Re: Thanks Marc. Actually, the typical gueidance is that...

Folks,

I would appreciate some "BTDT" or collective thinking re: compression tests. My question begins with two thoughts:

1. my 2016 readings vs 2014 readings are about 10-12lb loser across the board, but still clustered about 8% which is usually the guideline.
2. "farmer's math" does not get the the right projection as to what pressure readings should be.

let me back up. At the simplest, i would think that the pressure reading would be very close to:

compression ratio x ambient pressure, or in our case 11 x 14.7 = 162 lbs. But often it is more.
I have read ( and this was hard to find!) that this is normal, and is in part due to temperature rise as the air is compressed.

Moreover, all my readings are at or generally well over that number.

So, my question is, if i take a set of readings on one day, and another set at another place on another day, how much should I expect them to vary?

Roughly two years ago, my readings at shop A were #179 +/- 12%
This week, my readings, shop B, were #166 +/- 9%

The question is what i shoudl read into this... I am suspicious the absolute number (which is close to calculated) maybe fine and simply vary based on environment, engine heat, etc. But I have little data.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

It has been a lot of years since college physics and my scuba classes, but my guess is that Boyle's law comes into play here: PV=nRT

So the variables are Pressure, Volume, Temperature, and I forgot what the R is along with a constant "n" for the fluid. Pedro is probably better at this stuff, and he might be able to help, but in the meantime, Google Boyle's Law and see if it gets you anywhere.

I never put a lot of stock in compression tests other than to tell you that you might have a problem in a specific cylinder. I would use a leakdown tester to see what exactly the problem was - leakage around the valves or the rings.



Edited 1 time(s). Last edit at 09/28/2016 10:42AM by jg atl. (view changes)

Grant

gee-lenahan-at-gee-mail-dot-com

Since the readings involve equipment at "Shop A" and "Shop B" could the two different pieces of equipment account for any of the difference?

... clustered around 168 lbs, whcih is both good and very close to theoretical.

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
grant
Folks,

I would appreciate some "BTDT" or collective thinking re: compression tests. My question begins with two thoughts:

1. my 2016 readings vs 2014 readings are about 10-12lb loser across the board, but still clustered about 8% which is usually the guideline.
2. "farmer's math" does not get the the right projection as to what pressure readings should be.

let me back up. At the simplest, i would think that the pressure reading would be very close to:

compression ratio x ambient pressure, or in our case 11 x 14.7 = 162 lbs. But often it is more.
I have read ( and this was hard to find!) that this is normal, and is in part due to temperature rise as the air is compressed.

Moreover, all my readings are at or generally well over that number.

So, my question is, if i take a set of readings on one day, and another set at another place on another day, how much should I expect them to vary?

Roughly two years ago, my readings at shop A were #179 +/- 12%
This week, my readings, shop B, were #166 +/- 9%

The question is what i shoudl read into this... I am suspicious the absolute number (which is close to calculated) maybe fine and simply vary based on environment, engine heat, etc. But I have little data.

Grant

I wouldn't read too much into the numbers.

Well, other than if I read the numbers right the 1st compression test found a +/-12% difference between cylinder pressure readings? And the 2nd compression test found a +/- 9% difference between cylinder pressure readings?

If that's right that's not good at least based on my recollection. Generally the cylinder readings want to betweeen within 10% of one another with the numbers at or above a certain nominal pressure. I have not seen any numbers published by Porsche.

However, I wouldn't get too worried. If the deviation between cylinders was that bad I think the engine would be misfiring. The DME constantly monitors cylinder efficiency by measuring the speed each cylinder's power stroke imparts to the flywheel. If a cylinder doesn't do its job over a number of power strokes a misfire error code is logged and the CEL is turned on.

I suspect then what you are seeing is the variability inherent in your compression test setup/procedure.

About the only time I would use a compression test would be if the engine was misfiring and all the normal and usual things to address the misfires were unsuccessful then I'd compression test the engine looking for a weak cylinder. Then I'd leak down test if not all the cylinders the weak one and its neighbor or neighbors.

15% highest to lowest is the threshold for worry. The most recent one is 9% - everyone seems to agree that specific figure is pretty good. What i hear much variation on is whether its necessarily bad that the current readings are about 10 lbs lower across the board. Or if that's just variation due to temperature and equipment ( and even method... how many cranks before we declare it "pumped up"?). I did 5. What did Keith do in 2014? dunno.

