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Knifedging a crankshaft?

  #1  
Old 03-31-2005, 01:42 AM
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Default Knifedging a crankshaft?

I red about the advantages of knifedging a crankshaft in the sport compact cars. It makes the engine to rev faster, and it helps in the lubrication. I'm thinking to do this to my 1983 944's crank but, I was thinking... If something like this is done to the crank, it must be balance... it includes pistons(with rods), flywheel, and clucht. The stock crank it's kind a heavy, and lightening it it's a good idea, and balancing it is a good combination but, I was thinking about the flywheel. It's a heavy part too. Should it be lighten too? If so, How much? Should it be lighten relative to the weight of the crank? Does this affect the rear wheel torque, possitive or negative?

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  #2  
Old 11-03-2005, 03:56 AM
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Default RE: Knifedging a crankshaft?

dont know much about porsche but usually when you balance the rotating assy. you need front balancer,con rods,pistons,crank,and fly or flex plate.if you take your assembly to a machine shop and they dont know bout knife edge or balance walk out...with the whole assy. balance you can fell diff. the engine will rev quicker and of course its good for a few ponys.the knife edging and lightening of rotating assy. will creat more power less rotating weight.
 
  #3  
Old 11-04-2005, 11:39 AM
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Default RE: Knifedging a crankshaft?

My understanding is that "knife-edging" a crankshaft is really something you do only for wet-sump engines: it cuts "windage" losses particuarly when the crank and oil/oil mist are close. On a dry sump 911, etc., I'm not sure it would help much. Don't know specifics of a 944 engine.

Nearly every engine is different so you have to know the specifics. Some are internally balanced and most older designs were externally balanced (i.e., the harmonic balancer matters a lot - as in you can break the crankshaft otherwise). I do not know a lot about 944 engines per se, except that some got pretty big (4 cyl 3 liter!) so balance and vibration must have been quite an issue.

You can spend extra money and reduce reciprocating weight and generally it does lots of good things -reduces vibration, increases power and sacceleraiton slightly and ups the permissible red-line, cuts stress on the block and bearings. But this tends to get really expensive, and you can go too far: it can become difficult to shift smoothly without all that "internal inertia." And light weight can mean low strength which can be self defeating.

Overall, I would not spend a lot on light weight unless you are going to race at sustained high RPMs.

I am in to modifications in a big way but not on Porsches, and I've recently built one of each: an alumunum blcok and heads C5R 427 cubic inch V8 (LeMans engine) in a '02 Corvette which I had built professionally with very lightweight but strong parts - custom CP pistons that weighed only 70% of stock, titanium rods, a very lightweight "overbalanced" crank, lightweight AA flywheel, etc., etc. It cost $38,000 complete and in the car, and is bulletproof strong and in street tune with mufflers made 520 HP (to the rear wheels through a 6-spd manual) normally aspirated.
By son and I built a 408 cubic inch engine in the opposite mode for a '98 Camaro "street and strip" car: cast iron block and alum heads, Eagle rotating assembly (i.e., strong, but very heavy), everything else roughly the same as the 427. It cost only $11K complete and in the car, is just as bulletproof even though it weighs alot (and doesn't vibrate any more than the 427) and produces 510 rear wheel HP with a slightly nastier cam though what is essentialy the same drive train, but it has over twice the rotating weight.
There is a v e r y noticeable difference in the feel when you drive these cars: the lgithweight 427 revs quickly and requires more attention to revs and much more coordination to shift smoothly between gears; the heavy 408 is very easy to shift quicly between gears and feels much "lazier" when you blip the throttle to rev the engine in neutral.
The advantage of the heavier rotating assmebly is that it "stores" noticeable amounts of energy in the drag car to get it off the line quickly: rev it to 4500 RPM, drop the clutch (it has a really tough clutch) and it ifts the front wheels about 4 inches off the ground as it launches quickly.
 
  #4  
Old 01-14-2007, 05:27 AM
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Default RE: Knifedging a crankshaft?

Knife-edging a crankshaft will most likely help the crank cut throught the oil and enhance the reciprocating mass revs, but my question is will it weaken the crank? I know one way of strengthening a crankshaft is to bury in the ground for a year or so. Sounds crazy but it works. If i'm correct the balancer shafts spin at twice engine speed to counteract the harmonic pitch of the crank, so keep that in mind.
 
  #5  
Old 02-28-2007, 09:49 AM
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Default RE: Knifedging a crankshaft?

Knife-edging is not done for weight reduction, it's done to reduce windage. It will not weaken the crankshaft (if done correctly). It involves making the leading edge of the counterweights more aerodynamic. It is by definition only performed in areas that are non-load bearing. Crankshaft lightening is a different story. It requires removing metal from the entire crank, not just the counterweights. This weakens a crankshaft. Although the counterweights are the most attractive looking areas to shave off weight, they are there to balance the rotating assembly so they cannot be cut away on there own without having a negative affect an engine balance. When you've seen crankshafts that have been lightened in the counterweight area, it's because the assembly has also been lightened in the crank throws, con rods, and/or pistons.
Burying a crank in the ground sounds to me like an atempt at seasoning the iron. If it's steel, there's probably no advantage. And if there's oil poassages and machined surfaces, the will corrode ahich will weaken the part. If you get your hands on a bare casting, bury it. I'd opt for having a billet steel crank made. Much quicker.
 
  #6  
Old 01-23-2011, 09:48 AM
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It mostly works in the bigger and expensive cars, in low priced cars you ll hardly see anything like that.
 
  #7  
Old 10-04-2012, 08:32 PM
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The changes which you want to introduce in your vehicle might break it and also slower your performance therefore instead of making all these changes work on the style and engine factor of the car so that it will run smooth and fast without any effect of cracks.
 

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