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Old 11-24-2004, 08:36 PM
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Knife-edging crankshaft

I was wondering if anyone has some input on knife-edging a crank. Is it worth the time to do for the increase in hp? How hard is it to knife-edge a crankshaft, given you have the proper tools and competence to do so. How hard is it then to balance the crank after its been edged. Im just curious Ive heard some interesting things on the topic just seeing what everyone has to say about it.

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Old 11-24-2004, 09:01 PM
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knife edging the crank

If you are referring to a crank scraper, which mounts along the side of the block and peals the oil off the crank that is thrown out as it spins, you can contact any of the speed shops, and they will sometimes have them in stock. From your name, I would guess you are working on a Small block Ford, and I would go to Racer Walsh, a Ford specialist in Florida, and get their advise on them.
Scapars, dont actually increase horsepower but free up whats already their.
Smokey Yunick, several years ago, took a SBC and cut holes in various places in the block etc so he could observe oil in motion. He drove his test engine with an electric motor.
What he found was that oil didnt fly off the crank in drops and bounce back at the crank when it hit the block but actually wrapped itself around the crank like a rope. what the scraper does is scrapes off that oil much like a knife would peal off a potato or apple peal. You will need to maintain a certain amount of clearance for the scraper, which you would have mounted on the pan rail. With an engine turning clockwise, you would want the scraper on the passenger side of the block. It will have to be some pretty hard stuff to do the job. There are other options along with using a windage tray, which prevents oil from sloshing back against the rear crank throws and mains, submerging them in oil under hard acceleration.
A simple scaper could be some 1/8th inch steel plate that is cut out to match the throws on the crank. I would use cardboard to make a template. Carefully turning the crank, with rods attatched, cutting the cardboard to match or indenture the rotating assembly. It is a time consuming project.
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Old 11-24-2004, 09:18 PM
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Thanks for the reply. I was referring to when metal is ground off the counter-weights on the Crankshaft in the shape of a knife-edge, this would allow less resistance for the crank. It allows the weights to cut through the oil much easier. Not to mention it would end up taking a few pounds off the crank maker the rotating assembly that much lighter.
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Old 11-24-2004, 09:29 PM
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scraper

Thats exactly what I was talking about.

OOPS!!!!!!!!!! Thats not what I was talking about. Im not sure about cutting the crank itself. That would require a total rebalance of the engine, and it could structurally weaken the throws on the crank itself. your crankshaft under normal operating conditions doesnt dip down into the oil and bath, but swings above it.

If the crank were to swing down into the oil, that would be an extreme power robbing event, and it would be felt as the engine ran. It would also ariate the oil making it lose its substance and totally lose its ability to lubricate. It would be like stirring honey with an electric mixer. Try that sometime and you will see what would happen with oil if the crank was spinning through it.
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Old 11-25-2004, 05:10 AM
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Unless this is a big stroke engine or the engine is being used in a very competitive class, no its not worth your time. You'll have way more time and money in it then what performance gain you will get.

Chris
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Old 11-25-2004, 07:19 AM
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you'd be much better off addingcrank scrapers and windage sreen to your oil pan or even better to spend that money on a good moroso or milodon pan than is built with it.they do their jobs pretty well beside other benefits they offer.
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Old 11-25-2004, 07:32 AM
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Look at the 4340 pro comp crank from scat

http://www.scatcrankshafts.com


This crank are knife edge (the one i have buy for my new
sbc)..

Not sure how hard it is to do this on a std crank btw..
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Old 11-25-2004, 08:03 AM
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knife edged crank

Scat cranks are purpose cast and would be designed that way from the get go.
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Old 11-25-2004, 08:24 AM
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You can knife-edge your own crankshaft, but it's alot of work. A re-balance will be required when you are finished.

These types of operations really only make sense in class racing where the rules require a stock casting crank. Most class rules don't allow knife-edging anyway.

With the lowered price of aftermarket cranks, it doesn't make alot of sense to spend hours grinding on your crank's counterweights.

If you choose to do it just because you want to do it, it sure won't hurt anything. You'll want to thin the outer edges of your counterweights and grind a knife-edge point on the leading edges of the conterweights. Get a good, sharp carbide burr, and a heavy duty die grinder. Get your goggles and dust mask, you don't need to be breathing all of that iron.

You can finish your work with some sanding rolls.

One big advantage to doing this is that if done correctly , you can lighten up the crank quite a bit. Try to be even with your metal removal so you don't have to have mallory metal put in the crank when it's rebalanced. This will kind of ruin the cost effectiveness of doing it yourself.
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Old 11-25-2004, 09:32 AM
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Knife edging

I have knife edged a couple of steel cranks myself. One was a factory GM steel crank and the other was a Cola 4340 crank(hard grinding). If the crank is steel (i hope) you wont need a dust mask, just glasses and gloves because of the sharp little slivers.

It does help and quite a bit depending on the rpm range and oil pan type. The idea is to shape the leading edge of the counterweight to move thru the air/oil mixture easier.

I have seen some knifed to a point like a "V" and some rounded more like a wing leading edge. I prefer the rounding. Airplanes have rounded leading edges because that shape moves thru air easier. I also leave the trailing edge square to allow oil to come off at the edge. If the back edge is knifed too it encourages oil and air to close back in behind the counterweight only to be hit by the crank on the next revolution.

Polish smooth after grinding.

The crank will be just as strong since this is in a non-stressed area.

Re-balancing will be necessary of course. Knife-edging is best done before first balance, then it only needs done once.

Rick
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Old 11-25-2004, 07:44 PM
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There is virtually no effect from knife edging, the gain is from the weight reduction.
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Old 11-26-2004, 07:38 AM
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Quote:
There is virtually no effect from knife edging, the gain is from the weight reduction.
If the pistons and rods are the same, the balance bob-weight will be the same, which means the finished rotational inertia of the crank will be the same. Therefore, no HP would be gained from the weight removal.

Knife-edging is commonly done at the same time as lightening, but has its own merit.

Rick
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Old 11-26-2004, 07:49 AM
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Less restriction... More rpm quicker

knife Edge help to have less face that slow the rotation when hitting air or oil... And a bit less weight for sure but the weight removed is very minimal..
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