944 Rod Bearing - Fix?
08-31-2015, 01:56 AM
#32
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Hi Michael,
A couple of questions.
If you're upgrading rods to aftermarket of some sort. Do the same modifications apply as you'd still be using the factory bearings?
Can you bring the engine upto 7500 rpm and still use the stock oil pump?
A couple of questions.
If you're upgrading rods to aftermarket of some sort. Do the same modifications apply as you'd still be using the factory bearings?
Can you bring the engine upto 7500 rpm and still use the stock oil pump?
08-31-2015, 10:14 AM
#33
Rainman
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As I recall one of the theories as to the cause of the #2 bearing problem is due to the shape of the oil passage drilled into the crank. Oil is fed through the main bearing first and then through a drilled passage to the rod bearing where, IIRC, it has to make an abrupt turn which is more easily navigated by air bubbles than oil under some circumstances...could the drilled passage be slightly enlarged, or maybe re-drilled at another (I guess you'd say tangential to the apex?) angle to "round off" or widen the corner for the oil to more easily pass?
08-31-2015, 08:10 PM
#34
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I don't want to come across as speaking for michael here, but I have another related idea/question on this.
As I recall one of the theories as to the cause of the #2 bearing problem is due to the shape of the oil passage drilled into the crank. Oil is fed through the main bearing first and then through a drilled passage to the rod bearing where, IIRC, it has to make an abrupt turn which is more easily navigated by air bubbles than oil under some circumstances...could the drilled passage be slightly enlarged, or maybe re-drilled at another (I guess you'd say tangential to the apex?) angle to "round off" or widen the corner for the oil to more easily pass?
As I recall one of the theories as to the cause of the #2 bearing problem is due to the shape of the oil passage drilled into the crank. Oil is fed through the main bearing first and then through a drilled passage to the rod bearing where, IIRC, it has to make an abrupt turn which is more easily navigated by air bubbles than oil under some circumstances...could the drilled passage be slightly enlarged, or maybe re-drilled at another (I guess you'd say tangential to the apex?) angle to "round off" or widen the corner for the oil to more easily pass?
08-31-2015, 09:59 PM
#36
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Late to the party as usual...did a word search and didn't see the cross drilling of the number 2 bearing crankshaft journal.
Is this already considered in addition to this information? Works well in the 928 community...
Is this already considered in addition to this information? Works well in the 928 community...
09-01-2015, 09:10 AM
#37
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It's my understanding that the cross-drilling the main in the way its done for these Porsches requires a relatively high oil pressure. The large main journal diameter further increases the require oil pressure. This is because oil first needs to be accelerated to the surface speed of the main journal and then pushed against the centrifugal (centripetal for those with a bug up their a$$es) force to the crank centerline. Smaller the main journal diameter and the larger the minimum radius of the oil passage relative to the crank centerline, the lower the required oil pressure. Fortunately for us with stock oil pumps, the supply pressure is high enough to overcome these effects but if you're planning to get the maximum power out of the engine and run an external oil pump with lowest possible supply pressure, a stroker crank with a different oil passage scheme may give one some (marginal) efficiency gains.
It's also my understanding that the drilling modification that is popular for the 928 crankshafts is not really cross drilling (it's already cross drilled from the factory) but web drilling the crankshaft. The web drilling of the crankshaft doesn't really help as long as there's clean oil in the main gallery. It may however help when the pickup sucks a little air. This is because the way the main gallery geometry works is that a disproportionate fraction of air gets fed to the #2 main and thus to the 2/6 rod journal. Web drilling the crankshaft adds an oil supply from the center main and connects all oil passages in the crankshaft. This doesn't reduce the total amount of air in the oil, but the theory is that it will distribute the air more evenly between different rod throws and therefore give one a little more margin of safety when air gets into the oil circuit. Whether the theory is correct is anyone's guess.
Not pretending to be an expert here, this is just what I've read and how I understand it to work.
On the rod journal modification that is the topic of this thread: This seems to be a no-brainer modification for anyone rebuilding the engine on a budget and reusing stock rods. There's a lot of bang for the buck in terms of increased reliability relative to Glyco bearings. Just an opinion.
Last edited by ptuomov; 09-01-2015 at 09:27 AM.
09-01-2015, 09:27 AM
#38
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They were cross drilled on some early 2.5 engines but those are very rare. Mostly they are not cross drilled.
89 3.0
&
89 2.5
Interesting drilling pattern inconsistency between rod and main journals between these two.
89 3.0
&
89 2.5
Interesting drilling pattern inconsistency between rod and main journals between these two.
09-01-2015, 10:47 PM
#39
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FWIW..it seems that Chevy used to cross drill the cranks, but it was discovered that the oil would pack up at the rear of the crank at high RPMs and starve the rods (esp the front ones) from oil..after they quit crossdrilling the cranks, the problem went away. At least that is what I've been told by a VERY knowledgeable guy who works with Lots of Chevys. Could this be our problem also? and would that perhaps be a reason that Porsche quit crossdrilling the cranks? 2 cents worth..
