Liquid oil in the exhaust.... and other oddities

[911monty]

Originally Posted by maytag

Okay, I like this working theory (thanks, by the way). The only question it leaves unanswered for me immediately, is probably born from not having a full-scope view of the AOS system in it's entirety: if it's wide-open, as you've postulated, why is it ingesting oil and not air? Do I have a drain-back that's blocked somewhere, keeping the "intake" of the AOS fed with oil?

Short answer is the AOS is ingesting both. If you look closely at the AOS the bellows at the bottom of the de-mister section is directly attached to bank 1 right over the camshaft drive chain. It is attached to bank 2 through the long convoluted tube that crosses over the engine. Due to pressure drop across that tube the AOS will preferentially draw from bank 1. The million dollar question is why did the plugs blow out of bank 2? Still no answer to did the engine spin backwards, but I'm going to say probably. My theory basically is that one cylinder had recently fired but did not get to exhaust valve open. When the piston reversed, the compression ring lost seal on the piston ring-land and dumped as blowby into the crank case. Excess pressure in the crankcase popped the plugs before it could bleed through that long tube to the AOS which had no where to dump due to loss of vacuum in the intake tract when engine quit running. Pressure then relieved through the first weak point. I don't have the dimensions of the plug that popped out but I'd estimate about 3" diameter? Doing the math that puts it ~7 square inches. A cylinder head pressure of only 2 psi would exert 14 lbs of force against that plug.

[maytag]

Originally Posted by 911monty

Still no answer to did the engine spin backwards, but I'm going to say probably.

Sorry, I didn't mean to leave this hanging. the truth is that I don't know. It's quite possible. Everything happened very quickly. This wasn't a slow spin from powering through a corner, that I was trying to save. This was a very fast LOOP, caused by a previously oiled-down corner on the track. The fact that the motor died as a consequence of the spin could suggest that I didn't get the clutch in fast enough, and so yes, it could have turned backwards.
I realize that's an unsatisfactory answer, and probably shows my novice-level at the track... since this was, in point of fact, my FIRST spin (unless you count the day in 2006, when I took one of the school mustangs out in the snow with a Journalist who'd come to see the then-new track facilities, haha). I have to say though, unequivocally, that the spin in the Porsche was far preferable to the equivalent on a motorcycle.

Originally Posted by 911monty

My theory basically is that one cylinder had recently fired but did not get to exhaust valve open. When the piston reversed, the compression ring lost seal on the piston ring-land and dumped as blowby into the crank case. Excess pressure in the crankcase popped the plugs before it could bleed through that long tube to the AOS which had no where to dump due to loss of vacuum in the intake tract when engine quit running. Pressure then relieved through the first weak point. I don't have the dimensions of the plug that popped out but I'd estimate about 3" diameter? Doing the math that puts it ~7 square inches. A cylinder head pressure of only 2 psi would exert 14 lbs of force against that plug.

this all makes sense to me. What we'll hope then, is that I didn't break a ring-land in the process.

I'll try to run a leeakdown test before I start putting things back together.

[911monty]

No problemo. The oil on the track may have caused the spin but it's possible due to the lack of traction may minimized engine reverse rotation, and that's a good thing. When you do assemble and get ready to fire up, even though the AOS may be undamaged, I'd recommend disconnecting the tube and plug the port to the intake. Then take a hose and run the tube to a bucket. No sense in taking unnecessary risk.

[maytag]

Originally Posted by 911monty

My theory basically is that one cylinder had recently fired but did not get to exhaust valve open. When the piston reversed, the compression ring lost seal on the piston ring-land and dumped as blowby into the crank case.

I'm hoping to get back at it this week.
With this working theory, above, I want to do a leakdown test before i begin reassembly, just to be sure I didn't break a ring land or something.

My problem, or question that I'd appreciate some wisdom on, is this: I've always done leakdown tests on a hot motor, with the understanding that a cold motor won't seal anyway. So... I can't warm this motor up.... it isn't running right now. How useful IS a leakdown on a cold motor? And what sort of numbers are acceptable for a motor at 150k miles?

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[thstone]

Originally Posted by maytag

And what sort of numbers are acceptable for a motor at 150k miles?

Under 5% is preferable. But I'd say that anything less than 10% is probably ok on a well worn engine. This engine had 90K street miles when I installed it in my car. Then I had added 70 track hours of racing on top of those miles! That's got to be equal to 150K or more street miles.

If you see anything like 15% or 25% leak down, its time to worry. With low compression and poor leak down numbers in cyls 4 and 5, I knew that this engine was on its way out. It finally failed 11 track hours after this test.

The fact that your compression numbers are good brings hope that the leak down numbers will also be good. Best of luck.

[maytag]

Originally Posted by thstone

The fact that your compression numbers are good brings hope that the leak down numbers will also be good. Best of luck.

That's what I'm hoping, too.
Thanks for the response. Any thoughts on cold vs hot test?


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[Misfire]

I am new to the site and new to Porsche.

If I understand this event, the vehicle spun and the engine may have turned backwards.

Image one of the cylinders just fired the plug and all the valves are closed while combustion is taking place. There is pressure on the piston to push it down, however the weight of the vehicle (going backwards) does not allow the piston to go down.

The piston is forced up against combustion pressure and as the crankshaft begins to rotate a few degrees backwards, the intake valve opens,(motor is turning the wrong way). The cylinder is above max combustion pressure and this pressure is released into the intake manifold. The throttle valve is shut. The pressure takes the path of least resistance, travels past the AOS into the crankcase. The pressure expands, inside the crankcase, just enough to blow out one cam plug and unseat the others.

Any takers on this "small bang" theory?

[maytag]

Originally Posted by Misfire

Any takers on this "small bang" theory?

Plausible.
Either way, it seems that combustion gases pressurized the crankcase.

I'ma do the Leakdown. I think I'll find that things are fine (I probably just jinxed it). Then I'll feel confident buying new parts to reassemble (I'm going to do fresh coils, plugs, probably a baffle in the sump, etc etc).

Thanks again for everyone's help.