Due to much discussion regarding why the Porsche 944 engine can tend to trash the #2 rod bearing, I took the opportunity to look closely at an engine to see how the oiling system worked. As you look at the oiling system layout, you can begin to understand how the #2 bearing might be at risk. And to jump to the conclusion, #2 appears to be at risk for seeing an oil supply with a high percentage of oil with air bubbles in it. Stay tuned and all will be revealed.
Note: A picture is worth many words so I’m going to try and minimize words and maximize pictures. Note that most of the pictures are of an engine turned upside down so don’t let that confuse you. The engine shown is from a 1984 car.
Most of us understand that the oil starts its journey in the sump. Pooled in the bottom of the sump, it enters the oil pickup tube.
Oil pickup tube with screen. Oil return tube (no screen)
In the above picture, the oil return tube is in the foreground and the oil pickup tube is in the background. Both tubes attach to the “crank girdle”. In the next picture we’ll see the passageway that lies under the crank girdle, on the bottom side of the block, that transfers the oil from the pickup tube to the oil pump mounted on the front of the engine. I have used red wire to highlight the passageway. This picture is looking at the front of the block, without the oil pump mounted in position.
There are a couple of things to note about the above picture. The oil pump is not mounted but if it were, it would suck the oil from the passageway and insert it under pressure into the hole in the front of the block where the red wire disappears into. The green wires are stuck into the oil passageways for the crankshaft main bearings. We’ll come back to them.
In the above picture the red wire enters the front of the block. Near its entry point is a round hole. This hole is normally plugged with a cap. This hole passes through the length of the block and is the main “oil gallery”. The following pictures show the cap. There are caps at both ends of the block so if you remove them both, you can give the gallery a really good cleaning. I highly recommend this. The caps are destroyed upon their removal but new ones are like $2 a piece.
Oil gallery cap
Now we’ll look around the side of the block where the housing for the oil cooler (on a N/A engine) and the oil filter attach. Its hard to show but I have used a red wire to show the hot uncooled oil and a green wire to show the cooled filtered oil. It is important to note that the flow back into the block is pointing right at the #1 crankshaft bearing, with no turns or changes in direction required. Conversely and very important, the oil for the rest of the engine has to go around the first of several 90 degree corners to get into the oil gallery. The second picture shows the fitting for the oil filter.
So let’s go back to that photo of the block without the crank shaft or bearings installed. There are 5 crankshaft bearings on a 944, a very robust design. Note that each bearing has a passageway drilled from the bearing back to the oil gallery. These are shown by the green wires in the photo. If you could see the other end of the green wires, you would see them sticking into the oil gallery, at a right angle.
Green wires show the drilled passages for the crankshaft bearing oil supply.
Here is another picture with the bearing shell ready to be installed. You can see how the oil is allowed to get through the bearing and flood the narrow space between the bearing and the “journal” on the crankshaft. A “journal” is the smooth shiny very hard surface on the crankshaft that serves as 1/2 of the bearing surface. The other half of the bearing surface is formed by the bearing shell, which is hopefully the wear item that can be replaced during an engine rebuild.
So the oil floods the space between the bearing and the crankshaft journal. Where does it go from there? The answer is revealed by a close examination of the crankshaft. This is a little hard to show because all the passages are internal to the crank. First, each crankshaft journal is drilled with a hole radially towards its mid-point. This hole is intercepted by a diagonal hole that heads off towards the rod bearing journal. There is a hole drilled radially through the rod bearing journal that intercepts the diagonal hole. Note that the diagonal hole is plugged at its end, once drilled, to blank it off and force the oil into the rod bearing journal area. So the oil goes radially inward from the crankshaft journal, then diagonally outward towards the rod bearing journal, then radially outward to the surface of the rod bearing journal. Whew! Here are a few pictures.