One thing I'm looking to understand is whether blow by ,might be reducing crank case vacuum (like making it pressure) and contributing to my oil usage and smoke under very specific conditions (sustained high rpm + sustained high-g right hand turn). Not left. Not shorter hard turns. It needs to be all of those together for a sustained period.

Somewhat weird. Two classic cases are the top of the esses at WGI (looong right hand under WOT at or above 6k) or Pocono turn 3 (ditto)

Pocono was a great test case because we drove three configurations - allowing me to isolate conditions. Data, rare in these situations.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
grant
15% highest to lowest is the threshold for worry. The most recent one is 9% - everyone seems to agree that specific figure is pretty good. What i hear much variation on is whether its necessarily bad that the current readings are about 10 lbs lower across the board. Or if that's just variation due to temperature and equipment ( and even method... how many cranks before we declare it "pumped up"?). I did 5. What did Keith do in 2014? dunno.

One thing I'm looking to understand is whether blow by ,might be reducing crank case vacuum (like making it pressure) and contributing to my oil usage and smoke under very specific conditions (sustained high rpm + sustained high-g right hand turn). Not left. Not shorter hard turns. It needs to be all of those together for a sustained period.

Somewhat weird. Two classic cases are the top of the esses at WGI (looong right hand under WOT at or above 6k) or Pocono turn 3 (ditto)

Pocono was a great test case because we drove three configurations - allowing me to isolate conditions. Data, rare in these situations.

Grant

Well, it has been a while since I did any compression testing and I used the numbers available from the auto maker and 10% is what I recall. If it is 15% for the Boxster engine then that's what it is. Still a 15% difference is quite a bit.

It is hard to maintain consistency. The engine had better start out dead cold. So you crank the engine and get a reading and note that reading. You move to the next cylinder and repeat the process. In the meantime the cylinders are getting better oil dispersion as in the case of the Boxster engine the oil jets do their thing. The 1st cylinder likely starts out "dry". The 2nd less so and so on. There is also the unknown of how promptly the oil jets begin to spray oil. Some jets -- maybe those further away from the oil pump -- or the oil feed to the crankshaft oil galley -- might not spray right away while others spray much sooner.

Also, if you are using an unassisted battery there is the concern about the battery's condition at the test progresses. Perhaps the battery because it is getting run down some is spinning the engine slower? It doesn't spin the engine that fast anyhow which brings with it the variability of the ring/piston/cylinder sealing effectiveness at real low engine RPMs.

Blow by can be a problem, but if it were enough to cause a problem with oil usage and smoking you'd see it under less stressful conditions on the street. A "test" I've used in the past and continue to use once in a great while is with the engine fully warmed up and with a low sun at one's back to accelerate hard through a gear or two and observe for any haze/smoke in the sunlight. Believe it or not I do not see any from either engine. Another test involves running the vehicle speed up to some number in gear then letting off the throttle and letting the engine braking slow the car on closed throttle. In some cases I have done this coasting for a long distance -- when coming down a down grade. At some low -- but not too low RPM -- then floor the gas pedal. If one sees smoke then there is blow by -- at least some -- or one or more valve stem seals are leaking. Again neither engine has manifested any smoking.

What I suspect is happening is on the track the engine (naturally) sees full throttle and of course you mention you are WOT. Under full throttle intake pressure goes up and can approach atmospheric. As a result the engine crankcase experiences as high as a pressure as it ever will -- maybe even over atmospheric with no ventilation due to the intake being at high pressure and with some blow-by which is always present -- and this can force oil pass the rings on the intake stroke.

I have to mention the oil being used can possibly play a role. Some oils, those with higher detergent levels, or just more additives, can be more prone to foaming -- with an increase in oil vapor volume -- at high engine RPMs. Pretty much at high engine speeds the oil under the camshaft covers is more in a vapor form than liquid form. There is the issue of how full of oil the engine is and how fresh the oil is. Oil with miles on it and thus a build up of contamination -- mainly water -- can foam more and as I mentioned above can result in more oil vapor. As soon as you lift off the throttle intake pressure falls and this then results in a decrease in crankcase pressure. Along with this is a rush of crankcase fumes through the AOS and we all know how lousy the AOS can be even if it is not failing and as a result the fumes carry a big load of oil vapor which pass through the AOS and on into the engine which you may see as smoke. Over time it will certainly result in oil consumption. The Boxster is not too bad in this regard but the Turbo engine can consume quite a bit of oil when pressed hard. The rings and valve seals are fine. I think the oil escapes via the Turbo seals.

