DDK - Die Deutschen Klassiker

Hello all.

I know we are priviledged to have at least one very pukka metallurgist round these parts...and would be interested to see how we might get a bit further with Fetuhoe's thread on cranks..

In particular, it is accepted dogma that the no-counterweighted 66mm cranks are made from "cast steel", and the counterweighted forgings..

However, I have subjected both to my hi-tec analytical systems and find no difference on the Particle Oxidation Spectrophotometry (Sparks-from-Grinder), nor in the Acoustic Resonance Decay Spectral Analysis ( Ring-from-Hammer) tests..

I am dimly aware that there are a few Modern New-Fangled Tests some folk like which might provide more data..

I am particlularly encouraged to persue this because the supply of usable 66mm cranks must be drying up soon and we need to establish economical and reliable means of reconditioning them.

Grady Clay ,over on Pelican, says that he has never heard of any 66mm non-CW crank breaking in racing use...probably not too surprising in view of its very large journal overlaps..and intriguenigly he also says the caes do not seem to suffer despite what one would assume are much greater offset loads on the mains....

Kind regards
David

I know very little about 911 cranks but if the non-CW are forged can you not just weld on counterwieghts a la VW beetle cranks & then get the lump dynamically balanced?
Or am I missing the point?

Hello there.

I guess there were several points implied in the posting, ..and that is one of them!

The dogma is that the T cranks were cast not forged.

But who knows for sure and how to tell?

And, if they do not break, and do not knock out the mains, why bother to add rotating weight and windage with counterweights?

kind regards
david

davidppp wrote:The dogma is that the T cranks were cast not forged.

But who knows for sure and how to tell?

And, if they do not break, and do not knock out the mains, why bother to add rotating weight and windage with counterweights?

Ah. OK. I get it now.
Would a major sportscar manufacturer bother to make 2 (3?) different cranks? Cast T (& forged non CW?) & forged CW'd.
That seems alot of expense & effort for little cost benefit at the risk of a broken engine.
I too cannot see the point of the CWing other than Porsche didn't have the benefit of hindsight.
See, I told you I knew very little about the cranks

David, very interesting but does need some thinking about.

The counterweighted crank vs non-counterweighted must be to do with control of torsional vibration as the additional weight will change the mass elastics of the crank.

The T engines without counterweights also had different pistons and con rods and it could just be that the complete rotating assembly produced an inertia torque that didn't excite any nasty vibrations in the operating rev range.

Interetingly the heavier crank can be dynamically balanced to a higher standard than the lighter crank. The counterweighted crank will also produce higher radial loads at the main bearings. The standard of lateral balance is, however, unlikely to improve a cranks fatigue life buy may stop blocks from cracking up.

It is also possible that the counterweighted cranks did let the engine run more smoothly at higher rpm and so it may have been an early NVH decision rather than just durability.

The material and metallurgy is a little more complex.

I think that we have already established that the forged crank is made from a moderate strength low alloy steel that has been subjected to a Tenifer treatment rather than an nitrided crank manufactured from EN40B or equivalent.

I am unaware that 2 litre cranks have a particularly bad reputation for fatigue failures but 2.7 cranks do suffer this problem and it must be due to torsionals.

Chemically analysing the material is relatively painless and currently would have a cost of about £25 using a suitable Polyvac.

When we turn our attention to 'cast' cranks it does become even more complex.

In the beginning Ford used cast cranks in the 105E and other engines and these were simply cast iron and not too clever, they were very limited in terms of rpm and commonly suffered fatigue failures when revved.

Cast steel cranks were then developed using a range of medium carbon and low alloy steels. These materials are certainly better than cast iron and are normally 'sized' after casting by giving them a light blow in a forging tool.

These cranks may be quite good but the weakness will be that the cast structure, which doesn't offer great fatigue resistance, will not be broken up by the sizing operation and once a fatigue crack intitiates failure will be almost instantaneous. Again analysis would be about £25. The only way to tell if they are cast rather than forged is to prepare a section and have a look with a good quality microscope at 250-500 x magnification, the cast structure would be quite obvious.

The fatigue resistance of a cast steel crank would be improved by Tuftriding (570 degC)

About 20 years ago BCIRA in Avlechurch developed a new alloy which is known as Austenic Spheroidal Graphite Cast Iron which is impressively strong and relatively ductile, which implies good fatigue resistance.

This material is now commonly used with excellent results.

Welding on counterwights to a crank just isn't a great idea, even assuming that you can easily establish the correct welding conditions.

To obtain sound welds which will tolerate the stresses involved will probaly mean that the crank would distort during welding. This would mean straightening with all its implied problems.

My conclusion is that depending on the grade of cast steel and the surface treatment used I think the only downside would be increased torsional vibration. This could be measured using Bently probe (Accelerometers just don't give enough data) on the journals at each end of the crank and lookng at the spectrum analysis.


It is possible to carry out torsional analysis on the engine but cost would be high as the software needed to carry out this work has a cost of about £9000 if you want to look aty both the inertia torque and the gas torques.

It would probably take a week to 10 days to set up a model and run this type of programme.

Given a little information about the chemistry of a T crank I think the risk of problems is quite small. I am sure that the engine would be slightly harsher but not too fragile.

If you have a scrap T crank I can easily arrange to have it analysed.