Crankshaft - Rod Journal diameter and width: 2.4/2.7 vs SC

[blue72s]

While I understand why Porsche had to reduce the rod journal diameter from 57mm to 52mm to achieve the 70.4mm stroke (otherwise wouldn't be possible) for 2.4 models but why did they also widen the journal width from 22mm to 24mm (thinning the webs from 7mm to 6mm in the process), and then go back to 22mm width and 7mm webs again in 1978?

Can 66mm cranks be machined to 70.4mm stroke with SC spec rod journals? If so, why didn't the factory racing department do this when they experienced vibration problems with the 2.5ST long-stroke and the 2.8RSR?

[blue72s]

EDIT!!!!

Sorry, I had got the specs from Wayne's book and there's a mistake there. The SC rod journals are not 52mm dia x 22mm width.

Here is the correct version:

Diameter of SC journals: 53mm
SC rod width at big-end: 22.6mm

So, the thickness of SC webs are about half-way between the 66mm and 2.4/2.7 cranks.

If I'm calculating right, the 66mm crank CANNOT be machined to SC rod journal spec, because it is short of shaft material by 0.2mm.

[BURN-BROS]

SC rod journal width is 22mm. The 66mm rod journal can be ground down to diameter and width of the 3.0 liter journal. We do it all the time.

[Henry Schmidt]

Originally Posted by BURN-BROS

SC rod journal width is 22mm. The 66mm rod journal can be ground down to diameter and width of the 3.0 liter journal. We do it all the time.

The conversion became more viable when a production GT3 rod became available.
Previously we needed a custom rod to suit this conversion.

[blue72s]

Thanks.

From Frere's book:

3.3 Turbo and 3.2 Carrera (74.4mm stroke)
Rod journal 21.8mm width x 55mm dia.

3.0 SC (70.4mm stroke)
Rod journal 22.6mm width x 53mm dia.

Another question:
If the GT3 rod journal is 21.8 width x 53mm dia, then the surface bearing area is smaller than all its predecessors (2.0 to 3.2 Carrera).
Why's that?

[BURN-BROS]

The GT3 rod big end width is 21.9mm
The SC rod big end width is 21.8
The SC journal width is 22mm
The 2.0 liter journal width is 22.1mm

The journal diameter of 53mm came about around the 936/956 engines. The width was settled with the 9 bolt 935 engines/ 911SC 3 liter

Porsche used this dimension on all the competition cars...935,962,GT1,GT3.

[blue72s]

3.3 Turbo


3.0 SC

[BURN-BROS]

It's a typo. All the measurements I gave you came from actual parts in the shop.....

[blue72s]

Thanks for the clarification.

[blue72s]

What is the radius of the fillets on the GT3 rod journals? (So, I'd know what is the maximum stroke length can be achieved with the 66mm crank)

[BURN-BROS]

I do not know that dim.
All you will be able to get is a 2mm offset grind...maybe. So 4mm of additional stroke max would be the result. 69mm stroke may be all you could do conservatively. Your crank grinder would have to tell you what he would be comfortable doing.