Friday, 17 May 2013

Gearbox latest

I few weeks ago I took my gearbox to Jez at Carrera Performance. I knew synchromesh on 3rd gear was very tired, and his brief was to open the unit up, and let me know what was needed for a rebuild. I've been on tenderhooks ever since; bits for these units can be very expensive, and severe wear can render the casing scrap. 

So it was relief I learned this week that the gearbox internals were in good condition, and all it need was a few synchromesh rings - in fact about the least I could have hoped to get away with. It should be back with me in a couple of weeks. 

An aside I was interested to hear a comment Jez made when we were inspecting the 'box, comparing the weight favourably to the later G50 units. From what I'm able to tell, the various weights as follows;

  • G50 gearbox used in the later Carrera 3.2s                                                        ~70kg
  • 915 gearbox with aluminium cases used in the SCs and first 3.2s                ~58kgs
  • 915 gearbox with magnesium cases used in the '72 and '73 911s                ~50kgs
The stronger aluminium cases were adopted when Porsche started to worry about the torque of the bigger engines, but from what I can tell from other owner's experiences, with sympathetic use the mag  alloy cases do just fine. Certainly losing 20-odd kilograms from the tail of the car can only be a good thing.



Pat said...

For comparison, the weight for a 2012 Carrera S auto unit is 116.9kg. It is amazing that the driver can actually see the road and not just the clouds these days.

Shoestring7 said...

That's interesting Pat. I assume you mean the PDK 'box? Its a big lump of metal, even though most of it is well within the wheelbase of a 991.

I'd also be interested to know how much 18-way power seats weigh!


Bert said...

Hi Charles,

The mag 915 cases were used up until approximately april 1977- around that time Porsche was tooling up for the SC and the alloy gearbox case was stronger, but also cheaper to produce.

I noticed the changeover when researching the last of the Carrera 3 cars on the register: they were already equipped with the alloy-cased 915 gearboxes.
All the best,