Rod bearing analysis of race engine

[snowman]

The following photos are of the rod bearings in my 912 race car engine. They are after 25 hours of 8000 rpm plus operation and at least a couple of excursions to 10,000 rpm!. The overall condition is excellent, ie the initial bearing clearence was 2.4 thousands, and the final 2.6 thousands of an inch. Note even the tin plating is still visible over a large part of the bearing.

HOWEVER also note the small area where "worms" are starting to grow in the bearing. At this point I attribute the smallness of the worm area to the following:
Redline race oil, 30 wt
Scat Crankshaft
Near perfact engine balance

On earlier engines I have done, using original Porsche cranks and regular race oil and Mobil 1 type oils, I have seen the worms grow to half the rod bearing area in the same amount of time at slightly lower rpms.

Is this improvement due to:
1 Redline oil
2 Scat crank
3. both
4. small sample size
5. other

Heres the pictures:


My theory is that the pressure is so great that the bearing is overloaded. In other words the oil pressure from the load pushes so hard on the bearing material that it acutally starts to seperate it from itself, causing the "worming" effect. Or simply bearing overload. Solution is for a larger bearing surface or stronger bearing material. Thats my GUESS. So what say the metal people that know what they are talking about??

[camgrinder]

Is the wear pattern tapered to one side of the rod? Its hard to tell from the picture, but the fatigued section of the bearing looks like it is closer to the camera. I would check the crank journal for taper, and the check to see if the connecting rods are still straight.
I was told a softer bearing material has better embedabilty. It can trap small peices of dirt. I had an old Vandervall bearing catalog that had a very good technical article on bearings, but I couldnt find it.
How round are the big end housing bores after those 10k runs?

[cgarr]

How are you filtering your engine oil?

[cstreit]

Debris was my first thought, but it usually stays embedded in the bearing. That looks almost like galling to me...

[asphaltgambler]

Looks like the bearing was subjected to excessive heat, then started to fail.

[Tar356]

So many variables....

Maybe this was a better batch of bearings than your previous engines? Who knows?

Hard to tell from the pictures....some of the holes appear to be an embedment of some sort (dirt) rather that torn (wear).

It's difficult to gain hard data from just one rebuild and so many gray areas....rpm, load at a given rpm, temperature, etc. I guess after a few engine rebuilds you could start to establish a trend.

I will admit to seeing a "trend" in better bearing life in recent years which I'm willing to attribute to modern oils.....for lack of any other reasonable explaination.

With regards to your comment of needing a "bigger bearing".....maybe not necessarily bigger, but maybe thinner. That's been a long problem of the SC/912 bearings.....they're thick, therefore they're taking the load instead of the rod. Thinner stronger bearing shells won't deform as easily, and will back the bearing surface better. Obviously, this would require custom rods and bearings.....it's cheaper and easier to just replace the stock bearings on a regular basis.

At 8,000-10,000 rpm I'm curious to hear how the valvetrain is holding up!?

Todd

[mpdevelopment]

Those bearings are showing signs of spalling. This is due to mechanical overload. At the rpm you are turning the loading on the rod cap bearing due to inertia is too great for the bearing material. If you had a chevy size journal you could get a bearing with a harder overlay that would handle the loads. Check out Federal Moguls site for info on engine bearings. If you can't get better bearings you could try lighter pistons and or titanium rods to reduce the loads, but 25hrs use at 8000+ rpm is a long time and that may be the limit anyway.

[snowman]

I agree with MPDEvelopment. I used to see much worse when I used ordinary engine oil (the ester base oils are hard to beat) and also when I was using Porsche cranks. Then the "worms" grew much larger. The worms are on the rod side of the caps. Why they are on one side and not the other, don't know. Could be related to direction of rotation, some extreemly small taper??? Carillo rods are hard to tweek or bend and the bearing wear is uniform. So just don't know.

No dirt is involved. There are tiny specs of dirt, but not very many and none in my pictures. A full flow oil system is used with Fram HP 1 filter.

All parts are in as new condition, round. The bearing wear measures 0.0001" on the cap and 0.0002" on the rod side. No measurable taper. The rods are still perfactly round well within my ability to measure it with a Sunnen rod resizing machine that can see 0.00001" wiggles. Crank journals are as new.

[cgarr]

what does the hp 1 take the filtering down to? and do they hold up to 100 psi? I am using the canton filters in my 912 which are about a 8 micron set up to for as a flow filter, whats nice is you can take the filter apart and see what you catching if anything? Are you getting any oil starving in turns by any chance?