Anyhow, as long as the engine doesn't misfire or manifest any other signs of engine distress I don't think you have anything to worry about. And if you are worried about the differences in compression readings some time ago vs. more recent I think you are chasing a phantom problem.

You wrote:

Quote
marc
What I suspect is happening is on the track the engine (naturally) sees full throttle and of course you mention you are WOT. Under full throttle intake pressure goes up and can approach atmospheric. As a result the engine crankcase experiences as high as a pressure as it ever will -- maybe even over atmospheric with no ventilation due to the intake being at high pressure and with some blow-by which is always present -- and this can force oil pass the rings on the intake stroke.

All true. But would that method cause the oil fouling and misfires (clearly resulting from oil ingestion)...? (maybe..)

Quote
marc goes on
As soon as you lift off the throttle intake pressure falls and this then results in a decrease in crankcase pressure. Along with this is a rush of crankcase fumes through the AOS and we all know how lousy the AOS can be even if it is not failing and as a result the fumes carry a big load of oil vapor which pass through the AOS and on into the engine which you may see as smoke.

This is precisely what I think is happening, but the question is "why?". This explanation alone is not enough, or every well-driven 986 ont he track would have the same problem, which most don't. I'd like to think I'm just that much faster :-)

Also weigh in that it only really happens on hard, sustained right hand turns - so it has to do with pooling to the left side of the car.

Not an easy problem, so I appreciate the thoughtful and clear input....

Grant

gee-lenahan-at-gee-mail-dot-com

If it makes you feel any better, I had the same problem on my 02S at Road Atlanta at turn 3 - a hard right turn. I even got black flagged once because I was putting out so much smoke. But it was just smoke, and my read was that the oil was pooling up in the oil pan and overwhelming the AOS. I was using the stock setup - no deep pan or baffle. I modified my driving technique through the turn to go through it a bit less aggressively and the problem went away (obviously not what you would want in a competition environment).

Are you using the stock AOS? There is a competition one that us mucho expensive. I would think the Spec Boxster community would have seen this and has a solution. I know that Pedro runs an Accusump, that is more for preventing oil starvation in situations such as this.

Quote
jg atl
If it makes you feel any better, I had the same problem on my 02S at Road Atlanta at turn 3 - a hard right turn. I even got black flagged once because I was putting out so much smoke. But it was just smoke, and my read was that the oil was pooling up in the oil pan and overwhelming the AOS. I was using the stock setup - no deep pan or baffle. I modified my driving technique through the turn to go through it a bit less aggressively and the problem went away (obviously not what you would want in a competition environment).

Are you using the stock AOS? There is a competition one that us mucho expensive. I would think the Spec Boxster community would have seen this and has a solution. I know that Pedro runs an Accusump, that is more for preventing oil starvation in situations such as this.

well, check that. Oil does pool in the oil pan that's what the oil pan/sump is there for but that is not where the AOS gets overwhelmed.

The oil pools/gathers under the camshaft covers. I forget which direction it is now but a downhill high speed/sharp turn with braking -- but a right or left turn I can't recall -- causes oil under one camshaft cover in particular to migrate at the other end of the camshaft cover away from the oil scavenge pump.

As a result oil builds up under the cover. In the meantime the rapidly spinning cams whip this oil to a very dense oil mist.

When the turn completes and the pedal is pressed down hard this oil then migrates to the scavenge pump. With some positive pressure in the crankcase and some low pressure in the intake crankcase fumes -- heavy with oil mist/vapor -- will flow from the crankcase to the intake through the AOS. The problem is several fold: One is the crankcase fume flow rate. The fumes do not travel very fast through the AOS -- because the pressure differential is not that high -- and not all the vapor is slung out of the fumes by centrifugal force as the fumes pass through the AOS. The other problem is there is a lot of oil in vapor/mist form in the fumes and the AOS is not real good in dealing with lesser loads of oil vapor.

The result of all this is a big load of oil in the form of vapor/mist flows through the AOS and carries a considerable load of oil to the intake. As the fumes make a sharp turn the intake wall gathers the oil vapor/mist and it becomes liquid again. At some point maybe a distance away from the offending turn the oil then enters the engine and this accounts for the smoking as the engine receives a big dose of oil.

The word about the motorsports AOS is it is supposedly more robust -- to better withstand the rigors of competition (but at a pretty high cost). However, the motorsports AOS uses the same basic design as the standard AOS and is subject to the same problem of being in-efficient at separating oil vapor from crankcase fumes.

If I were tracking my Boxster I'd be inclined to stay with the standard AOS. It will wear out (so too can the motorsports AOS) but it is cheaper to replace.

