Catch-tank to solve oil burning issue
Discussion
My 928S has a tendancy to burn quite a bit of oil at high rpm. Over the weekend, I had the intake system in bits to fix a different issue, but looking at the state of the intake it is clear that the oil is being injested via the breather system. Talking to other 928 owners, seems that this is a common problem. I've previously had the compression checked out and that was fine.
So if this is "just one of those things", I was wondering about a practical solution as it's no fun leaving more smoke than a badly tuned diesel!
The most obvious solution is a catch-tank to take the current breather hose from the engine and filter the oil. I could then just block the throttle-body input.
Anyone tried this before to solve this sort of issue? Did it work?
Sheepy
So if this is "just one of those things", I was wondering about a practical solution as it's no fun leaving more smoke than a badly tuned diesel!
The most obvious solution is a catch-tank to take the current breather hose from the engine and filter the oil. I could then just block the throttle-body input.
Anyone tried this before to solve this sort of issue? Did it work?
Sheepy
Surely, the problem is not that the oil is being injested at the inlet manifold, but that it's being blown out of the crankcase breather in the first place? An oil catch tank would simply collect the oil ratehr than allowing it to be burnt off?
Sounds to me as though there is a more fundamental problem elsewhere (blocked breather? piston ring blow-by?).
Sounds to me as though there is a more fundamental problem elsewhere (blocked breather? piston ring blow-by?).
Sam_68 said:
Surely, the problem is not that the oil is being injested at the inlet manifold, but that it's being blown out of the crankcase breather in the first place? An oil catch tank would simply collect the oil ratehr than allowing it to be burnt off?
Sounds to me as though there is a more fundamental problem elsewhere (blocked breather? piston ring blow-by?).
If it was just my car, then I'd agree. However after I asked about it on a 928 email list and found lots of owners with the same symptoms, I'm more inclined to think that this is something to do with the design. My suspicion is that under wide-open throttle, the car sucks so hard that it is pulling oil vapour in though the breather mechanism (rather than being caused by piston-ring blow-by). Certainly the compression measurments done for me by Racing Developments in MK were very good, which does suggest that there isn't a specific piston problem. Sounds to me as though there is a more fundamental problem elsewhere (blocked breather? piston ring blow-by?).
I guess the test for this would be to temporarily disconnect the breather hose, sealing the input to the throttle body and terminating a hose from the crank-case end in a container and see what happens when I give the car some stick! Oil in bottle would imply blow-by and no oil would imply my engine sucks
Might try this at the weekend. Was hoping that someone on here might have had similar issues with a track or race car and looked into a solution.
hmmmmmm - I still think your bores are shagged, but your test should prove which is the cause......was the compression test done when the engine is hot?....... the bores are not conventional in the 928........they are aluminium......called alusil, whereby they are machined, then acid etched to raise the silicon, which is what the rings run on........they are notorious for scuffing the bore.......but fortunatley for you......
go to boosteds website and look in the news area
www.boostperformance.co.uk/
heres the text
"During two Porsche engine builds we discovered a viable way to recondition Alusil cylinder bores as used in engines like the Porsche 944 Turbo. Alusil bores can't be bored in the conventional manner due to the presence of silicon in the aluminium. We need to assess your engine block for wear or damage before we decide to etch the cylinders in readiness for new piston rings. This has to be done to allow the rings to bed in properly, thus sealing the cylinders and restoring compression. For an additional labour charge we can restore piston ring gaps to minimum factory specifications using aftermarket 'file fit' piston rings and then fit them to your pistons. Inspection and assessment costs £50. Etching the cylinders is £75 per cylinder and there is a charge of £40 for block cleaning after treatments. Piston ring prices vary depending on type, £200 is a maximum price for Total Seals, Porsche items are cheaper. We can also provide lighter weight piston pins with taper walls for £25 each. This work can be carried out on other Alusil engines such as Lexus, BMW and Mercedes subject to inspection and final costing."
