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I'm assembling a 5.9 liter engine, with one of my custom crankshafts and thought I'd share some pictures of what my latest crankshafts look like.
Actually, to casually call this a crankshaft, might be a sin against all things mechanical. This is pretty close to art.
Starts life as a vacuum formed solid billet chunk of steel and ends up like this:
This crank has a stock stroke of 85.9mm (stock GTS) and weighs 54 pounds, versus a stock GTS crank at 62.75 pounds. 4 Kilos less total weight!
It's an 8 counterweight crankshaft (the 928 GTS cranks have 8 counterweights also), with the two center counterweights slightly smaller (Nascar Style Counterweights....which have the exact same function, without the mass.)
The rod journals are hollow to reduce mass.
Pendulum cut counterweights (lighter) and center drilled all the way through (lighter and stiffer.)
I use 2" rod journals with these crankshafts to make them stiffer and allow us to use a much stronger rod bearing than the stock 928 bearings, plus reduce the bearing speed.
High speed oiling (each connecting rod is fed individually from a main bearing. No blind holes, no plugs, no shared oil passages.) Oil passages are "surface drilled", not center drilled, like in the stock 928 crankshafts.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Surface drilled ? Are the holes in the big ends off centre as they appear? Is this for mass distribution reasons, or to enable the oil drillings, or both?
jp 83 Euro S AT 57k
Beautiful work! They can assume many art forms. Haven't seen a Porsche crank break like this but I'm sure they have. This was out of my BMW CR Prepared car.
Wow... Be interesting and informative for many of us if GB felt like posting a little (or a lot) about the "journey" to get a product like this from his imagination to being ready to put into an engine. Can't imagine the sourcing, design, process, and testing iterations it took to make this a reality.
Surface drilled ? Are the holes in the big ends off centre as they appear? Is this for mass distribution reasons, or to enable the oil drillings, or both?
jp 83 Euro S AT 57k
Great question!
The stock crankshafts are cross drilled on the main bearings (the oil galley passage in the main bearing is drilled all the way through the middle to the other side.) In the center of this passage, the feed passage to the rod bearing is drilled at a 90 degree angle angle (Picture a 'tee' with 1" legs.) This means that the oil has to pump to the center of the crankshaft (fighting centrifugal force) and make a 90 degree turn to get to the rod bearing. Obviously works pretty good for a stock engine run at stock rpms (stock 928 engines rarely have bearing issues from oiling.) However, as the rpms go up, it gets tougher and tougher to deliver oil to the rod bearings.
Here's a picture of one of my oil passages with a piece of welding rod through it:
The main bearing is not cross drilled. The oil passage to the rod bearing is drilled just below the surface of the main bearing directly to the rod bearing, so the oil doesn't have to "fight" against centrifugal force. This style of drilling actually uses centrifugal force as a miniature oil pump!
In this style of crank drilling, I don't run a fully grooved main bearing, but a bearing with about 220 degrees of groove.....which seems entirely counter intuitive, since the oil passage to the rod bearing is going to be "covered" 140 degrees of crank rotation (no exposed oil passage once the oil hole in the main bearing hits the non-grooved part of the main bearing.
However, I use this to my advantage.
The oil passage hole to the rod bearing has to be drilled in the exact correct place, so that the oil flow is "shut down" during the non load part of the combustion cycle. When the oil passage is "covered" by the main bearing, Centrifugal force forces the oil toward the rod bearing....creating a vacuum in the oil passage (between the oil and the main bearing passage.) As soon as the oil passage rotates enough to be open to the grooved part of the main bearing, the vacuum sucks oil into the passage, while the oil pressure feeds the oil passage, at the same time. This provides more oil, to the rod bearing, than I can get with oil pressure alone (under the high bearing loading of the combustion cycle.)
Beautiful work! They can assume many art forms. Haven't seen a Porsche crank break like this but I'm sure they have. This was out of my BMW CR Prepared car.
Cast iron crankshafts are economical to make, but the weakest of all types of crankshafts.
I'm a bit surprised that they would used a cast iron crankshaft in this level of a vehicle.....the "bean counters" have taken control!
A stock GTS crank is now $6873.38 retail. Used ones are very difficult to find and frequently have thrust bearing damage, which requires some serious effort to repair.
This crankshaft is a drop down "Walmart" bargain, compared to that.
10-09-2018, 08:52 PM
Crankshaft ****
