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Knife edging a crankshaft

General Engine Building Discussion

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I’m after the pros and cons of knife edging and crankshaft. I’ve read quite a few threads on other forums claiming that it can actually do more harm than good and can cause premature big end failure due to removing weight that is actually there to counter balance the piston/rod assembly, even after properly balancing.

If also read that the power gains are very small if at all and not really worth the money.

Any input would be much appreciated.

Thanks!

If it was going to cause a failure due to out of balance/reduced crank rigidity wouldn't it be main bearings being beaten up, harmonic damper or crank cracking? The forces on the big end bearings would for practical puposes ignoring tiny variances in possible torsional crank harmonics still be the same unbalanced piston/rod inertia and forces due to differential pressure across the piston. I'm guessing seting up a partial vacuum system and or going to a dry sump would likely liberate more power if you are going for marginal gains. Probably only worth it in limited race classes or if the engine type is known to have oil foaming problems that are hard to get around?

I've knife edged some of our 4G63 drag cranks and couldn't really tell you if it offered a gain in performance. I haven't however seen a downside in terms of crankshaft reliability. Many of the billet cranks that are available now for popular engines are designed with knife edged counter weights.

Thank you both for the input, I appreciate your help

I’ve built a couple of relatively high revving traditional Mini engines. Both with heavily wedged and bladed cranks or just lightly reworked ones. The crankshafts in these engines share the same space and oil as the gearbox which increases wind age. They also only have 3 main bearing caps which isn’t ideal. All of which tends to amplify pros and cons of certain modifications.

There does seem a benefit in taking a vast amounts of material off the bottom end but no where near as much as by simply rounding or putting a nice radius on the corners and edges of the webs.