How Much Can 12 cwt Crank Be Lightened & Knife Edged ?

[Cheromaniac]

My brother told me the wrong weight. His scale might be off, but he weighed it to be:

So for all that effort he only managed to shave off a mere 2lb?

[jsawduste]

Here is a little bit of info that folks can chew on.

It's from Vizard and written for the enthusiast crowd. While it focuses on wheels and tires the principles apply to crankshafts also.

BA, you will have to slow down and read through this a couple times to grasp the concept.

http://hpwizard.com/rotational-inertia.html

[gradon]

505 had the edging, off-set grinding, and balancing done. The engine is wrapped up sitting in a shed and a poor eliminator is collecting dust. . .

[dwg86]

Hey Gradon, I thought the crank was ground that way for piston clearance and not for weight reduction?

[jsawduste]

Hey Gradon, I thought the crank was ground that way for piston clearance and not for weight reduction?

That would seem logical.

[Russ Pottenger]

If a crankshaft counterweight is ground for piston clearance for reasons of increased stroke or too short of a rod, the area of the counterweight needing clearancing will be below the piston when it's at TDC.

Knife edging crankshaft counterweights is primarily for crankcase windage at hi rpm's.
That's why I thought it was bit odd for a Jeep 12 counterweight application.

[gradon]

It is for a high rev race motor, as is the "most aggressive" roller cam spec'd for 6500rpm. One day we'll see what it can do. . .

[gradon]

I'm forgetful, because years ago I posted: "The weights were shaved like that for piston clearance." viewtopic.php?f=8&t=1355

[6TIME]

I've ran both cranks (4cw short snout and 12cw) in the same motor, everything was weight matched and balanced. I opted for the 12 as it ran smoother in my setup over 3500rpm. The 4cw had some harmonics coming in at 3500ish. Piston weight/reciprocating weight could have an affect on this as well. So it will probably vary with the particular motor. With the initial build and 4cw crank it revved real snappy with Lunati cam and worked head.. was impressive. I got tired of the vibes though while towing and playing, so I narrowed it down to trying the 12cw crank. I had it balanced and dressed up then installed it and noticed no difference in how quickly it revved, but it ran much smoother in my application. I was suprised considering the extra 20lbs added on... No these aren't race motors so I'm sure you'd notice a difference cutting/adding weight in that realm.. But I think the earlier statement is very valid in the crank mass being close to the crank CL not having a huge affect on rev speed with a fairly long stroke motor. In comparison to flywheel weight which is much further out from the crank CL, It has a much more pronounced affect on this by varying the mass just a few pounds.

[SIXPAK]

I too have tried the 4cw crank. The only thing i managed to do in that drag motor was to break 4 main caps in two pieces. Been there and done that. Motor revs were 5000-6800