Any difference between 915 gearboxes?

[smiles11]

Are there any notable differences between a 915 gearbox fitted to say, a 1973 car with 2.4L & a 1974 car with a 2.7L? Or are the gearboxes identical?

[gled49]

Same box except 4th & 5th are a little taller in the 74. Same as 73RS ratios.

[Chuck Miller]

Thanks Gordon... Learning every day...

[911MRP]

In addition to the ratios as mentioned previously some RS got an oil cooler in front cover of the transmission. Not all of first RS cars ( for around the first month of production) got these as insufficient transmission coolers were available at that time. Sometimes the early ones were reported to have a whistle this was fettled as a small modification early in calendar 73.

Around time RS homologation was completed — springtime of 73 — later RS ( roughly third series ) didn’t get this transmission oil cooler. This was yet another example of those special limited edition RS homolgation spec things for the first circa 1000 homologation cars that was eliminated for cost cutting reasons — presumably to consolidate / simplify and make more margin when those later cars became essentially productionized. The M491 RSRs however did continued to get the oil cooler throughout.

(Early) Carrera RS had a different clutch pressure plate too — the RS unique pressure plate part was not used on TES. I’m not certain if this part was also consolidated to have just a single production version of pressure plate for later series RS once homologation was completed. Seems likely once the bean counters got to streamline such things once the focus on homologation was complete so the racers / engineers stepped back — can’t recall if I’ve seen the elimination of that unique pressure plate documented.


Steve

[Jon B]

In addition to the ratios as mentioned previously some RS got an oil cooler in front cover of the transmission...

An oil pump, not an oil cooler.

Jon B.
Vista, CA

[911MRP]

Yes … it’s an oil pump… that’s what I was referring to when I wrote “cooler”.

They stopped fitting these transmission oil pumps to the 73 RS by about mid March 1973 so their fitment ended a before the 1000 homologation examples of RS had been built. Maybe the volume built with these pumps was near enough to the 1000 examples required by FIA they could stop fitting it by that date around mid March? Or possibly the pump was just a requirement of the 500 examples of Special GT Group 4? I understand the 528th RS example was inspected for certification purposes in the last week of Feb 73 so they would’ve had information regarding the original 500 to give approval of that series by March. On the other hand why having built so many with that equipment take a risk of an oil pump’s omission being queried by FiA for the sake of a hundred or or two more RS examples, if in fact the pump was still required for the other Group?



Unless I’m mistaken by around mid March 73 they’d have been getting pretty close to the target 500 plus 500 more figure required for the other FiA Group so why stop fitting them right at that point ? To me this suggests the pump was not deemed essential equipment for the second RS series but had been deemed part of necessary equipment for the originally conceived 500er series. Pretty clear in the detailed homologation papers (end even to general public per the the press snippet) requirement of FiA was 500 then doubled to 1000) — dropping this pump in between seems no man’s land. Porsche don’t seem to have shied from the cost and complexity of building the necessary RS quantity so the timing seems a bit odd?

Maybe it wasn’t a homologation spec thing perhaps by that point in the programme they’d determined the 915 in RS application was OK without the oil pump, except for those examples receiving the racing spec? I’m not an old Porsche transmission specialist. Not sure if the transmission oil pump was fitted on 2.4 as an option for 72/3? Perhaps this difference of oil pump (or not) being used on some 73 RS with the new 2.7 is a bit niche for a question that was asking about differences on 915 74 2.7 vs 73 2.4. Maybe it is a NA market centric question? Nevertheless it was for a while one difference between 2.4 and the earlier 2.7 transmission that had appears in some markets — those markets that got the 73 RS model.


Steve

[gled49]

860 still had an oil pump and the early input start seal. The change of the seal can be found in the parts manual by trans number. Oil pumps stoped around 875. I did a 72 ST trans with an oil pump, mostly prototype parts. RS and RSR trans pumps.

[911MRP]

Interesting regarding the cutoff. I realise they’re not built in strict Vin number however using that sequence as an approximation I had estimated they had stopped using these pumps in the 915 transmission of RS around the (mid) 800 to 900 vin range — which your comments confirms might be about right. I hadn’t looked 72/3 parts books for the change point.

Per my prior post it struck me as curious for factory to bail out after fitting so many more pumps if the target was just 500? In fact they ceased not many short if the target had been the 1000. I am sure of date when fitment was stopped — except for M491. I recall it was about middle of March 73 because it had previously made me wonder why fell at that point in the overall RS vin sequence / three series. Often changes to the model don’t coincide with series of course. So plenty of of pragmatic simple reasons beyond a homologation rationale for ceasing to use pumps on all but RSR of course, but if it is correct that the pump was only RS/R model equipment in that year it does seem point in that direction. Particularly as it is well documented once homologation was achieved there were the numerous quite deliberate changes made to the spec and build procedures whereby the factory could now sell / make the RS simpler, cheaper, prudictionized — presumably better margin than the early ones for homologation. No longer under FiA scrutiny they pointedly eliminated a number of the limited edition homologation spec stuff (with a some exceptions) but charged more because of the success in motorsport and stronger than expected demand. The transmission pump wasn’t on the items I’ve seen in internal correspondence referencing changing completion of homolgaton (deliberately simplifying (cheapening) the spec and the build process) - things like the panel thickness, forged crossmember etc were deliberate change in such documents triggered by homologation being complete. Maybe no mention of pump makes sense if done before the 1000 were completed. So maybe unrelated to the homologation at of 1000? Or possibly it was triggered when the vin 528 passed inspection finally got confirmed in March any over being to use up parts or ran on as contingency sonce it is known some very early RS that had no pump at start of production? One of this many little quirks.

Steve

[gled49]

They did continue to type stamp the gearboxes 915/08 even without oil pump.