Original muffler (exhaust silencer) material

[911MRP]

In his book Frere writes this on a
photo caption referring to corrosion protection improvements :

It seems to imply full stainless steel exhaust was used as a durability rust reduction measure*

I have an original stainless steel Bischoff muffler (exhaust-silencer) .
It has the correct part number per 1972/3 factory spare parts book (maroon paper edition III — not PET as latter not always helpful in tracking upgrades and supercession changes)



It clearly stamped 1072 after the slash of part number (October 1972 manufacture) so model year 73.


Helpfully, Bischoff used the Porsche part number as their stamping but abbreviate it (in this case by omitting .111.)

Four layers are visible along seam

It is stamped 911/1 version

Allowing for inbound supply lead-time, fitted around end calendar 72. Too early and wrong shape outlet for G series onwards. It still has the correct square edge chrome tip — not so relevant here other than it’s presence along with factory welds an indication the outlet is original and not messed with in nearly five decades.

I have another similar original Bischoff with the same maker and version stamps also same part number stamping before the slash. However this other one is regular steel. It is only four-months older — stamped 672 after the slash (June 1972 manufacture).

Both mine are the normal OEM 911/1 series part number is per factory 72/3 parts book, they’re the usual for period :two in, one out format — not some special twin outlet / modified sports version. Both ok but condition of 1072 example other than a few small cosmetic dings testament benefits of stainless steel. The other has some cosmetic rust but not perforated.

Haven’t weighed them. I don’t own any original Leistritz who were the other OEM supplier back then so can’t compare their contemporary product or its evolution.

Never seen an M-Option in model 73 accessory lists to choose stainless vs regular steel.
As shown above only one part listed on 72/3 spare parts book so no indication of stainless type for different models regular steel for other models; neither does parts book have (sw) annotation indicating a special order version was available as sometimes the case on some other parts. Despite my two examples isn’t a 1 or 2 annotation on the book showing difference for 72 vs 73 whatcha is sometimes the case. Parts books are useful reference but not always correct — in this case they don’t seem to give an answer to the two materials.

Question: Was there a change of material for this Bischoff OEM muffler ( exhaust silencer) part during model year 73? Possibly around beginning of model 73. Possibly so based on just these two obviously limited data points June and October 72? Or was there a way of specifying an option to have stainless vs regular steel at that time or as seems more likely was it a running change introduced for durability and rust protection for cars at this upmarket price point ? Probably not immediately apparent difference visually in any period photos of brand new 73 cars if all were painted grey.

Steve

* don’t think Frere’s comments can be used to date introduction as he refers a series of examples introduced over time but it was only mention I found so far on material used for exhaust component. Maybe other mentions elsewhere in his or other reference books or workshop manuals might comment but I’ve not read those on detail for a while.

[Arne]

I'm not sure date has much to do with it. My early '72T (8/71 build) has a replacement Bischoff muffler. It was probably installed in the mid-70s - the car had some accident damage in the left rear and I suspect the original muffler was replaced as part of the repair.

The replacement on my car has the exact same markings as yours, other than the date (1073).

But it is only 2 layers at the seam, and is not stainless as there are a few indications of rust. So even though mine is later than yours, it is definitely not stainless.

[911MRP]

Thanks for input, Arne. It might prove interesting to see if others post to help establish timing for stainless vs normal steel versions. From Frere’s comments it seems at some point there was a deliberate move to use stainless for the entire exhaust system although that might have been during impact bumper era and with different outlet to suit the revised rear.

Maybe there was a way of specifying stainless as an option without M # or another reason for different material being used on 911 in calendar 72. The corrosion / durability point seems obvious but not clear how it would have been specified to use one vs other. As my two examples show it seems to have been possible as both clearly exist with x72 stamp.

I’ve read some allude to a Sport stainless steel variant on long-hood era but never clear what that was. I assumed these were maybe different to stock OEM spec e.g. dual outlet or had internal changes. Maybe here some remember how it was in the day? Also the significance of 911/x stamp beyond the part number, the ones I’ve owned have had 911/1.

So from what you are saying it might not be a date in calendar 72 where regular steel was superseded by the stainless steel like my 1072 example. Although with yours known to be a later replacement probably following the accident maybe factory fit vs what was in network for collision repair could be a factor?

[Arne]

I don't know the replacement history on my muffler for sure, but since the muffler date is over 2 years later than the car, it is obviously not original. I'm just guessing that it was replaced as part of the LR corner repair.

It is also obvious that it was made as a replacement part, since the impact bumper cars were already in production by the time my muffler was built (10/73).

I can also say for certain that the original muffler on my '84 Carrera was stainless.

[gled49]

Arne, I see 4 layers on the end shot of that muffler. It’s stainless to me.

[911MRP]

I took Arne comment at face value but on close inspection his photo seems four layers as GLED spotted and pointed out. I believe the stainless is a wrap over regular steel so magnet test would not reveal in such areas.

So maybe there was a supercession to stainless around 72 for model 73? Although I don’t know on the absence of different part numbers between model 72 and 73 and M numbers in the papers a running change to bring the Better on service corrosion resistance might seem logical

[Arne]

I went back out for another look up close and personal. Looking around the back side (next to the engine) it is more obvious that there are indeed 4 layers. I tried to take a couple more pictures in an effort to show better what I see in person, but I con't get the phone close enough to capture the details and so the pictures don't really show it well.

But I note 2 or 3 very small pinholes in the outer shell that appear to be the early stages of rust-through. So I still don't think mine is stainless, despite the 4-layer construction.

[911MRP]

Mine was easy to see stainless steel outer layer as paint removed. Maybe try small magnet in areas not double wrapped ?

[Arne]

Magnet sticks everywhere I tried it.

[911MRP]

Fair enough maybe others have some data-points beyond our three examples