and then sealing the hole with a screw if the Cat's OK

[Flat Six]

Originally Posted by Ken Mikos

Thanks for the advise..... Leaning toward a cat bypass.

Flat Six, i am located east of Pittsburgh PA.

I have an extra, low-mile cat I'd loan to you if you were in the Las Vegas area . . . .

[josephvman]

That’s strange. When I first installed my cat bypass I had that smell for the first few weeks but it has gradually diminished to no smell at all after a few months. I assume this is from heat cycling and better sealing. I’m very sensitive to smell and nothing now.

Originally Posted by Arne2

I noticed no power difference in my previous '84 between the catalyst and a pre-muffler. No difference in sound either.

But a huge difference in smells. With the catalyst removed the exhaust stink was bad, smelled the garage up something horrible. Probably has something to do with the chemical stew we have these days instead of real gas. But in any case, the catalyst cleared that all up, no more smelly garage.

[Ken Mikos]

Thanks for the clarification Darrin.

[Arne2]

Originally Posted by josephvman

That’s strange. When I first installed my cat bypass I had that smell for the first few weeks but it has gradually diminished to no smell at all after a few months. I assume this is from heat cycling and better sealing. I’m very sensitive to smell and nothing now.

My '84 had a pre-muffler on it when I bought it. Premuffler was install several years prior to when I got the car. I drove it that way for over a year, the smell was bad the whole time. Went away immediately when I reinstalled the original cat (which came with the car in a box).