Counter Balanced Crankshafts

[BBausser]

Didn't see these mentioned in Bruce Andersons Book but in the recent Excellence article on the short throw engines it mentions them as a necessity for any engine that regulary sees 7,000 RPM and up. Was there a cutoff date by engine number or type when they began to be used regularly? I've heard that some engines have them, some don't but would like to know if there is some eay of ID'ing them short of tearing the engine down?

Thanks,

Bill

[Steve@Rennsport]

Hi Bill,

Only the 2.2T engines used the non-counterweighted crankshafts; the rest of them used the "normal" counterweighted ones. You can check engine type and serial numbers to confirm what you are looking at.

That said, the non-counterweighted cranks are significantly lighter so for some applications such as racing, they do make a noticable difference in how quickly the engine spins up. One does see more fretting on the case parting lines, even with shuffle-pins so it does put a bit more stress on the case and main bearings.

Remember, these cranks are ALL forged steel and very strong.

[72tii]

I thought the '68 and '69 911T 2.0 liter also used the non-counterweighted crankshaft. Is that not correct?

[BBausser]

Good responses, Thanks. I knew about the change to Counter Balanced Cranks in 356's and had figured that all 911's had them as part of the progresive improvements in their engineering of cars. I'm surprized that the some exceptions were made on the Early T's. I wonder what the benfit was other than the cost factor since the T construed a mind set of lower RPM's. I never though of Porsche as comprimising technologly for $. Seems sort of like planned obsolescence like the big 3 did.

[chris_seven]

As stated the T cranks are lighter and hence must have a lower polar moment of intertia and should spin up more quickly and I guess that this may increase radial bearing loads but I am not totally convinced.

From a lateral balance consideration I don't think counterweights would be too important on a Boxer 6 as both primary and secondary balance are good.

I think that the counterweights are there to improve high rpm fatigue life as there will be some high order torsional vibration due to gas torque and these will produce a torsional vibration which is very damaging and difficult to sense.

It is fairly easy to measure lateral vibration on a dyno as an accelorometer will measure the g that is produced.

A torsional would need soemting like a Bently Nevada Key Phasor and I wish I had the time and money to do the measurements.

I would always favour using a counterwighted crank on a high rpm engine as I believe torsional vibration is an issue and without measurements or detailed torsional analysis I just think it is safer.

[Zithlord]

You can take of the sump cover to look inside the engine if you want to check if you have a counter balanced crank or not.

John