Laust Pedersen

What's your oil pressure when it drops? Sounds like it's too hot which would easily cause the oil pressure drop. If you have the stock oil cooler, and I don't know that you do, you can easily see 250F-260F on the track. That alone will cause a huge pressure drop... not to mention drastically lowering the engine life.

Just a thought.

TonyG

 

I've been talking with an engine builder in Atlanta and his recommendation is to not drill the crank but rather obtain & install the other half grooved main bearing shells, (which don't come on 944's), to give a full grooved bearing effect.

He states that the full grooved bearing flows more oil onto the crank thus feeding more oil to the rod bearings.

Thoughts?

 

Guys I am not telling you what I think just quoting what the factory had to say about rod bearing oiling problems (they had it back in the late 60's with the 908 and then the 911). There solution on the 917 was to send the oil down the center of the crank from either end. This was possible with the 917 because the power takeoff was from the center of the crank. The fact that they only needed 34LBS of pressure to run 10,300 rpm indicates that Bengt is correct about the oil from the center of the crank being forced to the rods by the centripetal acceleration. However the fact that the engines with "normal" oiling of the mains took much more pressure (90-100lbs) indicates that it is difficult to get the oil to the center of the crank. The article I listed was from a mid 70's Panorama. It had nice drawings showing the relative pressure at each journal on the crank. I am not saying this is the cause of 944 number 2 problem just food for thought.

john

john

 

I thought the problem was getting oil from the center of the rod journal to the journal face. If oil is being feed in the opposite direction of crank rotation, there would be centripetal forces acting on oil flow. And since that leg of the oil passage is at the the farthest point from the crank centerline. It has the more, centripetal force,per unit, than the centrifugal forces acting one the rest of the oil in the crank shaft. (except the feed points on the main journals).

If only centripetal forces were being taken into account. Cross drilling should solve any rod journal oiling problems. But, Perp drilling may take advantage of a few different things other than centrifugal forces. Possibly introducing oil to a portion of the surface that is not loaded the same way (less load=larger oil gap=higher potential oil flow?) Just a thought.

 

Fortysixandtwo,
The centripetal forces are directed towards the rotation centre (and the imaginary centrifugal force in opposite direction). Any tangential forces would be based on accelerating or decellerating the crank rotation but these forces are much smaller and can be disregarded.

I calculated the size of the forces ( correct me if I'm wrong, it is some time since I went to school).

Centripetal force = mass X square(speed)/radius
This equals mass X square(angle speed) X radius
Mass = density X cross section area X dr
Integrated from center to surface ( 0-R) gives
Force=Density X Area X Square(angle speed) X square (radius)/2
Pressure = force/Area

Pressure = Density X Square ( Angle Speed) X Square (radius)/2

I guessed some data

Density 800kg/m3, rpm 6500, radius of mains 0,04m
This gives 3 bars at the surface. I interpret this as if we force feed with less than 3 bars there will be negative pressure which could lead to forming of gas bubbles and broken oil supply.

I haven't double checked this ( just some lunch time thinking).

Bengt

 

TonyG,
As mentioned, my oil pressure drops to about 3.0 bar (at high rpm) after some laps on Willow Springs, contrary to daily driving where it is 4.5 bar (at high rpm).
I do use a standard oil cooler and 5-50 Castrol Syntec. If you have any suggestions beyond going to 15-50 oil I am all ears. Btw I did use my custom water injection, which didn’t feel like much power gain, but the coolant temperature stayed at normal all the time, contrary to prior occasions, where it was close to max.

Bengt,
I assume that your numbers are correct, but believe it is more relevant to think in terms of flow and resistance. Imagine for example that the 4cm oil column at 6500 rpm is closed at the end (zero bearing clearance) and the oil pump pressure is 4.5 bar, then the pressure at the bearing would be a healthy 3+4.5=7.5 bar. As the bearing opens up, the bearing pressure will decrease depending on the volume flow and the resistance on the way to the bearing compared to the resistance out of the bearing.
It is interesting to compare the cross-sectional area for the oil leakage out of the bearing to the diameter of the oil supply hole. Remembering that the bearing leaks to both sides the cross-sectional area for the oil leakage is: A = 2 * [((D+c)/2)^2 - (D/2)^2]*PI and the equivalent hole diameter is d = 2*SQRT(A/PI). Plugging in 50mm for the bearing diameter (I am guessing here) and 0.05mm for the clearance (double oil film) A = 7.86 mm2 which gives an equivalent hole diameter of d = 3.16 mm.
The rule of thumb is to have a bearing clearance between 0.1% and 0.2% of the journal diameter so if the clearance is at the upper end, the equivalent hole diameter is d = 4.47 mm.
Even if the two areas were to be the same, the bearing would have more resistance due to the shear forces between the journal, oil and bearing.
This was really more than I wanted to write on the subject and I am sure that Porsche have lengthy computer programs that takes more things into consideration and calculate these things more accurately, but also remember that they have to assume that everything works close to intention, such as oil pump flow and pressure, consistent oil supply, pressure relief valve, no clogging of channels, etc.

