A knife edge crank is one that has been modified to have sharp edges near where the crank connects to the con-rods. It allows the the crank to "cut" through the oil with less friction than if the sides are blunt. Of course a better way is to have a dry sump so there are no losses at all due to the crank in the oil.
Knife-edging the crank reduces rotating mass, allowing for a faster revving, higher rpm engine. The thinner counterweights also cut parasitic power losses normally encountered when rotating through splashing oil in the oil pan.
Inside an oil pan at speed it's HELL. Oil is all over the place as vapor, droplets, and on some motors as a thick rope that drags on the crank spinning off the throws. Knife edging puts a bit of an aerodynamic contour to the crank (as I understand it) so that it will cut through the thick atmosphere inside the pan. Polishing up the crank will also remove many potential stress risers and allow oil to free itself most easily. Reshaping the crank in this manner will effect balance but if you're at this point you're going to have things balanced anyway so no biggie so long as strength isn't compromised. Crank scrapers, baffles, anything that can be done to keep the oil where it belongs out of the crank's way will all help.
I'm not sure that knife edging by itself will get you much horsepower but once you get to a certain point with any engine every little bit helps! If you're to the point of having everything torn apart this, like replacing rods with stronger pieces, is one of the things to consider NOW as opposed to after putting it back together and iwshing you had dome something or other that you skipped.
Hrm, any aftermarket oil pans for the Supra or has the stock pan got very good oil control from the factory? Not ever heard of one losing oil pressure on the skidpad or anything...
This is a knife-edge crank. It's from my 7M though.
Remember to have it fully rebalanced when you put it back in the car. And I must admit, it spins like hell. It doesn't make the whole assembly stronger obviously, but most cranks are strong enough. Expect to invest quite some money btw, think of 800 USD for the crank job.
I would think with a knife-edge crank, you'll gain maybe 5-10 hp and a quicker rev (which is nice to have for turbo), but the process removes some of the strength around the rod connector. For a turbo car, I'd rather have a stronger crank and be able to run higher boost, than a quicker rev and the possibility of a crank failure.
If you do get it knife-edged, I'd get it shotpeened and polished to re-strengthen it (which means more money) and then completely balanced.
Rod connector?! Umm, it's the counterweights that are knife edged - nothing else shoudl be getting material removed unless you polish up everything else which takes little metal and removes stress risers...
I believe most stock cranks are "flat ended" all around (no knife edge). That means if you want a true knife-edge, you'll have to shave both ends. If you look at the picture of the crank above, you'll see what I mean. The rod connector side is also knifed. Besides, why would you want to knife-edge only one half the rotating mass? You'll only be helping half the rotation cut through the oil, as well as messing up the balance. If the rod connector side was already knifed, then you'll be starting with a weak crank for a turbo car.
I though tyou meant someone was messing with the rod journals I now see what you mean. I'm also not sure that rounding off those edges loses strength from the crank. I would think that the rod is trying to pull the journal or stretch the crank - I'm not sure that the metal we're talking about has much effect. Any engine builders around to answer?