Engine teardown results

[altiain]

Well, I got a call from John Day this morning. The diagnosis: #2 & #4 rod bearings were damaged by oil starvation.

There was plenty of oil in the pan, but I think I've changed the oil since the noise started. I do recall seeing comparatively low oil pressure (I have a converted guage) after a set of runs at one of the last brutally hot S2k events. The combination of long runs, hot ambient temps, and a slightly low oil level probably did my motor in. This is with 0w40 Mobil1, btw.

Long story short, probably not a bad idea to overfill your crankcase by a half a quart, especially at something like a track day or midsummer S2k autocross.

We had a short discussion on oil pickups and lateral loads in autocross. He said that if the Miata were a circle track car, he'd recommend dry sumping based on the peak lateral Gs I've datalogged in the past!

[MadMerv]

That's odd that it took out #2 and # 4 and left #3 ok. I would expect that all four would show some damage if it was starvation. I can't imagine that an autocross course would give you enough time in a left turn, then right turn to do that either. I can see it more easily in Wagonwheel at the Ranch, the carousel at TWS, or the Roval at TMS, but have never seen oil starvation on any of those in a Spec Miata, Sunny(92), Shelby(94), or Luna(99), and when I dropped a valve in Speedie, all the bearings were still good after 7600 RACING miles. I think we are missing something here..... You think maybe that 0w40 might be too thin in the heat?

[TheChadVIII]

Man it sucks so bad that not having a few cents worth of some crap you'd hate to get on your pants can cost you so much money.

[POS Racing]

That's it I'm dumping a can of Restore in the car!

[altiain]

That's odd that it took out #2 and # 4 and left #3 ok. I would expect that all four would show some damage if it was starvation. I can't imagine that an autocross course would give you enough time in a left turn, then right turn to do that either. I can see it more easily in Wagonwheel at the Ranch, the carousel at TWS, or the Roval at TMS, but have never seen oil starvation on any of those in a Spec Miata, Sunny(92), Shelby(94), or Luna(99), and when I dropped a valve in Speedie, all the bearings were still good after 7600 RACING miles. I think we are missing something here..... You think maybe that 0w40 might be too thin in the heat?

Merv, would you suggest running 50 weight oil?

One thing to keep in mind about autocrossing is that it develops an entirely different set of thermal strains on an engine than road racing. In road racing there is generally a cool down period, and during the intervals of high engine RPM there is typically a lot of cooling flow over the radiator, oil cooler, and engine components in general. By contrast, autocross cars are often subjected to short but intense bouts of engine usage with minimal cool down periods. At a format like the S2000 events, the engine is given just about enough time between runs to heatsoak, and then forced to run again. Perhaps not such a good format...

For what it's worth, I've experienced a whole lot more incidents of creeping radiator temps autocrossing than track driving, just because there is greater opportunity for heatsoak.

Let's also keep in mind that this engine had 95,000 miles, the last 65,000 of which were at my hands, with a couple of thousand track miles and over 600 autocross runs. I think I'll definitely overfill the crankcase and be a lot more diligent about monitoring the oil level from now on, but other than that I'm not sure what else I can do.

[Billy Blaze]

Iain, why do you use 0w40 instead of another weight?...I'm just ignorant and curious.

[Titus]

Iain, why do you use 0w40 instead of another weight?...I'm just ignorant and curious.

The first number is the cold number. The lower it is, the thinner the oil. You want the oil really thin so that it can quickly get distributed throughout the engine when you start it. This thin condition is also good for the Miata's common HLA noise, as the oil gets in there and calms them down quickly.

The second number is how thin the oil is at temperature (I think it is 210?). The higher the number the thicker, but here thick is good. The thick oil provides more protection from the high temperatures.

[Billy Blaze]

Thanks Titus, I was curious as I have never seen a 0w the lowest I have seen in 5w.
I knew the lower the 1st # was better but was clear on why, makes a lot of sense.