Laust

 

Laust
I am not sure which situation you refer to.
I made a quick drawing here of what I tried to state.
If the centrifugal forces on the oil column in the main (F1) is bigger than the oil pump pressure then F1 and F2 will drag the oil apart at the center like a bursting weak rope. If the oil is strong enough ( it can take some negative pressure) then F2 will suck oil to the rod journal. If the oil contains dilluted gases ( which is normal) then gas bubbles can form which breaks the oil column and then no oil can be sucked.

 

If it's the #2 rod bearing that always spins then there has to be a difference in the oiling between the bearings.
My guess, without looking at the crank, is that the oil feed from the mains to the rod bearings is pretty much the same for all bearings.
This would indicate that there is a difference in the delivery of oil to the mains. Has someone studied the routing of the oil channels to the main bearings?

If it's always #2 that fails then the problem is likely to be in the oil delivery to that main bearing.
If it's completely random which bearing that fails, then the problem is likely to be in delivering enough volume of oil, and with correct viscosity to the mains.
If #2 fails more frequently than the others then there is probably a combination between these problems.


I don't belive in cross or perp drilling of rod journals.
The holes in the rod journal are placed where they are to deliver the oil at the correct moment, from what I have seen other cranks have them in the same position as a 944.
Perhaps perp drilling works but the technical explanation on how it's working is definitely flawed.

Laust's ideas of the area (diameter) of the oil holes in the crank could be worth checking up, how do the size compare to other cranks with the same journal diameter and stroke?

Tomas

 

The oil delivery to the mains are from a common large diameter (15mm or so) tube along the block to which oil supply holes are drilled from the mains. All main journals that feed rods have bearings with recess groove in the upper shell and cross drilling, so the crank is always fed with oil pressure. The center main is not connected to any rod bearing.

I agree that too much play and you would loose more oil to the sides of the bearing than you can supply from a small hole. Higher viscosity will slow down the escaping oil but will also cause more flow restriction in the supply holes. So perhaps widening the holes and running higher viscosities would help? Higher viscosity will give higher oil pressure but since that is measured in the supply pipe, the pipe must be wide enough to deliver that pressure. I can't see any drawbacks with widening the holes.

Bengt

 

Bengt,
I was going through a few exercises on pressure, flow and clearances at the rod bearing.
I think I understand what you are saying though and in essence you cannot (statically) pull more on a liquid than 1 bar before it separates, meaning that oil pressure (OP) has to overcome F1 minus 1bar, assuming that (F2 - F1) > 1 bar or said a little more technically OP > F1 – MIN{(F2-F1), 1}. So in essence F2 is “helping” the oil pressure by up to 1 bar.
For oscillating pressure, such as during high intensity sound transmission, you can momentarily pull significantly more than more 1 bar on a liquid until cavitation occurs and that threshold is typically proportional to the square root of the frequency and the presence of micro bubbles. The reason for the higher (negative) pressure is, that a liquid has some inertia and it takes a (short) while to pull it apart, but this is a different subject.
Laust

 

there is lots of technical theory here but no real answers i think I might start another 'cheery' not thread.
Tony

 

I tracked a 944t for six years (25 days a year) with no engine problems, and courtesy of Jon Milledge's advice, here are the rules:
-You need 10 psi per 1,000 rpm, so at 6,000 rpm, your oil pressure should NEVER drop below 60 psi or 4 bar.
-To achieve this, you need to avoid Mobil 1 and use Amsoil or Redline synthetic, and have a large oil cooler. The standard 944t oil cooler is too small for the track.
-Also install a hinged baffle in the sump to avoid a pressure drop on left hand turns as the oil runs up the side of the sump. And install a collar around the oil pump pickup.
-Adding an air/oil separator will help (although I didn't have one)
-The only true solution is a dry sump system - anything else will eventually fail
-Change your rod bearings regularly

I retired from track after two serious crashes, but that was my experience FWIW.

 

Excellent input thankyou,
Tony

 

Here's a pic of my perp drilled crank. I drilled all the rod journals at the end of throw. I did this myself oa mill was very easy to do. Most any competent engine shop should do it and chamfer the holes for 75 or so.

 

were is the pic?
and hey smokey you had no full groove bearingings and no drillings. no one is realy confident the drilling works ? think i might skip it seems the mony is better spent on an oil cooler than drilling