Crankshaft question

[Puma]

Can a non-counter weighted crankshaft be altered to counter weighted?

Specifically, a 2.2 T crank made to work in a 2.2 with S Pistons and E cam.

Thanks,

Puma

[Puma]

If possible, how much should it cost?

[tharbert]

I'd bet if it could be done, it would cost a lot more than sourcing a good used one. If your non-CB crank is Std/Std, I'd bet you could probably sell it and break even getting a counter balanced crank.

From what I understand, your non-CB crank weighing less helps the engine build RPM's quicker. You sure you want to get rid of it?

[faapgar]

2.2 T cranks are cast not forged therefore not as strong.Ok for 125 H.P.Fred

[chris_seven]

The 66mm T cranks do have a lower Polar Moment of Inertia and for a given torque will accelerate more quickly which may ne noticable in 1st or 2nd Gear but less so in any of the higher gears.

They will be more likley to suffer from fatigue cracks at higher revs or a few of reasons.

The first is that cast Iron has a lower fatigue threshold stress than steel so will crack more readily if there is a sufficiently high shear stress.

The second is that the Cast Iron has a lower Modulus of Elasticity than steel and for a given force the crank will suffer more torsional deformation and be more likley to initiate fatigue cracks.

Finally the lack of counterweights means that the crank is less able to absorb the high order torsional vibrations produced by the combustion process.

having sadi all of that I am sure that for normal road use a T crank will work resonably well but would worry that for sustained high rpm use in race engines they will be much more likley to break than a counter-weighted steel crank.

I am n0t sure trying to bolt on additional weights would be a very good idea.

[Steve@Rennsport]

Originally Posted by faapgar

2.2 T cranks are cast not forged therefore not as strong.Ok for 125 H.P.Fred

Au contraire, Sir!

ALL 911 cranks are forged; those particular T cranks simply lack counterweights.

While these cranks can be hard on cases due to vibration and the resultant fretting, they have not given us any additional issues in race engines operated to 8600 RPM.

[356RS]

Words of knowledge and history from Sir Steve Weiner.

[Trackrash]

I hotrodded my 2,2 T motor in '84. 90mm P&Cs, E cams and Webers. After 100K miles, dozens of autocrosses and DEs the motor still runs strong to 7300rpm. I imagine the case is shot, oh well.

[chris_seven]

Interesting - Bruce Anderson's Book quite clearly says that early 911T Cranks are cast iron.

If the vibrations cause the cases to fret they must be fairly significant and can only be due to the lack of counterweights and the effect of gas torques.

I am surprised that you don't see increased main and thrust bearing wear as a consequence.

I am not sure 7300 rpm is fast enough as some basic hand calculations would indicate that the vibrations aren't too bad until round 7800rpm.

[Geneulm]

Rarely has one wrong statement in a good book had to be beaten down so many times. Steve performing a public service with, "ALL 911 cranks are forged; those particular T cranks simply lack counterweights."

Tons of stressed to the max racing motors have been made with these cranks. Particularly in the heyday of IMSA GTU 2.5 and SCCA.

[Trackrash]

FWIW. I was told by an "expert" before Bruce Anderson's book came out that T cranks were cast.

IMO if they were cast, we would be seeing failures. I have never heard of any.

[chris_seven]

It has been easy to belive that the early T had a cast crankshaft as most of the publicity seems to suggest that that the T was built as a 'budget' model and the changes to the engine specification were to save money.

To produce a completely new die and then to forge a revised crank to save a very small amount of money on the steel cost doesn't seem to make sense so there must be another reason.

If it had been a real high volume part then the economics woud be better but at Porsche's production levels it seems hard to justify.

The reasons for the change ae probably buried too deeply to ever find out why.

The fatigue of 'cast' cranks is also interesting.

Ford switched to cast cranks in their early Kent 4 cylinder engines and with the original 3 bearing design the cranks used to fail in a matter of weeks when used at more than 6500rpm.

When Ford switched to a 5 Bearing design the cast cranks were much better as they were better supported and the shear stresses reduced.

These cranks ran well and reliably to around 8000rpm - the rods used to fail and the flywheels and front pulleys which were also cast iron failed regulalry but when these were replaced by steel the cranks were quite good.

The early Lotus Elan Twin Cam engine which had a Ford blok and crank used a cast steel crank as an 'uograde' and these were really very good andwe used seveal of them in very reliable race engines.

In the late Seventies BCIRA in th UK prodcued a 'new' crankshaft material which was Basically a Spheroidal Graphite Iron which was very high strength, had good ductility and exceptional fatigue strength.

This beacmae a common material for crankshafts and was used in a very large number of high end engines.

I was involved in a Fatigue test programme (as the test equipment supplier) for this material for around 5 years and I graduated with the Reaearch Group leader some years before.

Most of the testing involved looking at basic material proprties but some comparitive work was done looking at basic steel cranks and EN40B Nitirded cranks.

In SG Iron Cranks generally perforemd as well as the EN40B Cranks.

Ford produced a number of Kent Cranks for Formula Ford Engines and I think thay they were sold as 'Duracast'.

[KTL]

Chris never fails to impress! Nice contribution, as always.