09-02-2015, 12:31 AM
#40
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Pardon my ignorance, but aren't the 928 crankshafts all already cross-drilled from the factory? Both the rod journal is cross drilled and the main journal is cross drilled in the 928 crankshaft, and one drilled passage connects those in the 944 crankshaft and two in the 928 crankshaft? In the 928 crankshaft the rod journal and main journal are cross drilled from the factory in all the cranks that I've seen, but I am not sure whether this is also the case for the 944, or whether the 944 drilling only goes from one side to the center. I think (but do not know) that the 944 crankshafts have the main journal cross drilled thru but not the rod journal.
It's my understanding that the cross-drilling the main in the way its done for these Porsches requires a relatively high oil pressure. The large main journal diameter further increases the require oil pressure. This is because oil first needs to be accelerated to the surface speed of the main journal and then pushed against the centrifugal (centripetal for those with a bug up their a$$es) force to the crank centerline. Smaller the main journal diameter and the larger the minimum radius of the oil passage relative to the crank centerline, the lower the required oil pressure. Fortunately for us with stock oil pumps, the supply pressure is high enough to overcome these effects but if you're planning to get the maximum power out of the engine and run an external oil pump with lowest possible supply pressure, a stroker crank with a different oil passage scheme may give one some (marginal) efficiency gains.
It's also my understanding that the drilling modification that is popular for the 928 crankshafts is not really cross drilling (it's already cross drilled from the factory) but web drilling the crankshaft. The web drilling of the crankshaft doesn't really help as long as there's clean oil in the main gallery. It may however help when the pickup sucks a little air. This is because the way the main gallery geometry works is that a disproportionate fraction of air gets fed to the #2 main and thus to the 2/6 rod journal. Web drilling the crankshaft adds an oil supply from the center main and connects all oil passages in the crankshaft. This doesn't reduce the total amount of air in the oil, but the theory is that it will distribute the air more evenly between different rod throws and therefore give one a little more margin of safety when air gets into the oil circuit. Whether the theory is correct is anyone's guess.
Not pretending to be an expert here, this is just what I've read and how I understand it to work.
On the rod journal modification that is the topic of this thread: This seems to be a no-brainer modification for anyone rebuilding the engine on a budget and reusing stock rods. There's a lot of bang for the buck in terms of increased reliability relative to Glyco bearings. Just an opinion.
It's my understanding that the cross-drilling the main in the way its done for these Porsches requires a relatively high oil pressure. The large main journal diameter further increases the require oil pressure. This is because oil first needs to be accelerated to the surface speed of the main journal and then pushed against the centrifugal (centripetal for those with a bug up their a$$es) force to the crank centerline. Smaller the main journal diameter and the larger the minimum radius of the oil passage relative to the crank centerline, the lower the required oil pressure. Fortunately for us with stock oil pumps, the supply pressure is high enough to overcome these effects but if you're planning to get the maximum power out of the engine and run an external oil pump with lowest possible supply pressure, a stroker crank with a different oil passage scheme may give one some (marginal) efficiency gains.
It's also my understanding that the drilling modification that is popular for the 928 crankshafts is not really cross drilling (it's already cross drilled from the factory) but web drilling the crankshaft. The web drilling of the crankshaft doesn't really help as long as there's clean oil in the main gallery. It may however help when the pickup sucks a little air. This is because the way the main gallery geometry works is that a disproportionate fraction of air gets fed to the #2 main and thus to the 2/6 rod journal. Web drilling the crankshaft adds an oil supply from the center main and connects all oil passages in the crankshaft. This doesn't reduce the total amount of air in the oil, but the theory is that it will distribute the air more evenly between different rod throws and therefore give one a little more margin of safety when air gets into the oil circuit. Whether the theory is correct is anyone's guess.
Not pretending to be an expert here, this is just what I've read and how I understand it to work.
On the rod journal modification that is the topic of this thread: This seems to be a no-brainer modification for anyone rebuilding the engine on a budget and reusing stock rods. There's a lot of bang for the buck in terms of increased reliability relative to Glyco bearings. Just an opinion.
Just offering what I recall from my 928 ownership days. However, a simple search on the 928 boards will reveal a number of posts and questions & responses to those who have built motors or stroked motors as well as just trying to ensure that they don't experience the dreaded #2 and #6 crankshaft bearing failure.
A common solution was to cross drill the crankshaft journal in the same direction as the 'Chevy' route.
Not saying this is the fix, nor am I saying that I have first hand experience with this mod.
Not interested in a debate on the topic. Merely passing on information - I suggest if you, or anyone else is interested or curious regarding this topic to use the search function on the 928 board here. Read and decide for yourself.
YMMV
09-04-2015, 11:18 PM
#41
Rainman
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Can anyone tell me if the replacement bearings for this service are available in 0.50mm undersize? Or any undersize?
Got a (few) crank(s) that might be salvageable at undersize, but Glyco only makes 944 in 0.25mm, don't know if it's enough.
Got a (few) crank(s) that might be salvageable at undersize, but Glyco only makes 944 in 0.25mm, don't know if it's enough.
