66mm 935 crank for magnesium case ?

[buster73]

Still going through the options for my 2,4 project based on a 73 2.4 7R case. Now I was offered a brand new 66mm 935 crankshaft complete with titanium rods and bearings. Did some research and seems most 935 engines were based on early turbo cases, so in theory the crank should also fit a 2.4 magnesium case ?

However my gut feeling is that this crank would probably be „over-kill“ for an early case and with all limitations of case and other components (heads…) you can not really take full advantage of the potential of a 935 crank. Probably no significant benefit compared to a 66mm 2,2 S crank unless you go really wild on the engine specs.

Appreciate your advice.

[Matt Monson]

I can’t imagine that’s cost effective in any way. And yes, overkill.

[Henry Schmidt]

I would have to agree with Matt. A factory race crank with titanium rods would be designed for performance far exceeding the limitations of a 2.4/2.7 magnesium case.

I will also add, I have never seen a 66 mm, 6 bolt, 935 crank. 9 bolt sure, they were used in 962, 956 as well. I doubt it exists.

Please post picture of the crank and ti rods.

[Rosco_NZ]

I understand the 2.1 turbo RSR was built on Mg cases (& 915 trans). I know these days we know the limitations but weren’t the 2.8 RSR’s also Mg cases?

[Geneulm]

This has to be a 9 bolt crank (won't fit). If 6 bolt it's 906. (Will fit)

[Henry Schmidt]

Originally Posted by Rosco_NZ

I understand the 2.1 turbo RSR was built on Mg cases (& 915 trans). I know these days we know the limitations but weren’t the 2.8 RSR’s also Mg cases?

Porsche race engineers were enamored by the weight saving of magnesium. The original sand cast 2.0 cast was a pig.
They even tried sand casting a magnesium version for the 906 and those engines would crack everywhere.
Like most racing developments you build out until it fails. The 2.7 street engine and the 2.8 RSR are perfect examples.
The 2.8 suffered crank failures, harmonics that spit the flywheel off and insufficiency in case stability. Cracking everywhere.
Those issues facilitated the need for a purpose built crank and a sand cast 3.0 case that closely mimicked the 2.0 sand cast cast.

Here's a 906 case we repaired.

[Henry Schmidt]

Sand cast, aluminum 3.0 RSR

[buster73]

Thanks for your advice. Confirms my doubts that this crank is no option for my project. The owner was racing RSRs in historic racing for many years and has this crank in storage since. At the moment he does not have more infos than it is a 66mm crank. Will ask him for more details and pictures once he found it in his stash of parts. Of course we also have not talked about pricing yet and guess even a „friendship price“ would blow my budget.

[PeteKz]

Add this: If the crank and rods were used in racing, you don't know how much abuse they have already endured, and how much useful life they have left.