Tom. M

That had crossed my mind...but I ask you?
Glyco...

or Porsche?

GlenL

I use Glyco because they're less expensive.

And aren't the Porsche parts Glyco in a better box?

GregBBRD

Tough to tell everything, from a picture.

The bearings have "touched" the crankshaft and have "moved" the first layer of metal around some.

It would be tough to do this much damage at idle or at cruise....you will virtually never see a bearing this bad in a completely stock street engine...regardless of temperature or age.

Bearings can "touch" the crankshaft for a variety of reasons:

1. "Knocks" can "push" the rod backwards hard enough to "pound" the bearing through the oil film and onto the crank surface. This "shows" up as a loss of "crush" in the bearing cap very quickly. How much "crush" do you have, in these bearings, compared to a new bearing? Note that any "reduction" in octane increasaes knocks. Oil in the intake system reduces octane....significantly. What are you doing to actively keep this from happening?

2. People "joke" about oil...but this is very serious stuff. What kind of oil was in this engine?

3. Obviously, anytime the engine runs "low" on oil (at the pick-up) and the oil pump sees oil/air mixtures, the bearings can touch the crankshaft. You are actively doing things to improve what the oil in the pan is "exposed" to, by adding in an early pick-up, lowering the oil level, and adding in windage trays. If you are spending "extended" periods of time at high rpms, you should do whatever you can to improve the oil return from the cylinder heads.

4. Bearing clearance. "Higher" bearing clearance tends to "allow" more oil between the crank and the bearing. If you get lucky and get a production run of "loose" Glycos....you can run them. If you get a "set" of "tight" Glycos, you will have serious grief very quickly. Porsche "provides" the bearings with "size" mrakings. The difference between each size is generally .0004". So, you can use a set of "red" bearings (if you are lucky enough for them to send you some) and "pick-up" almost .001" of additional clearance over a set of blue bearings. Last time I ordered bearings, from Porsche, they were shipping "two yellows" in a box. Not "great", but better than a set of "tight" Glycos.

5. These bearings (both stock and Glycos) are very soft. Have them coated. Note that coating "reduces" the bearing clearance. You need to "actively compensate" for this.

Note that the above is only my opinion, based upon my personal experiences with the 928 engine. Others (with more experience) may not agree or have different opinions.

Tom. M

My responses/questions in red...

85euro928

Will the person or persons with more experience than Greg Brown please step forward?

PorKen

James Bailey

I propose we all just take a step back

IcemanG17

Tom
This setup will work VERY well....... Depending on your HP level you might consider an oil cooler and even an accusump.....

If you are running a 32V engine (I think you are) then you need to take a serious look at crankcase breathing......16V is much easier...just a vent-separator works fine..... But the stock oil filler on 32V engines is HORRIBLE.....just a pure near straight up vent....no built in separator like the 16V....

Brian

GregBBRD

Actually, the entire breathing, ventilation, and oil return problems generated by the lack of correct breathing and ventilation are absolutely terrible. Porsche really fell asleep on this aspect of the car....and then continued making the problem worse, as the years went by.

By the time they made the GTS engine, they had completely messed this entire system up so badly, that they simply gave up. They decided to "allow" the crankcase to build pressure and compensated for that by not drilling the oil return holes in the pistons. (Yes, that was not an oversight, but intentional.)

To get all of these things to work, in harmony, took some serious engineering. I literally spent months working on these systems to "solve" the various issues. The solution was not simple. Yes, one can easily "separate" the oil out of the air returning to the intake.....that's simple. But to fix the "entire" set of problems took some significant changes and the "kit" to fix it is very complex. Literally, there is nothing left in the ventilation or breathing system that remains "stock"....because, Porsche "missed" what the problems were, from the very beginning. They just kept making changes to the "same base problems" and never went back to the very beginning. My "dry erase" board had a entire list of the problems, gleened from years of looking at these engines and their associated problems. I was forced to literally start with a clean sheet of paper and work my way forward, from the very beginning.

