lightened crank

[litoslit]

Hello Guy, delighted to welcome back and thank you for your professionalism.
What is your view on lightened crank?

Greetings and thanks Guy.

[Guy Croft]

My feeling is the time and money is better spent on other things.

I have super-tuned the 84.60mm 2024cc TC at 200 and have dyno data from colleagues at 86mm bore and 210 bhp - the former with and the latter without a lightened crank, both with light steel flywheels, and frankly I couldn't assert any useful noticeable difference in throttle response between them. They were both lightning fast engines.

Now that's a very general statement and typical of me when it comes to niceties like crank lightening, but I do have to consider time and cost most carefully on every job and make sure the owner is really getting value for money and not 'dreams'. That said of course a reduction in mass on rotating parts does (in theory) lead to a reduction in frictional loss and inertia, so the pickup should be faster with a lightened item. Of course this is readily apparent from fw lightening but rather hard to discern in a lightened crank.

There is plenty of evidence that reducing the windage (air effects) around the crank is reduced by reshaping the crank webs and designers of pure race engines (eg: F1) of course devote much attention to this. At our level, so to speak, there have been numerous reports on this site of folks both lightening and 'wedging/knife-edging cranks' with great success.

I would make the point however that it's easy to make the crank so out-of-balance by machining in the wrong places that it cannot be corrected, and the crank on an must be done to an excellent standard of dynamic balance (and fw too and the assembly as a pair) before fitting, which rather suggests (to me anyway) that the person who does the lightening machining ought maybe to be the balancing expert.

And finally - anyone who thinks he can say with certainty that this or that engine was way quicker with lightened/modified crank better have a full set of back-to-back BENCH dyno curves (including step tests) to hand!

GC