66mm crankshafts

[Tar356]

Looking for opinions on using a "T" crank in a vintage race 2.0 motor.

Anyone?

Thanks,

Todd

[chris_seven]

Todd,

I am interested in this idea as I have heard it is common practice but I have two specific concerns which I have not resolved.

Firstly I understand that 2.0 litre T cranks are cast iron rather than Tenifer treated steel and this worries me that fatigue life may be an issue.

Secondly I don't think that the T has counterweights and whilst this may give some benefit of reduced inertia I would worry about the higher order (4th, 4.5th, 5th) order torsional vibrations that are produced by gas torques also reducing fatigue life.

[Steve@Rennsport]

Gents:

The 66mm "T", non-counterweighted cranks are OK for a race engine. In fact, they spin up quicker.

For the record: ALL,........ALL 911 cranks are forged;.....they never sold a 911 with a cast crank. These things are quite strong. In 30+ years of racing 911's, I've never seen a broken non-counterweighted 66mm crank.

For street use, they shake a bit more than counterweighted ones and that does affect case life. I would recommend that ALL of these mag cases get shuffle-pinned to reduce fretting on the case half parting lines and extend main bearing life.

[Plavan]

From our experience, no problems. My Dad has a T crank in his race motor. Not an issue. I am building another 2.5L race motor and am using a T crank this time for the weight benefits. It is based on an aluminum case.

[MBruns]

We do alot of engine dyno work for several shops that run at the front of the pack in HSR 2.0 group, and being on the throttle end ( direct cable from engine to operator)of the engines that have the non CW cranks the harmonics are VERY noticable at the 7800 and up range some to the point that the dyno gets upset and the feel in the throttle handle is a severe buzz.

Mike Bruns JBRacing.com

[chris_seven]

Mike,

Interesting.

Could this be a coupling vibration between the engine and the load? I guess if the counterweighted crank is smoother in its operation that would tend to eliminate any coupling issues.


It would be interesting to fit an encoder and try to estimate the order of the vibration and I guess it may be an odd order.

Do you ever see any fretting of the coupling shafts?

[MBruns]

I have operated this DTS dyno aprox 400 hours over the last 5 years, and 80 % of the engines are 911 based engines, granted the 2.0 are the ones that operate above 8500 normaly. but consistantly the non C/W cranks can be detected with out prior knowlege of the combo, on some days we will do several race engines from the same shop ranging from 2.0, 2.5, 3.0,3.8 and when those 2.0 go thru that range it is alarming almost like an electric shock/Buzz in the throttle lever. Just imagine what that does to the fuel and ultimately the fuel mixture in the carbs.
We have found that leaving the full C/W cranks in and going with alum. flywheels and 5 1/2 " racing style clutches the assembly ends up being lighter, smoother and the moment of inertia is much lower.

Mike Bruns JBRacing.com