[altiain]

The first number is the cold number. The lower it is, the thinner the oil. You want the oil really thin so that it can quickly get distributed throughout the engine when you start it. This thin condition is also good for the Miata's common HLA noise, as the oil gets in there and calms them down quickly.

The second number is how thin the oil is at temperature (I think it is 210?). The higher the number the thicker, but here thick is good. The thick oil provides more protection from the high temperatures.

For multi-viscosity oils, the first number is the oil's relative viscosity at 0 degrees Celsuis (32 degrees Fahrenheit or the freezing point of pure water). The w denotes "winter" weight.

The second number denotes the relative viscosity of the oil at 100 degrees Celsius (212 degrees Fahrenheit, or the boiling point of pure water).

The reasoning Titus gave is the same one I use when choosing a motor oil viscosity.

[Billy Blaze]

Iain am I right in guessing you are an engineer?

[altiain]

Iain am I right in guessing you are an engineer?

Yessir. Mechanical Design Engineer.

Why? Is my pocket protector showing?

[altiain]

BTW, in an email I got from Gary today, he mentioned the fact that the damage to my motor is nearly identical to the damage majik saw in his first motor.

Hmm... I wonder if the problem is autocrossing, or Emerald Mica paint?

[POS Racing]

BTW, in an email I got from Gary today, he mentioned the fact that the damage to my motor is nearly identical to the damage majik saw in his first motor.

Hmm... I wonder if the problem is autocrossing, or Emerald Mica paint?

I'm hoping it's the Emerald Mica!

[altiain]

I'm hoping it's the Emerald Mica!

You shouldn't worry too much POS. You probably have to drive fast in order to do any damage.

[POS Racing]

You shouldn't worry too much POS. You probably have to drive fast in order to do any damage.

Aw now you went and hurt my fellers! I'm going to drive away slowly.........

So how did the rest of the engine look? Was it just the #2 & #4 rod bearings or were there other issues??

[Billy Blaze]

Because Iain, I ran into Engineers a lot in my last profession as an Optician.
I became pretty good at detecting certain clues about a person.
One being that Engineers are over stimulated by the way something works.
I always thought this to be a good thing since you guys make the world go around.
I have noticed that you are very precise and informative with your posts.

[MadMerv]

I would never recommend 50w oil, but I would recommend a single viscosity. It seems to be more stable. The W should really be determined by bearing clearences. The tighter the clearences, the lighter you can go. The engines that seem to make the most power are loose, so need heavier oil, of course they don't last very long. I prefer a bit tighter tolerence and thinner synthetic oil. I still think it is odd that the only bearings that seem to be hurt are 2 and 4. Just trying to help, that's all.

[POS Racing]

MadMerv what oil weight and brand do you run in your cars??

[Miatamoto]

Iain am I right in guessing you are an engineer?

They prefer "enginerds"

[altiain]

I would never recommend 50w oil, but I would recommend a single viscosity. It seems to be more stable. The W should really be determined by bearing clearences. The tighter the clearences, the lighter you can go. The engines that seem to make the most power are loose, so need heavier oil, of course they don't last very long. I prefer a bit tighter tolerence and thinner synthetic oil. I still think it is odd that the only bearings that seem to be hurt are 2 and 4. Just trying to help, that's all.

No problem Merv, and no offense taken to your suggestions or your input. In fact I welcome it, as you've got a lot more knowledge about track-induced engine damage than I do. Any idea who makes single weight synthetic oils?

In an email conversation with Gary this afternoon, another possible autocross-related explanation for oil starvation occurred to me. One situation that is found to a much greater extent in autocrossing than in road racing or track driving is high frequency transitional manuvers - things like slaloms and offset gates. Sloshing all of the oil back and forth in the pan through a series of slalom sections - especially with the extremely high lateral G loads that can be achieved on modern R compound tires - might be enough to foam it to the point where the oil pump's pickup isn't getting a reliable supply of oil.

Unfortunately, all of the potential solutions to this problem - baffling the pan, modifying the windage tray, converting to dry sump or adding an accusump - are all illegal in Stock.