Then about all one can do is ensure the oil is fresh, the "right" oil, (oils that have a bigger additive package can be more prone to foaming) and the engine is not overfilled.

This overfilling issue is tricky. Remember the engine gets plenty hot on the track and the oil level that is OK under normal operating conditions and engine/engine oil temperatures can be too high at track temperatures. The hot oil level can be possibly a bar higher and thus the engine is overfilled at the elevated track temperature.

Marc is onto a very interesting scenarios here, which I've also been trying to piece together (again though, why just me and not all hard driven 986s?):

Oil does pool in the oil pan that's what the oil pan/sump is there for but that is not where the AOS gets overwhelmed.

Quote
Marc
The oil pools/gathers under the camshaft covers. I forget which direction it is now but a downhill high speed/sharp turn with braking -- but a right or left turn I can't recall -- causes oil under one camshaft cover in particular to migrate at the other end of the camshaft cover away from the oil scavenge pump.

As a result oil builds up under the cover. In the meantime the rapidly spinning cams whip this oil to a very dense oil mist.

When the turn completes and the pedal is pressed down hard this oil then migrates to the scavenge pump. With some positive pressure in the crankcase and some low pressure in the intake crankcase fumes -- heavy with oil mist/vapor -- will flow from the crankcase to the intake through the AOS. The problem is several fold: One is the crankcase fume flow rate. The fumes do not travel very fast through the AOS -- because the pressure differential is not that high -- and not all the vapor is slung out of the fumes by centrifugal force as the fumes pass through the AOS. The other problem is there is a lot of oil in vapor/mist form in the fumes and the AOS is not real good in dealing with lesser loads of oil vapor.

The result of all this is a big load of oil in the form of vapor/mist flows through the AOS and carries a considerable load of oil to the intake. As the fumes make a sharp turn the intake wall gathers the oil vapor/mist and it becomes liquid again. At some point maybe a distance away from the offending turn the oil then enters the engine and this accounts for the smoking as the engine receives a big dose of oil.

Weak scavange pump? More pressure in the crankcase than usual (and in fact, under low-load conditions I can measure this with a pressure meter connected to the oil fill tube). That's why i asked about the compression - rule out, or not, excessive blow-by.

Continued thanks for all this group thinking.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

... catch can.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

to JG,

Well aware of motorsports AOS. But that is still a band-aide. Remember we have incorrect readings on crankcase pressure as well.

May run motorsports AOS - or another (there are some aftermarket items in development), but first i want to understand what's up with my car. This is using LARGE amounts of oil under these circumstances - AoS burps create tons of smoke, but its generally only a few drops (amazing how much smoke it can create).

Also see below, i think Marc is going down the right path, we've been speculating and wondering also if a weak scavange pump int he head might contirbute. Also looking at AoS return line.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
grant
to JG,

Well aware of motorsports AOS. But that is still a band-aide. Remember we have incorrect readings on crankcase pressure as well.

May run motorsports AOS - or another (there are some aftermarket items in development), but first i want to understand what's up with my car. This is using LARGE amounts of oil under these circumstances - AoS burps create tons of smoke, but its generally only a few drops (amazing how much smoke it can create).

Also see below, i think Marc is going down the right path, we've been speculating and wondering also if a weak scavange pump int he head might contirbute. Also looking at AoS return line.

Grant

Hard to imagine a weak scavenge pump. The pump is very simple and about the only way it could get "weak" is wear but the pump impellers I believe do not make contact thus there should be no wear. I've never encounted anyone who has had to replace a scavenge pump due to wear.

I bet the AOS is passing more oil -- in the form of vapor -- than you suspect. The techs tell me invariably when they have the intake exposed the inside of the intake is wet with oil. I suspect a considerable amount of oil consumption with these engines is due to oil being lost via the AOS rather than making it past the rings or valve seals. As I touched upon in another post I've never seen any smoking from my Boxster's engine when giving the engine the old hard acceleration or coast down then full throttle business.

But I sure as heck see the engine smoke upon startup once in a while. That's AOS.

Pedro offers "catch can". I've suggested this a few times myself. There are problems with this though. Mounting. The catch can wants to be secure. And it requires servicing. Since there will be no return of the oil collected to the crankcase -- at least not without some serious plumbing -- the catch can will get full. The can has to be emptied and before there is any risk of any collected oil sloshing to the point it gets pulled into the intake. Oil vapor is bad enough but have the engine inhale a cup or two of oil and you will regret it.