go to boosteds website and look in the news area
www.boostperformance.co.uk/
heres the text
"During two Porsche engine builds we discovered a viable way to recondition Alusil cylinder bores as used in engines like the Porsche 944 Turbo. Alusil bores can't be bored in the conventional manner due to the presence of silicon in the aluminium. We need to assess your engine block for wear or damage before we decide to etch the cylinders in readiness for new piston rings. This has to be done to allow the rings to bed in properly, thus sealing the cylinders and restoring compression. For an additional labour charge we can restore piston ring gaps to minimum factory specifications using aftermarket 'file fit' piston rings and then fit them to your pistons. Inspection and assessment costs £50. Etching the cylinders is £75 per cylinder and there is a charge of £40 for block cleaning after treatments. Piston ring prices vary depending on type, £200 is a maximum price for Total Seals, Porsche items are cheaper. We can also provide lighter weight piston pins with taper walls for £25 each. This work can be carried out on other Alusil engines such as Lexus, BMW and Mercedes subject to inspection and final costing."
Edited by knightly on Monday 19th February 15:12
Edited by knightly on Monday 19th February 15:13
I don't know whether the blow-by is a sign of a problem, or just a characteristic of that engine. Given that the blow-by is occuring, it seems that you need to either take the breather into a separate catch tank, or fit an oil separator. If you can package it, this could be arranged so that the catch tank and/or separator drain back to the sump so no need to empty it yourself.
The 4.6 rv8 in my westfield would blow oil out of the rocker cover in to the pancake air filter under really hard use. This resulted mainly in the air filter element getting soaked with oil rather than oil ingestion in to the engine.
I blocked off the input to the air filter and ran a line from the rocker cover to a home made oil catch bottle which had a small air filter to atmosphere. Under normal use it would not collect any oil at all but under really hard use would collect a bit, the air filter would remain clear and my engine wouldn't ingest oil vapour.
My bores were not worn in the slightest as the engine was a new John Eales build with a few thousand miles on it. The westfield club guys seemed to think this was fairly normal behaviour with the setup I had and an ok solution.
As long as you're happy that this isn't a symptom of engine wear in your case I can't see how you'd be introducing any problems as all you're doing is collecting the oil rather than allowing it to be burned off in the engine.
I blocked off the input to the air filter and ran a line from the rocker cover to a home made oil catch bottle which had a small air filter to atmosphere. Under normal use it would not collect any oil at all but under really hard use would collect a bit, the air filter would remain clear and my engine wouldn't ingest oil vapour.
My bores were not worn in the slightest as the engine was a new John Eales build with a few thousand miles on it. The westfield club guys seemed to think this was fairly normal behaviour with the setup I had and an ok solution.
As long as you're happy that this isn't a symptom of engine wear in your case I can't see how you'd be introducing any problems as all you're doing is collecting the oil rather than allowing it to be burned off in the engine.
Engine was tested after being warmed up, but I'm wondering if because I only see this effect when I hit high rpm (4.5k+), this might not show up in a simple test.
If my bores are shagged, then I'm not the only one judging by the number of folks saying "mine does that too".
Actually if it was the bores, would it not show as a bit of smoke at all times, just getting worse as the rpm rises? My car's behaviour is really like a switch has been thrown to turn on the smoke machine!!
I guess I'm going to go down the route of a home-made tank first and see what happens. For what it is worth, the breather line on the 928 comes into the intake after the air-filter, not before so i don't see oil fouling of the filter itself.
If my bores are shagged, then I'm not the only one judging by the number of folks saying "mine does that too".
Actually if it was the bores, would it not show as a bit of smoke at all times, just getting worse as the rpm rises? My car's behaviour is really like a switch has been thrown to turn on the smoke machine!!
I guess I'm going to go down the route of a home-made tank first and see what happens. For what it is worth, the breather line on the 928 comes into the intake after the air-filter, not before so i don't see oil fouling of the filter itself.