[cstreit]

Originally posted by mpdevelopment
Those bearings are showing signs of spalling. .

That's the word I was looking for, not galling.

To add to this, you can see that the damage to the bearing is at the bottom, directly below the force transfer of the piston/rod assembly. It's essentially hammering on the bearing.

I'd also check your crank journal for damage. I've seen the bearing actually extruded out of the seat in extreme cases. Down to a paper thin ribbon sitting in the sump.

Bigger bearings would help, otherwise I'd look into refreshing the motor down to the bottom end a bit more frequently than you just did...

[mpdevelopment]

One of the bearings looks like some dirt or other material put a grove in it. The grove will reduce the load carrying capacity of the bearing by over 50 percent. Also if this spalling is on the rod side bearing you may have a detonation problem that is fatiguing the bearing. Do you see any other signs of detonation? damaged pistion ringlands etc. How do the back sides of the bearings look? Do they show signs of metal tranfer to the rod, are they galled and black?

[snowman]

Back sides of bearings are black. No other signs of damage. THere are no grooves in the bearings. NO damage to the journals of the crank. I don't think its to long between changes as no damage was done to the crank and the bearings were a long long way from destruction. I am interested in what black back sides of bearings means.

I am interested in detonation. No damage to rings or ring lands or piston. It could be first signs though, as my compression ratio is 12:1 and I am only running 105 Plus octane gas. Thats a mix of 110 octane race gas with 91 octane unleaded, ie 50/50. I could easily go to straight 110 octane to see if it makes a difference. Up to 120 octane is available, if necessary.

Since I am going to a 330 degree cam I may see more of this next time. We will see. I am looking at what 200 HP? Currently am seeing an actual 140hp at ther rear wheels, continuous, thats what? Between 160 and 185hp at the crank. Note the generator is running and the cooling fan has all the blades (but a "power" pully).

[cstreit]

What ignition are you running? Single or twin plug? What advance?

How's the quality of the 91 Octane you're using to mix?

[rcwaldo]

Originally posted by mpdevelopment
Those bearings are showing signs of spalling. This is due to mechanical overload. At the rpm you are turning the loading on the rod cap bearing due to inertia is too great for the bearing material. If you had a chevy size journal you could get a bearing with a harder overlay that would handle the loads. Check out Federal Moguls site for info on engine bearings. If you can't get better bearings you could try lighter pistons and or titanium rods to reduce the loads, but 25hrs use at 8000+ rpm is a long time and that may be the limit anyway.

I would have to agree. It does seem inertia related.

Not certain how this could relate to a bearing surface, but we discovered an anomaly with an early gearbox design of ours five years ago. I wont bore anyone with the details. PM sent to snowman.

In a nut shell, is the oil being ignited by way of Diesel effect? Small focussed combustion?

[mpdevelopment]

Since you cannot test the octane rating of your mixed fuel don't use it. Use a fuel from ERC called 110k with high convective cooling and 105 motor octane. This fuel will help cool the combustion chamber something any aircooled engine needs. The black on the back of the bearings can indicate a general bearing overload. You may have some detonation, I may not have been correct in talking about inertia because the spalling is on the rod half of the bearing not the cap half where the larger inertia loads are concentrated. I would think you have 160+ hp at the crank 200 with a 912 even with major port-combustion chamber work and giant carbs may be a pipe dream.

[cstreit]

Originally posted by mpdevelopment
...black on the back of the bearings can indicate a general bearing overload...

Can you elaborate on this? Interesting topic.

[WERK I]

Snowman,
Is this a normally aspirated engine?

[cgarr]

Are you talking about movement of the bearing causing the black? It would make sense, this is how you can spot a loose rivets on aluminum aircraft, you get a black line or (hot rivet) when they work loose in the aluminum..

[mpdevelopment]

Don't know what the mechanism that causes the black deposits on the back of some overloaded bearings, but I have seen it on damaged bearings used in big block drag motors. May be due to heat combined with movement of the shell in the rod housing? I often wonder if Porsche used titanium connecting rods in the turbo race engines as an energy absorber of sorts not just for the reciprocating weight reduction. Titanium with a tensile strength = to high strength steel but an elastic modulus of approx 1/2 of steel can absorb a lot of energy. Energy in the form of detonation.

[cstreit]

Interesting. Thanks!