Given the effort to expose the top of the engine I think a catch can is not really a viable solution. However, if there is a way to mount it a distance away from the top of the engine maybe locate it down low to where one could get at the drain from under the car this could change things. Maybe.

... in the trunk.
This is for racecars.
Easy to get to and service (empty).
Happy Boxstering
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

I have not seen the scavenge pump, i mentioned it since a pretty noted mechanic had that thought...so not it moves down on y mental priority list.

I was going down the catch can route some time back, and maybe i must return to it. I'd have to locate it somewhere accessible - trunk, or maybe bottom of motor (not a bad place since it will flow down)

I know that MOST of it is passing through the AoS - I know that because I've opened the inlet hose and seen what's there - lots of oil. The question is only "why?. I also know that the "why?" is in part due to oil levels being higher than they should be and (hard to prove) more aerated or thick vapor. I know this due to some issues with the sump/baffle arrangement that, when fixed, resulted in vastly lower use (but still lots and still a problem). Net out - oil was begin trapped above the windage tray, and under high side loads finding its way into the intake. Note that without high rpm and side loads the oil usage seems to be low. I say seems because, as a track car, i don't have enough regular driving to be absolutely sure. I speculate that the high rpm part contributes in two ways:

1. more oil volume to deal with
2. more ability to aerate oil or even have components churn it up....

So the side load (hard, sustained turns) mean the oil gets pulled to the side and potentially up the side, and the rpm puts more of it there, and in a less viscous state.

Thanks again. I do need to look into that new, un-named AoS. But I'd also like to retain the oil where it should be!

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
grant
I have not seen the scavenge pump, i mentioned it since a pretty noted mechanic had that thought...so not it moves down on y mental priority list.

I was going down the catch can route some time back, and maybe i must return to it. I'd have to locate it somewhere accessible - trunk, or maybe bottom of motor (not a bad place since it will flow down)

I know that MOST of it is passing through the AoS - I know that because I've opened the inlet hose and seen what's there - lots of oil. The question is only "why?. I also know that the "why?" is in part due to oil levels being higher than they should be and (hard to prove) more aerated or thick vapor. I know this due to some issues with the sump/baffle arrangement that, when fixed, resulted in vastly lower use (but still lots and still a problem). Net out - oil was begin trapped above the windage tray, and under high side loads finding its way into the intake. Note that without high rpm and side loads the oil usage seems to be low. I say seems because, as a track car, i don't have enough regular driving to be absolutely sure. I speculate that the high rpm part contributes in two ways:

1. more oil volume to deal with
2. more ability to aerate oil or even have components churn it up....

So the side load (hard, sustained turns) mean the oil gets pulled to the side and potentially up the side, and the rpm puts more of it there, and in a less viscous state.

Thanks again. I do need to look into that new, un-named AoS. But I'd also like to retain the oil where it should be!

Grant

Why?

The answer is simple enough. The AOS sucks at removing oil vapor from crankcase fumes. With a hard working engine with dual over head cams the crankcase is going to be filled with oil vapor. This is unavoidable in all areas of the crankcase but possibly on purpose at least under the camshaft covers as the cam lobes do not have oil holes to lube the lifter bucket faces. Oil that leaks out from around the oil lifter lubes the lifter/lobe inteface.

(As an aside I observed techs setting the valve lash on OHV engines with the valve cover removed and the engine running. As long as the engine RPMs were at idle there was no mess. By way of comparison when I had my Datsun 510 with its SOHC engine with the cam cover removed just cranking the engine made a mess. This was from the copious amount of oil the cam lobe oil holes emitted even at cranking speed.)

In the Boxster engine under the intake lifter buckets are cast alum. fingers that project out that I suspect are intended to collect the oil that is slung off the intake lobe and then let it run/drip down to add to the oil that leaks out around the exhaust lifter bucket to lube the exhaust lifter/bucket interface. The exhaust lifter buckets probably run a bit hotter naturally and the extra and cooler oil from the intake lifter hardware helps keep the exhaust lifter buckets cooler than they would be otherwise.

I use the word "leaks" but this suggests the oil coming out is doing so in a gentle manner. Once the engine is warmed up the oil from around the lifter bucket probably sprays out. The fit of the lifter bucket in its bore is a close sliding fit. IOWs there not much clearance/gap between the bucket and its bore. The hot oil is under some pressure from the pump side. As the lobe begins to press down on the lifter bucket this closes off the oil inlet from the pump side and at the same time this compresses the oil under the bucket and increases the pressure that probably has the oil come out around the lifter bucket in a violent spray of oil in mist form.

