IcemanG17

I would do the test with the girdle installed...to show the effects of how high the fluids will climb the sump...... another huge factor is TIME....since the fluids will reach the same level with the same amount of inclination if sitting long enough....and we are talking about seconds at most.....

the S4 pan will climb the fastest....followed by the OB sump, but by how much....I dunno

James Bailey

Just a refresher on what track use looks like .... www.youtube.com/watch?v=fv53RbvgfGc‎

Rob Edwards

Yeah, my right wrist is strong, but not that strong.

James Bailey

that video will make you question how a front sump can possibly work ????

docmirror

You are explaining clear enough, but your terminology needs work. The oil would not cavitate as that is due to the disturbance by a surface boundary layer tension(water on the low pressure side of a prop blade will cavitate). However, what you're describing sounds suspiciously like the oil would reach it's vapor pressure due to the slinging force of the oil in the passage on it's way to the rod journal. So, the theory goes that the oil will reach the vapor pressure where it undergoes a state change from liquid to gas, while in an enclosed pressure vessel, pushed by the oil pump.

This begins to make a bit more sense, and I could potentially see this happening with the caveat that we have oil entrained with air bubble(vaporization being improved by emulsified oil and air mixture) at the molecular level.

This is also pretty difficult to quantify, as one would need to calculate a lot of various vapor pressures, and I'm not sure the tribology in this case supports an actual vaporization model so much as a loss of film strength due to the increased journal pressure squeezing the entrained air bubbles so small that the oil film becomes compromised.

Maybe this is a lot of dancing on the head of a pin, but these really are two distinct failure modes. One can be solved by an increase in pressure, the other by mitigation of entrained air, and they are also cross enabled as the entrained air leads to vaporization, and the decreasing oil pressure leads to larger air bubbles(less pressure) which could allow for greater squish out and loss of film strength.

Complex tribology indeed.

ptuomov

You do know that cavitation is the exact same thing as a fluid reaching it's vaporization pressure, right? At least that's what my Finglish dictionary says. Or are you reserving cavitation word to the collapse of the vapor bubbles after the pressure conditions change?

In any case, the oil passage in a crankshaft pulls in a very low pressure gas bubble at the innermost crankshaft radius and the oil flow stops.

The vaporization pressure depends on, among other things, the amount of air bubbles in the oil. More air bubbles, higher the vaporization pressure and higher the required oil supply pressure at the mains.

GlenL

I'll guess that no one read the post I linked.

Back in 2006 I wondered about this and ran the numbers. There's more acceleration to the sides but more volume towards the back and a lot more time in an acceleration than in a corner, at least at most tracks.

This is why I went with the I-J kit. The sump has a cover for controlling the oil in corners and accelerating. There are trap doors along the back of the sump to hold oil in while accelerating. Works great. Never an oil pressure bobble. It's the next best thing to a dry sump.

Accusumps compensate for lower pressures but the air is sucked into the system.

And didn't someone (Mike Schmidt?) already do the oil tipping exercise.

The shallow front sump is a huge mistake on a track car.

The Fixer

I'll try an dumb this down for us regular folks..It's i think what Tuomo was saying a few back.

back story:
Cross drilled cranks have oil feeds drilled completely through the journals, mains and throws, sometimes just mains but usually both nowadays. Then a straight hole is drilled on center line of the crank to connect the two. Engine Builders feel this will ensure oiling - as both ends have oil supply and the journal will always have oil but it does not ensure this.

Pressurized oil must enter the main journal and overcome high rpms to reach the center of the crank, before the throw gets any oil.
I think this is what is referred to as the 'crack and whip effect'

This is an obvious case of there not being enough oil pressure to overcome this high rpm oiling effect the throws create.

This is why i shift no later than 5500 rpm in my beloved '83S.

Sorry to see your motor Iceman, good luck with the next one.

GregBBRD

Notes above, in blue.

