Crankshaft Lightening
12-31-2007, 08:15 PM
#46
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12-31-2007, 09:43 PM
#47
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http://www.circletrack.com/techartic...ogy/index.html
Calculation formula for four cylinder crank is little different than V8 but basic idea is same. Crank must counter rod+piston weight or engine will shake too much. Higher rewing engine more important this is. Normal Porsche piston weight tolerance is +/-4 grams. Thus cranks are designed to work with stock part weights. When ever weight is removed from piston or rod, crank should be lightened accordingly. Or build 3L turbo engine with heavier 951 rods and mallory weights need to be put into crank to counter this too.
For example Moldex in Dearborn Heights does custom cranks. 95.25mm (3.75") stroker for 928 engine is something like $2500 depending on specs. They can probably make four cylinder cranks too as long as order is detailed enough.
http://www.macraesbluebook.com/searc...company=465269
Calculation formula for four cylinder crank is little different than V8 but basic idea is same. Crank must counter rod+piston weight or engine will shake too much. Higher rewing engine more important this is. Normal Porsche piston weight tolerance is +/-4 grams. Thus cranks are designed to work with stock part weights. When ever weight is removed from piston or rod, crank should be lightened accordingly. Or build 3L turbo engine with heavier 951 rods and mallory weights need to be put into crank to counter this too.
For example Moldex in Dearborn Heights does custom cranks. 95.25mm (3.75") stroker for 928 engine is something like $2500 depending on specs. They can probably make four cylinder cranks too as long as order is detailed enough.
http://www.macraesbluebook.com/searc...company=465269
12-31-2007, 09:54 PM
#48
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Well the extreme case would be:
-full weight crank/fly/PP/rods/pistons
-dry sump
-large plenum.
I don't know if it would slow down 100 RPM's between shifts if you did it like this!
Conversely:
-lightened everything
-wet sump
-stock intake
Like this, the engine would all but die between shifts.
Choose a point in between to tune to your likings.
-full weight crank/fly/PP/rods/pistons
-dry sump
-large plenum.
I don't know if it would slow down 100 RPM's between shifts if you did it like this!
Conversely:
-lightened everything
-wet sump
-stock intake
Like this, the engine would all but die between shifts.
Choose a point in between to tune to your likings.
12-31-2007, 09:55 PM
#49
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http://www.circletrack.com/techartic...ogy/index.html
Calculation formula for four cylinder crank is little different than V8 but basic idea is same. Crank must counter rod+piston weight or engine will shake too much. Higher rewing engine more important this is. Normal Porsche piston weight tolerance is +/-4 grams. Thus cranks are designed to work with stock part weights. When ever weight is removed from piston or rod, crank should be lightened accordingly. Or build 3L turbo engine with heavier 951 rods and mallory weights need to be put into crank to counter this too.
For example Moldex in Dearborn Heights does custom cranks. 95.25mm (3.75") stroker for 928 engine is something like $2500 depending on specs. They can probably make four cylinder cranks too as long as order is detailed enough.
http://www.macraesbluebook.com/searc...company=465269
Calculation formula for four cylinder crank is little different than V8 but basic idea is same. Crank must counter rod+piston weight or engine will shake too much. Higher rewing engine more important this is. Normal Porsche piston weight tolerance is +/-4 grams. Thus cranks are designed to work with stock part weights. When ever weight is removed from piston or rod, crank should be lightened accordingly. Or build 3L turbo engine with heavier 951 rods and mallory weights need to be put into crank to counter this too.
For example Moldex in Dearborn Heights does custom cranks. 95.25mm (3.75") stroker for 928 engine is something like $2500 depending on specs. They can probably make four cylinder cranks too as long as order is detailed enough.
http://www.macraesbluebook.com/searc...company=465269
01-02-2008, 03:07 PM
#50
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Why is the 944 crank so heavy? Two reasons –
First - The heavier rotating mass makes for a smoother engine and one that easy for 1st gear slow starts.
Second – the heavy counter weights dampen the individual ‘impulse’ forces generated by each cylinder.