It's something that's fairly common for us rotary owners to fit as there tends to be quite a lot of oil under heavy load that makes it's way beck round through the intake system. In some cases a 500ml can can fill in less than 10 laps of somewhere like Silverstone! Mine's not quite so bad but it still passes enough after hard cornering (right hand bends) to be useful.
You could always regard the catch tank as a temporary test. See just how much oil you are burning by measuring how much is in the tank after a blast - you will probably find that it is quite small and nothing to worry about. Just for reference, my V6 smokes a little when red-lining. I fitted a catch tank and it stopped smoking but the catch tank is still empty after 500 miles so the smoke must have been generated by very little oil.
A catch tank is one of the first things that most engine tuners fit as the burned oil just messes up the mapping.
A catch tank is one of the first things that most engine tuners fit as the burned oil just messes up the mapping.
Edited by RazMan on Monday 19th February 21:32
You need to understand what the source of the problem is in order to correct it.
While the engine burns a lot of oil, if that is because the AIS is overly effective in pulling crankcase fumes then it may be better to change the AIS so it does not work on the jet-pump principle.
The next thing is to find out why the crancase generates fumes. It may not be that the bores are gone, but that the ring pack cannot cope with being revved without losing oil control function. But you will never fix that - so just deal with it.
Then you may have a problem with the crank frothing the oil - this will generate the fumes and potentially cause the problem. The solution is to experiment with oil levels. Overfilling can be a cause of the problem you have.
If the engine checks out then a catch tank is a fair solution, but I would have one that drains back to a cam cover so you recover caught oil.
While the engine burns a lot of oil, if that is because the AIS is overly effective in pulling crankcase fumes then it may be better to change the AIS so it does not work on the jet-pump principle.
The next thing is to find out why the crancase generates fumes. It may not be that the bores are gone, but that the ring pack cannot cope with being revved without losing oil control function. But you will never fix that - so just deal with it.
Then you may have a problem with the crank frothing the oil - this will generate the fumes and potentially cause the problem. The solution is to experiment with oil levels. Overfilling can be a cause of the problem you have.
If the engine checks out then a catch tank is a fair solution, but I would have one that drains back to a cam cover so you recover caught oil.
I was looking at this again last night pondering my options, when it dawned on me that I've actually got a catch-tank already. The way Porsche designed the oil system is that there is a little "tank" which is where you add the oil. This "tank" (or "separator" as the workshop manual insists on calling it) has connections to the sump, a hose from the crank-case, and the cap has the hose which goes to the intake. It was only when I was looking more into the catch-tank idea that I realised I already had one!!
Anyway, I've also now received an interesting bit of information. In '85 Porsche changed the design of the filter inside the separator because of oil being thrown up into the filter from the crankshaft (and thus oil being drawn into the intake and burnt). In '86 they also changed the way oil is returned to the pan from this separator because under high rpm oil was being pushed back up this pipe. I guess that this implies that even in a new engine sufficient gases were getting past the rings to cause problems.
Looks like a second catch-tank might be the way to go.
Anyway, I've also now received an interesting bit of information. In '85 Porsche changed the design of the filter inside the separator because of oil being thrown up into the filter from the crankshaft (and thus oil being drawn into the intake and burnt). In '86 they also changed the way oil is returned to the pan from this separator because under high rpm oil was being pushed back up this pipe. I guess that this implies that even in a new engine sufficient gases were getting past the rings to cause problems.
Looks like a second catch-tank might be the way to go.
GavinPearson said:
....
If the engine checks out then a catch tank is a fair solution, but I would have one that drains back to a cam cover so you recover caught oil.
If the engine checks out then a catch tank is a fair solution, but I would have one that drains back to a cam cover so you recover caught oil.
V interesting Gavin. I have a Cerbera AJP 4.5 with a catch-tank that gets pretty full pretty quickly. Never dawned on me to feed it back in..... DOH!
By way what is AIS - sorry
Cheers
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