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Join Date: Mar 2005
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Porsche Crest Knifedging a crankshaft?

I red about the benefits of knifedging a crankshaft in sport compacts. They help in reving an engine faster, and some other things. I'm planing to do this on my 944, because the crack is quite heavy, but got some questions about. I know that if the crack is knifedged, it should be balance. That should be done including the pistons(with rods), flywheel, and clutch. What I want to know is if the flywheel (because it's sorta heavy too)needs to be lighten acording to the crank's new weight, or if it could stay untouched? Would it help the car, or do it affect the car negatively?

PS. Sorry for my english.
Old 03-31-2005, 05:49 PM
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Yes, you can lighten the flywheel.

Or, better yet buy the Fidanza aluminum flywheel.



Does it add any hp? No, but it will help it rev a little quicker.

A very light reciprocating assembly takes a little getting used too when first starting out, (easy to stall).
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Old 03-31-2005, 08:48 PM
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With a lighter flywheel, you may have problems with it stalling at idle - I know a guy here in Memphis had that problem when he went to a really light flywheel on his street car.
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Ryan
'84 944 moneysucker edition (SFR, Corbeaus, etc.)
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Old 04-01-2005, 07:49 AM
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Don't lighten the flywheel for a street car.

Go ahead and knife-edge the crank, but don't lighten it much, or else you will have some vibration problems. While you're in there, cross/perp drill the crank too.
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Old 04-01-2005, 08:24 AM
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Cross drill the crank for longevity. As others have said, a light flywheel makes the car less friendly on the street. Not a big problem, you just have to pay more attention to every shift, both up and down. Step on the clutch, and the engine is at idle very quickly.
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Thumbs up

Thanks! this is the first forum where I got answered, and fast...

I'll be doing other things like regrinding the cam to 261 degrees and .435" lift. The people from Paeco told me this is the regrind for torque and acceleration for street use. Port matching, polishing the intake inside, maybe Extrude Hone. Use a 3mm larger honed throttle body... Not sure about a Nitrous system for 50-75 hp (for the stoke engine). What do you think?

Oh, I forgot crossdrill the crank...

Thanks again...
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Last edited by morbid; 04-01-2005 at 02:57 PM..
Old 04-01-2005, 02:54 PM
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If I didn't have lighter rod & piston assembly, I would not remove any weight from the crank counterweights. There is a math formula for the throw weight needed compared to jounal with big rod end & piston, pin & small rod end. Even with a light piston/rod setup I would be careful about removing too much weight.

Knife edgeing not only removes metal but is useful in making the crank more areodynamic, cutting through the pressurized air in a wet sump oil pan like on a 924. The oil is thrown off the knife edge better than a flat counter weight, also. This gives even less rotating weight to speed up & slow down.

Where are you located? David Brown at Speedwerks in Thomasville, NC is pretty sharp about oil setups. I heard him say that when he worked on a Nascar team, they put a lexan window on an oil pan to watch oil behavior. He said the oil looked like a big wad of cotton candy rotating with the crank. A knife edge & scraper can help a good bit.
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Old 04-04-2005, 02:48 PM
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I'm located in Puerto Rico. That sort of things I just want to hear. Want to know the formulas involved in this things of rotating weight movement 'n distribution. The relationship between the weight to be removed in the crank, flywheel, and/or pistons. I got some programs of engine building, and want to know all I can about bulding an efficient street/race engine. I need input for my brain drew1. Thaks in advance...

morbid
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Old 04-04-2005, 05:15 PM
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I'll look for the formula over the next couple of days. Out of my head comes this. (1/2 of Reciprocating Weight + Rotating Weight) * 1/2 stroke) = counter weight * (mass center ditance away from main bearing center)

Reciprocating weight = piston
pin
lock rings
piston rings
weight of rod small end

Rotating Weight = Weight of big rod end
Rod bearing

Let's do an example of an inline 4 Cylinder, 1 cylinder per throw with 2 counterweights per cylinder, like a 944.
Balancing units are grams.
Stroke = 80 mm
Old piston with pin, rings & lock rings 680 g
Old Rod Small end 240 g
Old Rod big end 400 g

New piston assy 590 g
New Rod small end 200 g
New Rod Big end 350 g

Rod bearing inserts are the same weight so not included in calculations.