The second reason (impulse forces) can twist the crankshaft and cause harmonic problems with the stability of the crank. This is not something that you will feel (like incorrect static balance) but it will fatigue the crank over long periods. The other way to address this is adding a crank damper to absorb some of the oscillating energy.
In an inline 4 cylinder the weight of the crank counter weights is not tied directly to the weight of the rods / pistons – the weight of the counter weights is determined by the need for rotational inertia. If you remove the counter weights and keep the static balance correct the system will not have any ‘additional’ imbalance vibrations.
I looked into getting some cranks made up a while ago – not cheap and I could not find a shop that could make a crank that would compare to the stock crank for strength. The stock forging is actually a real nicely made part (accept for the oiling system!). Most crank shops would start with a billet and machine the part – not as strong as forged crank. If you do get a quality shop to make some up let me know – I might be able to get the quantity up enough to make it work economically (you usually have to have a decent production run made up to make it worthwhile to get a forging process performed)
Chris
First - The heavier rotating mass makes for a smoother engine and one that easy for 1st gear slow starts.
Second – the heavy counter weights dampen the individual ‘impulse’ forces generated by each cylinder.
The second reason (impulse forces) can twist the crankshaft and cause harmonic problems with the stability of the crank. This is not something that you will feel (like incorrect static balance) but it will fatigue the crank over long periods. The other way to address this is adding a crank damper to absorb some of the oscillating energy.
In an inline 4 cylinder the weight of the crank counter weights is not tied directly to the weight of the rods / pistons – the weight of the counter weights is determined by the need for rotational inertia. If you remove the counter weights and keep the static balance correct the system will not have any ‘additional’ imbalance vibrations.
I looked into getting some cranks made up a while ago – not cheap and I could not find a shop that could make a crank that would compare to the stock crank for strength. The stock forging is actually a real nicely made part (accept for the oiling system!). Most crank shops would start with a billet and machine the part – not as strong as forged crank. If you do get a quality shop to make some up let me know – I might be able to get the quantity up enough to make it work economically (you usually have to have a decent production run made up to make it worthwhile to get a forging process performed)
Chris
01-02-2008, 03:10 PM
#51
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BTW – when you let a crank shop know that you plan to make 150hp+ per cylinder they will start thinking a little differently (especially for road race engines)!
01-02-2008, 04:21 PM
#52
Drifting
I don't agree that it's a nice part. It is adequate for the job it does in a stock motor. Just because a crank is forged doesn't make it superior. It depends on the material that it is made out of. In the case of the 944 crank, it's not made out of a high quality forging. It's a mass production piece made for a series of realatively low output engines.
Once you modify the engine and increase output you are now outside of the design parameters the crank was made to survive in. Failures happen quite often as we all know.
The solution is to make a better part. I had atleast three companies willing to work with me on the design of a better crank. All of them were told it needed to support as much as 200hp per cylinder.
Once you modify the engine and increase output you are now outside of the design parameters the crank was made to survive in. Failures happen quite often as we all know.
The solution is to make a better part. I had atleast three companies willing to work with me on the design of a better crank. All of them were told it needed to support as much as 200hp per cylinder.
01-02-2008, 04:35 PM
#53
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I don't agree that it's a nice part. It is adequate for the job it does in a stock motor. Just because a crank is forged doesn't make it superior. It depends on the material that it is made out of. In the case of the 944 crank, it's not made out of a high quality forging. It's a mass production piece made for a series of realatively low output engines.
Once you modify the engine and increase output you are now outside of the design parameters the crank was made to survive in. Failures happen quite often as we all know.
The solution is to make a better part. I had atleast three companies willing to work with me on the design of a better crank. All of them were told it needed to support as much as 200hp per cylinder.
Once you modify the engine and increase output you are now outside of the design parameters the crank was made to survive in. Failures happen quite often as we all know.
The solution is to make a better part. I had atleast three companies willing to work with me on the design of a better crank. All of them were told it needed to support as much as 200hp per cylinder.
The last crank deal that I worked on was for a minimum of 50 pcs @ $1-1.5K each. I passed on that one…. If you can get a better one made up I will more than happy to ‘buy in’ – I couldn’t justify the $50 -75K of product on the shelf!