I won't bore you with what is happening down in the crankcase from the oil from the main and rod bearings and the piston spray jets. And let us give a quick mention of the many chains which receive oil via an oil hole in the guide/tensioner rails. This contributes to the oil vapor/mist in the crankcase. Suffice it to say that a good amount of the nomimal approx. 9 quarts of oil is in the form of suspended vapor/mist inside the crankcase. Several quarts I believe is often the number quoted and I can believe it.

IOWs, in the Boxster engine above some engine speed a lot of oil mist/vapor in the crankcase is the norm. And when the crankcase fumes laden with this oil vapor flow through the AOS on the way to the intake and engine the AOS simply can't cope and some oil vapor passes through.

I can only guess and probably guess wrong at what possibly one could do with the AOS to improve its efficiency. I wonder if a more abrupt change in fume flow direction would help? IIRC the interior of the AOS is kind of like a shell of a Nautilus with a decreasing radius. The idea of course is to cause the fumes to change direction with the heavier oil vapor being slung out of the fume flow. Would a tighter radius help? What would this do to fume flow speed? If it slows it down the benefit of a tighter radius might be negated.

I don't know. I'm no expert on fluid flow dynamics.

I think though I'm on firmer ground when I say about all one can do is watch the oil level, keep the oil fresh, use the right oil with the right additive package --- avoid too high a detergent level -- and let the chips fall where they may. The AOS will invariably let some oil vapor past and one just has to deal with this in the form of higher than normal oil consumption and the ocasional cloud of oil smoke at almost certainly inappropriate times.

The car is being tracked and that's part of the cost of tracking a car/engine intended for the street.

You know after writing the above and thinking about this some more I'm willing to be a bit more forgiving of the AOS. It still sucks, but it has an heck of a job to do -- dealing with removing quarts of oil in the form of vapor and mist over and over again -- and that it does it as well as it does deserves some acknowlegement.

Changes in the oil sump and baffling should not effect oil vapor to any degree. Regardless of the volume of oil in the sump -- barring too low a level of course -- the oil pump at a given speed supplies X amount oil. I don't know what the oil delivery numbers are for the Boxster engine but for an older air-cooled Porsche engine at some elevated engine RPM (5K?) the oil pump delivered 60l per minute. Around a 1/3rd of this was routed to the heads to lube the valve hardware. I would expect the Boxster engine (and the 996 engine) to have similar numbers.

What you want to gain by any oil sump changes -- deeper oil sump -- and better baffling is to ensure there is always a good supply of air free oil around the oil pump pickup.

I've never observed an engine oil sump in a running engine but I have observed large machine tool hydraulic oil systems and the collection tank -- oil sump - and the oil discharged into the collection tank is still a bit aerated -- the oil is noticeably lighter in color -- even after having passed through a defoamer. The hot oil loses its air as it sits in the collection tank and by the time it is pulled into the oil pump it is pretty much free of any air.

Might mention this was with the oil up to temperature. Upon using the machine tool cold the used oil would return to the collection tank but not sink down as the cold oil under it was denser. What would happen is the oil pick up would create a depression in the oil and the used oil would run over to this depression and some of it would flow into the oil pump pick up. This was one reason why one ran the machine tool gently some time to "warm up" the oil to ensure that when the machine tool was operating a production levels it would receive a suitable supply of air free oil.

These collection tanks held gallons of oil. The tanks were stationary not subjected to any centrifugal forces.

A hard working Boxster engine though is another thing. A deeper sump with better baffling helps reduce the volume of air that is in the oil that is pulled in by the oil pump. The benefits of deeper is obvious. More oil without raising the oil level in the sump above its current level. But the baffling is a bit more complex. No matter which direction centrifugal force is being generated the baffling wants to prevent any oil from leaving the enclosed area around the oil pump pick up. This is obvious of course. But what may not be obvious is the baffling wants to use this centrifugal force to its advantage and let any oil enter the enclosed area but it wants to allow oil to enter this but only from the very bottom of the sump where the oil has collected as it has shed any air it might have had after the oil left the defoaming pot.

The oil at the very bottom of the sump is most free of any air. And this is very important.

1. "The AOS sucks at removing oil vapor from crankcase fumes. With a hard working engine with dual over head cams the crankcase is going to be filled with oil vapor. This is unavoidable"

Yes, but again, why doesn't EVERY tacked 986 then do this? They don't, only a few... something's different on those few.

2. "Changes in the oil sump and baffling should not effect oil vapor to any degree."