Again, these are just my findings and my opinions. Others may not agree and have other, better, opinions and ideas to solve these problems.

69gaugeman

The bearings are soft and for a reason. The bearings in an ideal world NEVER touch the journal. They in fact are separated by a film of oil. Greg is absolutely right that any wear is from contact that should never happen in theory. The problem is that in practice this is impossible to prevent. The single largest wear is at startup when it has sat for a significant length of time (hours).This is when all the oil has had time to drain away.


The bearings therefore are designed to try and minimise this by making the bearings of different layers. The layer closest to the journal is made of Babbitt. This is a very soft later that can touch and not damage the journal. It does however wear very quickly. The next layer is usually aluminum or sometimes Copper. The next layer is copper or steel and is the base layer.More and more Babbitt is not being used due to the lead content.


There are complex reasons for what they use which we don't need to go into here, but it is why oil analysis works so well. As the bearing wears it deposits its material into the oil. When you see copper or aluminum rise in the oil it's time to open up the engine.

69gaugeman

Greg is also correct in that too tight of a clearance is not good. Keep in mind that too much clearance allows the oil to be forced out and all hydro dynamic effect is lost. It is an art to get this right. Side clearance plays into this as well. Oil volume is the key here (and not pressure, to an extent) if the volume drops off you lose that film. Now when designing an engine you must take into account the flow and volume required to maintain the film in all the journals. This is mind numbingly difficult to due. Almost impossible in 1975 when the engine was designed and there were no computers around to help. Then you must design an oil pump to supply that and calculate losses from wall fr ction and flow patterns of fluid dynamics and temperature differences and viscosity changes and you can START to see the difficulties of engine design.now you must account for how much will be in suspension and where it goes with g forces etc, etc.


All this just for the oil....it is incredibly complex.

upstate bob

That bearing half looks like it encountered some crapola. I would open up the filter and rinse the element into a clean solvent can to examine. FWIW, I still like valvoline semi synthetic, and 928 motorsports' oil control kit worked wonders with my old '79. -74K mi., driven mostly in a stodgy manner.

GregBBRD

All very true.

Perhaps important to say that there are many different kinds of rod bearings. "Soft bearings" are designed to "absorb" dirt and debris, so the crankshaft doesn't get damaged by these things. Consequently, many OE engine manufacturers will put "soft" bearings into their "street car high intended mileage" engines.

There are also much "harder" rod bearings, which will not "absorb" dirt and debris, like a soft bearing, but will "stand up" to increased load levels.

Porsche used the "soft" street car bearings up to the development of the water cooled Cup Cars. These engines spewed the connecting rods out of the original versions, as fast as Porsche could put them together, and Porsche quickly replaced the crankshafts, rods, and bearings with a bearing design that was much "harder".

"Knocks" are very hard on "soft rod bearings". Many people learned this, the expensive, hard way. The "last" of the air cooled Porsche race engines (993 RSRs) had the soft "street car" rod bearings. These engines had very little "ignition timing" in them, to make them "live". There was quite a bit of "additional performance" to be obtained by increasing this very conservative timing (like there is in a 928.) The problem was (and is, when timing is added to any engine with soft bearings) that the rod bearings would deteriorate and a connecting rod would "spew" out of the engine at about 6 hours of running....from virtually any knocking.

Leon Speed

Great read in this thread, thanks for the insights.

jpitman2

Rod bearing crush - WSM for one car I had many years ago read in this area:-
Get a selection of bearing shells of correct sizes for the journals.
Fit a pair in a rod, torque the bolts/nuts
Undo ONE bolt/nut, check the size of the opening at the cap to rod joint - X is optimum. More means shells are too big for the eye (and the mating ends may bend inwards), less means shells are not being clamped enough. If incorrect clearance, select another pair of shells.

Thats my understanding anyway.

Am I correct in thinking the crank journal above looks OK?
jp 83 Euro S AT 55k