Adding a Catalytic Converter

[EV911S]

I have a 1976 911 with 2.7L CIS. The engine has been rebuilt and upgraded, 11 blade fan, chain tensioners, time-certs, head studs, new p&cs, new valves, etc. The thermal reactors were replaced with early heat exchangers and the air pump was removed.

The car runs well. Idle is 3.5% CO or 13.2 AFR. Cruise is low to mid 14’s AFR and WOT is 12.5 AFR as measured by my wideband O2 sensor.

Without the emissions equipment, the exhaust fumes are strong especially at idle. I would like to reduce the odor to make the car more enjoyable. I am considering adding some type of Catalytic converter to the exhaust to clean it up. I could either use a later stock 3.0L exhaust / heat exchanger set up wit a cat or do some type of custom set up off on my current heat exchangers.
Would this reduce exhaust odors? Has anyone ever added a cat to an early car? Any tips, pros, cons, etc.

I would like to be able to pass emissions test as well and use a standard registration, right now I have Historic which makes it emissions exempt. While the visual would be off, which I don’t think is an issue here in NJ, I figure the new cat would be as clean or cleaner than the thermal reactors and air pump were.

Erik

[mppickett]

One I like is the Magnaflow 59956 (less than $100). It flows well, is durable (metal substrate rather than ceramic) and is small enough to squeeze into tight spots if you're doing a custom mod. Cleans up the gas smell pretty well. You can get this with different inlet/outlet sizes. Here's the info:

2.5 " ID 300 cell density Metallic Substrate Universal Catalytic Converter Magnaflow 59956

Description: 2.5" ID ( 4" OD Body) High Flow 300 cell density Metallic Substrate Universal Magnaflow Catalytic Converter 59956. 3" Version flows in the 596 CFM range! Really unleashes any flow barrier in your Catalytic Converter.. 4" compact body for tight fitting areas.. Metallic substrate offers considerable longevity advantage over any ceramic substrate..EPA approved, CARB approval pending..

[longhornchris04]

Short answer: A cat converter really won't do a lot for you... not without improved fuel injection. You may be able to set it right at idle (using your O2 sensor) but w/o feedback, this setting will drift.

Long answer: A catalytic converter only works (or rather works as intended) if the air-fuel mixture is in a very narrow band, much narrower than the engine will actually operate. This was a major factor in the adoption of fuel injection over carburetors. Its also why bosch (and others) developed O2 sensors and feedback systems.

While the 78 & 79 SCs had/have cats on them, they only worked well if the CIS was properly adjusted. The later CIS, Mototronic, and everything afterward use O2 sensors to ensure the mixture is within the limits under idle and light operating conditions.

2-Way cat converters are functional over a decent range of AFRs. 3-Way cat converters require the AFR to be between 14.6 and 14.8. Your idle AFRs are much richer. Good for power but bad for emissions & fumes.

[lupin..the..3rd]

Idling odors??? You didn't just eat a burrito, did you? But seriously, if the exhaust at idle bothers you, stop idling so much. A 911 was meant to be driven, not sitting there idling.

And why the heck do you want to get rid of the historic tags in favor of regular registration?? That seems so backwards. I cant wait for the day my '87 qualifies as historic. As is I need to bolt the cat back on once per year to pass emissions, then off it comes again. Can't wait to leave it off permanently.

[tcar]

A cat will do nothing for odors.

Cat is mainly for CO emissions - odorless.

I'm guessing that you are running fairly rich.

[stlrj]

Re-connecting the air pump should do wonders as far as eliminating exhaust odors. How difficult would that be?
You might lean it out some since you are quite rich at idle too.

[DasBoot]

Originally Posted by lupin..the..3rd

And why the heck do you want to get rid of the historic tags in favor of regular registration?? That seems so backwards.

Because, in NJ where the OP is located, historic plates on your vehicle limit driving to "exhibition & educational purposes" only. With "regular" registration, he can use the vehicle as a daily driver if he so wishes.

[nineball]

Originally Posted by DasBoot

Because, in NJ where the OP is located, historic plates on your vehicle limit driving to "exhibition & educational purposes" only. With "regular" registration, he can use the vehicle as a daily driver if he so wishes.

same thing in ohio but they allow for "test runs", and since i am always doing something to the car it always needs a test run. to be honest i have never been stopped for having historical plates for the 12 years i have ran them on various cars.