Premuffler / catalytic converter question

kees · 11-27-2006, 01:19 AM

I have a 911 carrera coupe 1985 euro. I know for sure that I have no catalyst. So I assume that I have a premuffler instead of a catalytic converter although they look the same for me on the pictures I've found.

Is it correct that the premuffler doesn't have a cover and the catalytic converter does?

Please help me out?

Thanks,
Kees.

Oh Haha · 11-27-2006, 05:06 AM

Correct. The premuffler came with no heat shield unlike the cat. converters.

kees · 11-27-2006, 07:40 AM

Thank you Wayne,

Kees.

Wil Ferch · 11-27-2006, 07:46 AM

Pre-muff has no provision for an O2 sensor , either.....as a factory part.

- Wil

911 in SC · 12-13-2006, 06:20 AM

Wil, I just read your post. I believe you when you say that the euro pre-muffler (stock) doesn't have an O2 provision. My question is - do the euro cars (RoW) have an O2 sensor from teh factory, and where was it placed in the exhaust? Obviously, mine was 'federalized' and has an O2 sensor at the catalytic converter. (OK, it DID, now it has a Fabspeed euro pre-muffler).

Thanks,
Mike

Peterfrans · 12-13-2006, 06:55 AM

Did some work on my brother in law's 1987 carrera, and it has no cat and no O2 sensor.

fumanchu · 12-13-2006, 07:34 AM

I have a factory pre-muff (930 prefix) with an O2 bung on my '82. Of course its plugged, but it's there.

Wil Ferch · 12-13-2006, 07:41 AM

Think about it....the only reason for an O2 sensor is to keep the mixture under a VERY strict range....necessary for catalytic converter operation.

*Normally*...the Euro cars were delivered with no cats...hence they ran open loop with no O2 sensor. There was an option for Euro buyers...that would include a cat ( tax benefit), and those cars had the cat with the O2 sensor bung.

There would be no reason to have an O2 sensor/bung if there was no cat.

That being said...when I converted my 85, USA spec Carrera to run a Porsche pre-muffler... I had a bung added to allow use of the pre-existing O2 sensor.

No factory pre-muff came with an O2 sensor bung..if it exists, it was added later by the buyer.

- Wil

wcc · 12-13-2006, 08:19 AM

So for the US Spec cars, if you got rid of the CAT you wouldn't need run an O2 sensor because it's just like the Euro setup. Correct? But I've read on here that can cause premature valve guide wear cause of the full rich open loop. Is that true? If so, then is that a concern with the Euro cars? Or would that US Spec car have to have the computer re-programmed to Euro Specs, or would a SW chip do it?

On the flip side of that, I know people in our local club with US Spec cars and don't run a O2 sensor for years without problems. Or are they dodging a bullet so-to-speak?

fumanchu · 12-13-2006, 09:09 AM

Will

I am sure you are correct. Never really looked to see if the bung looked factory, not that you could tell. I was simply posting the application on my vehicle. Euro delivered 951's never came with O2 sensors and they came with Motronic so definately makes sense the same logic would apply to the 911.

wcc. Not sure I would run a Motronic equipped 911 without a o2 sensor(open loop) if it was designed to run with one(USA), ...CIS car... yes, but that is my uninformed seat of the pants guess.

Wil Ferch · 12-13-2006, 09:13 AM

wcc

No.....

The USA spec cars had DME units that expected feed-back control signals for O2 sensor operation. If you get rid of the cat.... there would be a lost signal that otherwise interfaces with an O2 sensor.

If you eliminate the O2 sensor signal for USA spec cars ( or tax-break Euro cars with cat...or Japanese spec cars with O2 sensors)..then the signal defaults to a full rich condition, as I recall.

The simple answer is that the DME used for USA spec cars is a closed-loop system...the DME used for normal Euro spec cars is an open-loop control system.

A careful re-read of my previous post should fill in any remaining blanks on questions, I would propose.

- Wil

911 in SC · 12-13-2006, 03:27 PM

I am trying to understand this the best I can, but I'm not 'functioning' at 100% (ready for vacation!!)...

So Wil, are you are saying that with the DME Unit in my car (RoW not US Spec) is an open looped system and therefore, I would not need to run a O2 Sensor in my Euro Pre-muffler?

Then again, when it was federalized in 1985 (and O2 Sensor and cat added), was the DME changed to a US Spec unit?

911 in SC · 12-27-2007, 07:55 PM

Sorry to bring a post back from the dead, but my car does, in fact, have a US DME w/ a 1 wire oxygen sensor.