Not so fast. It may not affect the vapor, but it may affect its location. A windage tray, essentially a horizontal baffle, can trap oil ABOVE the tray rather than BELOW, thus raising its level much closer to the intake to the AoS. I'm almost certain this WAS a problem, much reduced by modifications I made. Since it did improve after i made the modifications, it strongly suggests that my guess was right .... now, the question is "what's causing the remainder of the issue?"

IN fact, later you write: "More oil without raising the oil level in the sump above its current level. But the baffling is a bit more complex.".

And I'll point out that some designs - and these have real advantages in terms of containing the oil - can raise a portion of the oil - the oil in the process of returning to the sump, traversing the baffle. Bottom line is that a horizontal baffle not only prevents oil from climbing the walls out of the lower sump chamber, but also prevents it from draining - specifically under high-g circumstances when the oil is pulled the side, above the windage tray, and therefore away from the OEM plastic baffle and the primary return path (look at a M96 baffle - its a plastic tower with one-way window valves in it, that allow oil to flow from outside --> in but not the reverse, in most deep sumps this is relocated to above the windage tray...)

here's a picture, but only of OEM location. In deep apps, it is relocated ot the windage try. You care see the rubber flaps that act as one-way valves if you look closely and know what they are....
Baffle pic from fleabay

3. " I won't bore you"

This is driving me crazy. I won't be bored :-) (joke, let's move on)

4. "What you want to gain by any oil sump changes -- deeper oil sump -- and better baffling is to ensure there is always a good supply of air free oil around the oil pump pickup."

All this is very well understood, including the potential adverse effects of some deep designs. For the record, i have about 2 qts additional oil, the horizontal windage tray, and, now, some additional baffling to contain oil near the pickup. Pickup is extended of course.

5. "Suffice it to say that a good amount of the nominal approx. 9 quarts of oil is in the form of suspended vapor/mist inside the crankcase. Several quarts I believe is often the number quoted and I can believe it."

That's a very interesting conclusion. I understand all those sources, but would not have guessed the quantity to be that high. Certainly explains some of the issues. While we don't know your precision, ( and its may not matter) nice thinking.....

So you certainly have me thinking (or i shoudl say "back to thinking") primarily about the oil vapor - how its being handled byt he current AoS, how it could be handled, why I might have additional vapor, whether the vapor is sitting somewhat higher in my application....

Thanks as usual.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
grant
1. "The AOS sucks at removing oil vapor from crankcase fumes. With a hard working engine with dual over head cams the crankcase is going to be filled with oil vapor. This is unavoidable"

Yes, but again, why doesn't EVERY tacked 986 then do this? They don't, only a few... something's different on those few.

2. "Changes in the oil sump and baffling should not effect oil vapor to any degree."

Not so fast. It may not affect the vapor, but it may affect its location. A windage tray, essentially a horizontal baffle, can trap oil ABOVE the tray rather than BELOW, thus raising its level much closer to the intake to the AoS. I'm almost certain this WAS a problem, much reduced by modifications I made. Since it did improve after i made the modifications, it strongly suggests that my guess was right .... now, the question is "what's causing the remainder of the issue?"

IN fact, later you write: "More oil without raising the oil level in the sump above its current level. But the baffling is a bit more complex.".

And I'll point out that some designs - and these have real advantages in terms of containing the oil - can raise a portion of the oil - the oil in the process of returning to the sump, traversing the baffle. Bottom line is that a horizontal baffle not only prevents oil from climbing the walls out of the lower sump chamber, but also prevents it from draining - specifically under high-g circumstances when the oil is pulled the side, above the windage tray, and therefore away from the OEM plastic baffle and the primary return path (look at a M96 baffle - its a plastic tower with one-way window valves in it, that allow oil to flow from outside --> in but not the reverse, in most deep sumps this is relocated to above the windage tray...)

here's a picture, but only of OEM location. In deep apps, it is relocated ot the windage try. You care see the rubber flaps that act as one-way valves if you look closely and know what they are....
Baffle pic from fleabay

3. " I won't bore you"

This is driving me crazy. I won't be bored :-) (joke, let's move on)

4. "What you want to gain by any oil sump changes -- deeper oil sump -- and better baffling is to ensure there is always a good supply of air free oil around the oil pump pickup."

All this is very well understood, including the potential adverse effects of some deep designs. For the record, i have about 2 qts additional oil, the horizontal windage tray, and, now, some additional baffling to contain oil near the pickup. Pickup is extended of course.