Old Reciprocating = 680 + 240 or 920 g
Old Rotatating weight = 400g
(920/2)+400 = 860g
80/2 = 40
moment = 860g * 40 mm = 34400 g mm

New Reciprocating weight = 590 + 200 or 790 g
New Rotating weight = 350 g
(790/2) + 350 = 745g
745 g * 40 mm = 29800 g mm

Old moment - New moment
34400g mm - 29800 g mm = 9600 g mm



Now we need to figure how much metal we can take off the counterweight. Lets say the counterweigt is 80 mm wide & its outside is 120 mm from the centerline of the main bearing journal. We want to take metal away to give about 9200 gmm less moment on each pair of counterweights. 9200 is better than 9600 because it is good to sort of overbalance on a high revving motor & weight will be removed from crank during balancing.

Lets see what a 45 degree cut to the edge of the counter weights will give us. Since we are cutting 2 counterwiights we can calculate like it is a ring 80 mm wide by 80 mm thick.


Radius = 240 so the area of the cw swing would be PI * 240^2 or about 175000 mm^2. To get its volume, we multiply by 80 mm getting 14000000 mm^3 or 14000 cc.

80 mm off the radius gives PI * 160^2 or 25600 with an area of about 80000 mm^2 & a volume of 6400000 mm^3 or 6400 cc.

14000 - 6400 = 7600 CC for the ring. 120 degree / 360 degree is 1/3.
1/3 * 7600 = 2533 cc how much metal will be removed from a pair of counterweights.

Ferous metals are about 8 g per cc so the mass off of each pair is 317 grams. Since we are cutting triangle sections off with 80 mm legs the center of the mas is 1/3 of 80 from the side or 27 mm. This would make its center 120 - 27 or 93 mm from the main bearing center line.
The moment removed is 93 mm * 317 grams or 34881 g mm. This is about 4 times the momement we want we would have to calculate for less of an angle.

I have never knife edged a crank but I have lightened them by the method above (an angled cut in a lathe) but not having a sharp edge on the outside then had them ballanced & they do good. Knife edged cranks I have seen have a rounded surface leading to the sharp edge. The front & rear of the weights are cut too to give less air drag. I saw one for a Porsche flat 6 that the boys said was 6 lbs lighter than stock. I do know their motors run strong. I don't think a 911 piston/rod set weighs but about 1/2 lb. That tells me that maybe I'm too conservative, but with my budget it's better to be on the conservative side.

If you have light pistons made for your 944 (probably $600 to 800 from JE or Wiseco) you will have to get your cylinders sleeved since they won't have the coating to run in aluminum cylinders. Also, I'm not sure of how the 944 countershafts would work with a lightened rotating assymbly. Not running them would get you less rotating mass. To lighten a flywheel, I cut on the ouside toward the back of the motor. Cutting off the lip of a 944 flywheel where the pressure plate & ring gear bolt up & replacing it with an aluminum ring would give it less rotating inertia & might be a good idea. Things to watch out for in this is enough threads for the bolt engagement & where the TDC screw is.

For balancing I have an electronic postage scale that weighs to the gram. It is repetitive to the gram. This works on pistons & rods but I take the crank assembly, crank, flywheel, pressure plate, pulleys, etc to a shop that has a balancing machine.
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Old 04-04-2005, 10:21 PM
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I like that! I need a machine shop with a tech like you... Thanks a lot for the info my friend, It's that I want to know all about this to see if when I get my parts to the machine shop, they show they'll do the right job. Plus, I want to learn about. I'll be playing with this formulas. I got a disassembled engine here to take the info needed to calcule this out.
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Old 04-05-2005, 05:16 PM
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