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.
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