01-02-2008, 05:23 PM
#54
Nordschleife Master
I don't agree that it's a nice part. It is adequate for the job it does in a stock motor. Just because a crank is forged doesn't make it superior. It depends on the material that it is made out of. In the case of the 944 crank, it's not made out of a high quality forging. It's a mass production piece made for a series of realatively low output engines.
Once you modify the engine and increase output you are now outside of the design parameters the crank was made to survive in. Failures happen quite often as we all know.
The solution is to make a better part. I had atleast three companies willing to work with me on the design of a better crank. All of them were told it needed to support as much as 200hp per cylinder.
Once you modify the engine and increase output you are now outside of the design parameters the crank was made to survive in. Failures happen quite often as we all know.
The solution is to make a better part. I had atleast three companies willing to work with me on the design of a better crank. All of them were told it needed to support as much as 200hp per cylinder.
01-02-2008, 05:41 PM
#55
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Have you ever priced stock crank out form the dealer? A 3ltr crank is $3800. So, A custom"modern" crank @ $5k is worth it to me. That is the ball park for a custom "Billet" crank from an extremely well know F1 parts supplier. I'm having 3 made right now and they are all different strokes.
01-02-2008, 09:46 PM
#56
Drifting
I respect your opinion, I just don't agree. No sweat. You are happy with it, I have nothing to say.
As far as 6800rpm? Let's just say we will see about that.
01-02-2008, 09:52 PM
#57
Drifting
I wouldn't have touched that either. I guess they wanted you make sure you were serious....The guy I am dealing with would do just one if that's what I wanted. I have to pay for the design work and then are not held to a single unit if I wish. I can then shop it around if I want or go ahead and have then manufacture. They can do just about any stroke.
My buddy talked to them this morning but I didn't have a chance to find out where they stand.
01-02-2008, 11:05 PM
#58
Nordschleife Master
01-02-2008, 11:28 PM
#59
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I see cranks like this and I wonder. I have used one crank that was cut down like the cranks in this thread.
The whole idea is that the crank is balanced with the weight of the connecting rod, pin and piston in mind. It's not just hack some weight off and call it improved. IMO this is done blindly all too often by someone who does not have the proper understanding of what they are doing.
Now Porsche does some "interesting" things with crank weights. If you look at the crank in a GT3RS motor. The crank is light with small counter weights. They counter this by using Ti rods and small 2 ring pistons.
The crankshaft in a 944 is huge. It has large counterweights and weighs 58lbs. Why? Why does it have to weight 58lbs.? The crankshafts also suffer from less than ideal oiling. The common "fix" for this is not a solution but a mild improvement. The oiling hole in the rod journal is in the wrong place.
Lightening the crank is more complex. You need to get weight out of the rod end first then move to the counterweight. For every ounce out of the small end you can get two ounces out of the big end.
Currently I have a crank out to a manufacture to have it redesigned. New material, proper oiling and less mass.
The whole idea is that the crank is balanced with the weight of the connecting rod, pin and piston in mind. It's not just hack some weight off and call it improved. IMO this is done blindly all too often by someone who does not have the proper understanding of what they are doing.
Now Porsche does some "interesting" things with crank weights. If you look at the crank in a GT3RS motor. The crank is light with small counter weights. They counter this by using Ti rods and small 2 ring pistons.
The crankshaft in a 944 is huge. It has large counterweights and weighs 58lbs. Why? Why does it have to weight 58lbs.? The crankshafts also suffer from less than ideal oiling. The common "fix" for this is not a solution but a mild improvement. The oiling hole in the rod journal is in the wrong place.
Lightening the crank is more complex. You need to get weight out of the rod end first then move to the counterweight. For every ounce out of the small end you can get two ounces out of the big end.
Currently I have a crank out to a manufacture to have it redesigned. New material, proper oiling and less mass.
Raj
01-02-2008, 11:28 PM
#60
Drifting
Wow. I guess the stack of junk 944/51/68 cranks I had at another shop was a mirage......or a flashback. See I always thought I got cheated on the flashbacks.....