In the above 2 pictures, I have just run the green wire externally to the internal passages in the crank, to give you an idea of how they run. The actual passages are hidden. If you look closely to the right side of the above picture, there is a machined hole. This is where they inserted the drill to make the passage, then installed a plug. Also note that if the rod bearing journal is “cross drilled” the hole shown in the above picture extends all the way through the rod bearing journal. If it is not cross drilled, it only extends halfway through the rod bearing journal. Regardless of either design, 100% of the oil coming out of the diagonal passageway reaches the bearing. But being a zero sum game, I don’t see that cross drilling does anything to improve the oil supply, as there are so many passages before it that serve to limit the flow. Think of it like the interstate leading into Charlotte. Its 2 lanes north of town and then opens up to 4 lanes nearer to town. But during the morning rush hour, the 2 lane section is stop and go. Having the 4 lane section down the road really doesn’t help much because the 2 lane section is the restriction to flow. Indeed, I have seen that early cranks (83, 84) were cross drilled by Porsche but later cranks are not.
FYI, near the rear end of the oil gallery there is yet another 90 degree branch that shoots straight up, goes through the headgasket, and enters the cam housing to lubricate the cam. This oil returns to the sump via drain passages.
So finally the oil has reached the rod bearing. If all goes according to plan, the space between the rod bearing shell and its journal stays filled with a very thin layer of oil at all times and all is well.
Now a diagram would be in nice. But in words, we see that the oil to the 1st rod bearing has no 90 degree turns and two 45 degree turns. The oil to the #2 rod bearing has a 90 degree turn to get out of the oil gallery and then two 45 degree turns. The same is true for the #3 and #4 bearings. It is worth noting that the oil for all but the first crankshaft bearing also goes through a 90 degree turn.
So my theory (not original to me but I subscribe to it) is that the oil in the sump is “aerated” (air bubbles are introduced into the oil) by the crank beating the oil like an egg beater. This mixture of oil and trapped air bubbles runs through the path described. Every time it goes through a change in direction the air, being lighter, is happier to go around the corner than the oil is. That is not of consequence until there is a split or branch in the passageway. The air bubbles are willing to go around the corner, the oil not so much. As we have seen above, the oil coming out of the filter/cooler goes straight on to the 1st crank bearing (and eventually to the 1st rod bearing). It has to go around a 90 degree corner to enter the oil gallery so all the oil going to crank and rod bearings 2 through 4 has a higher concentration of air bubbles. When it gets to the first branch off the oil gallery going to the 2nd crank and rod bearing, the air once again is selected preferentially as it is more willing to go around the 90 degree corner. For the 3rd crank and rod bearing the phenomena is the same but the bulk of the air has been extracted. By the time it gets to the passage for the 4th crank and rod bearing, the oil has been de-aerated pretty well. Ditto for the oil going to the last crank bearing and up to the cam.
So I believe that the culprit is the air entrained in the oil and the design of the oil passageways. And what are some remedial solutions? Here is what I have come up with.
- Street use typically means lower rpms and less frothing of the oil. Don’t rev your engine so much!
- Hell, I want to rev my engine! So use a good quality oil that minimizes foaming. There is a long and opinionated discussion about what that oil should be. Check the archives or Bobs the Oil Guy. Personally I use Valvoline Racing oil.
- Use a synthetic oil.
- Use the correct viscosity. More discussion and opinions abound. I use 20W-50.
- Keep your oil filled to the mark but do not overfill. Overfilling just increases foaming.
- For heavy track use, use aggressive oil change intervals.
- Get a crank scraper to help minimize the foaming. Alternatively use an oil pan from a later model 944 that has a built in crank scraper. See article here.
Except for the crank scraper, all of the above can be done by any and all of us that want to track our cars without too much effort.
As I discussed above, I think cross drilling the crank has no value. I do think that oil pickup extenders and sump baffles are a good idea but more for preventing general oil starvation in long sweepers than anything else.
So there you go. The oiling system for the 944 is reasonably straight forward. And a good oil supply is the life blood of a high performance engine. Treat it well and it will hopefully treat you well.