5. "Suffice it to say that a good amount of the nominal approx. 9 quarts of oil is in the form of suspended vapor/mist inside the crankcase. Several quarts I believe is often the number quoted and I can believe it."

That's a very interesting conclusion. I understand all those sources, but would not have guessed the quantity to be that high. Certainly explains some of the issues. While we don't know your precision, ( and its may not matter) nice thinking.....

So you certainly have me thinking (or i shoudl say "back to thinking") primarily about the oil vapor - how its being handled byt he current AoS, how it could be handled, why I might have additional vapor, whether the vapor is sitting somewhat higher in my application....

Thanks as usual.

Grant

There can be differences in the effiiciency of the AOS. There are possibly differences in the engine, the number of cam chains for instance (3 vs. 5), also the efficiency of the scavenge pumps, engine oil, oil level, miles on oil, etc. And differences between drivers. I would hazard a guess the smoother drivers (and because of it probably faster drivers) see less smoking because they are better at braking before a turn and just over all car control. This is not to insinuate that you're not a good driver because you see smoking of course.

Not much of a windage tray if it traps oil above it. Another factor is the oil from the crankshaft and rod bearings is less broken up more liquid and as such tends to be slung away form the crankshaft and rods and collects on the crankcase walls and from there runs down and into the sump.

Besides the AOS's intake as you put it I believe is located quite a distance above the crankshaft center line. The AOS's intake wants to be high as possible to help ensure the AOS only pulls in the finest of the oil vapor. They heavier stuff being pulled out from gravity. 'course crankcase fumes heavily laden with oil mist/vapor and not under much low pressure then suddenly subjected to a low pressure in the intake this results in pulling out even some of the heavier oil vapor/mist.

The couple of quarts in suspension is something I picked up from some material from Porsche. OHC engines and especially DOHC engines are especially bad about filling the engine crankcase -- this includes the areas under the camshaft covers -- with oil droplets, mist and vapor. The "old" OHV engines didn't require much "top end" oiling and in fact the oil fed to the rockers -- some of it through the valve push rods -- was restricted to limit the amount of oil top side and to increase the amount of oil for the bottom end.

Even so even in "modern" versions of these engines I read once in a while about dyno testing where the engine output falls off unexpectedly at very high RPMs -- still below red line though -- and this in some cases is attributed to the oil level getting low enough the pump ingests oil with too much air in it and the hydraulic lifters collapse a bit and affect valve lift and even timing to the detriment of the engine's peak power output.

It is why with the Boxster engine when the engine is shut off and then one turns the key back on to check the oil the count down timer starts at IIRC around 60 seconds mark. If one leaves the engine off a few minutes -- like when filling the fuel tank -- and then goes to check the oil level the count down timer -- at least in the case of my Boxster -- starts at just 5 seconds.

In the Turbo I have to check the oil with the engine warmed up and idling and the car pretty level. In this case if I give the engine some throttle to raise its RPMs above idle then using the OBC stalk to trigger an oil level reading the countdown timer starts at 20 seconds provided the engine RPMs are back to idle. This 20 seconds gives time for the oil to drain to the sump where the scavenge pump then gets it and sends it back to the tank. In this regard the Turbo engine differs from the N/A engines in that it has a true dry sump system and a crankcase scavenge pump (and of course scavenge pumps in the heads) to remove any oil from the crankcase sump and send it back to the tank. Since the engine is still running there is still oil in the engine as liquid drops, mist, and vapor but after 20 seconds the assumption is this amount of oil is inconsequential.

Might mention when changing the Turbo oil and when draining the oil sump one can get a couple of quarts of oil from the sump drain, the rest comes from the oil tank drain. Not all this oil is residual oil that drained from the internal engine walls after the engine was shut of though. But some of it is.

In some engines the builder fits sheet metal that is more than a windage tray. It is positioned to "scrape" oil from the rotating crankshaft. By "scrape" I don't mean the sheet metal actually touches the crankshaft only that it is positioned very close to the rotating crankshaft to collect the oil as the crankshaft begins to rotate up past the oil pan/engine block interface (this is in good old 'merican engines) before the oil from the crankshaft can be slung off to the outer reaches of the crankcase walls. This results in a reduction in the amount of oil being whipped around in suspension by the crankshaft which means the oil level in the pan/sump is higher than it would be otherwise. It also helps to reduce oil aeration too. And as an added benefit it can reduce parasitic drag the crank experiences by having to rotate through this heavy cloud of oil droplets/mist. In some cases a 2% gain in HP at the rear wheels can be observed.