The Fixer

Well explained Greg thank you,
cross drilling has been proven to be ineffective over the years
and sometimes actually very harmful ( i think it can actually cause oil aeration)
Most cases i think it doesn't hurt though, but is old tech now.

GregBBRD

The really tough part is that no one completely agrees. You don't have to look very far back and the Chevy Factory Performance Books are telling you that all of the high performance crankshafts for both the small block and the big block are cross drilled (old style engines). They say if you want to use one of the cranks that are not cross drilled, you need to make sure that you do so.

Of course, the oil passages in those cranks are not the same as those in a 928 crank. The Chevy cranks have a very "shallow" drilling, in the main that goes to the rod journal, whereas the 928 crank feeds the rods from the center of the main journals.

However, cross drilled is cross drilled......the oil flow in the main journal is going to always be "confused" and moving in both directions.....especially on the "long" side of a Chevy crank, where oil has to completely pass across the main bearing drilling to reach the rod passage, on the other side

One interesting point worth considering is that Chevy suggests very low rev limits in almost all of their older small blocks and big blocks. Just like the 928 engine, anything that runs over 6,500 is pretty rare. I believe that the problems appear once any of these engines start pulling rpms over these rpms.....and up to that point, cross drilling is fine.

docmirror

Yes, caviation is a form of instantaneous vaporization which collapses as soon as the surface tension against the viscous media(water in the case of a prop) goes back below the point of vaporization. So, a cavitation is more accurately a state change from liquid, to vapor, and back to liquid in a very short time.

As for the vapor pressure, it always happens at exactly the same temp pressure gradient based on the fluid media, but the emulsification speeds it up due to the boundary layer breakdown at the molecular surface of the air bubble. This can be seen again with the example of the boat prop where cavitation occurs in perfectly pure water passing through the blades, but are disturbed by the blade itself. Without linking, one can google a cavitation burn on a prop and note that it will be in the inner/middle part near the hub because that's where the lowest pressure gradient location, and not out by the tip.

This also matches some of the theory on vaporization of the oil in the crank journal because the oil, like the water is being slung out rapidly. I think it has merit, but is one of those really, really hard things to prove. I'd be more inclined to go for ventilation where the air volume gets just too great for the film strength to overcome. What has me baffled is why the 2/6 journal? It's all spinning at the same rate, the gallery looks the suitable to deliver to the main, so I don't get why 2/6? Weird...

IcemanG17

clearly the 2/6 is the death kneel of 928's and no surprise the 2 is the death kneel of 944's....so obviously there is a flaw there.... Why do track or hard driven 928-944 both die at the same spot? Why is this?

Why I don't know for sure....I have an idea....I have spent a fair amount of time at the track...and I recall a lemons race at Thunderhill where I heard a BOOM...then a ding ding ding crack.... which was a 944 taking a hard corner...blowing the engine and ejecting a piston-rod that bounced along the track then hit the retaining wall by the paddock.... something I will never forget....even though I keep finding ways to almost copy it myself....unfortunately due to other issues we blew our engine as well, but it much less dramatic fashion....

Black Sea RD

Haven't read though this whole thread, but isn't this the reason one should dry sump an engine and do other modifications like cross drilling the crank if they want to go racing a 928?

Greg B. can correct me if I'm wrong, but I believe Greg B. built Mark A.'s 928 motor that lasted an incredible 5 race season's. Incredible feat especially given Mark A.'s race results with that engine.

Given this, Greg. B. should know the formula to make 928 race engines by now. If I were going 928 racing, I would listen to what Greg B. says about this topic.

No affiliation to Greg B. in saying this, but I don't know who else has this sort of track record (pun intended) on this forum.

Cheers,

heinrich

This failure is unrelated to standard failures of these engines. Oil / water mixture is not a symptom of the normal oil ingestion / starvation. So I say your loss is a one-off Brian. However ... I'm stull sorry to hear you lost this one.