Compression tests - questions re: consistency grant - 7 years ago |
Re: Compression tests - questions re: consistency jg wnc - 7 years ago |
Re: Compression tests - questions re: consistency MarcW - 7 years ago |
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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
Thanks Marc. Actually, the typical gueidance is that... grant - 7 years ago |
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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
Thanks again (as usual) a few cthoughts... grant - 7 years ago |
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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.
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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.
The oil is not pooling in the oil pan... MarcW - 7 years ago |
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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.
Re: The oil is not pooling in the oil pan... grant - 7 years ago |
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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.
Two words ... Pedro (Odessa, FL) - 7 years ago |
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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
We have located catch cans ... Pedro (Odessa, FL) - 7 years ago |
Lots of good comments. grant - 7 years ago |
Re: Lots of good comments. MarcW - 7 years ago |
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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
Great info, a few nits: grant - 7 years ago |
Re: Great info, a few nits: MarcW - 7 years ago |
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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