Turbo vent is dumping oil into the airbox (on the track only)

cowtown · 07:34 AM

Hi,
I had a really great day at SCCA's Thunderhill DE on Sunday with my '77 930 powered 911S. But after two 20-minute runs, I started seeing some oil smoke out the tailpipe. Pulled into the pits, and the car was belching oil out of the airbox.

I've searched, and see that this is a somewhat common problem related to inadequate turbo sump scavenging and/or blowby and/or too much oil to the turbo.

I suspect that the oil is coming into the airbox via the turbo oil tank breather.

The problem is, I can't get on the car hard enough to duplicate the problem on the street.

So here are my questions:

--Does anyone know how I can diagnose this problem without a racetrack?

--Since I suspect that the turbo sump tank is getting backed up, why can't I vent that tank to the main oil tank instead of the airbox? This would be a bandaid, but might keep me from getting black-flagged, and it only seems to be an issue under extremely hard engine use.

cowtown · 08-10-2004, 03:29 PM

Bump...after some more searching, it looks like a catch can might be a step in the right direction.

Where are the Turbo Junkies today?

beepbeep · 08-10-2004, 03:38 PM

930 have small oil sump underneath the turbo that is supposed to collect the oil before scavenge pump takes care of it.
This is quite old-school thinking as modern cars just have a gravity-drain pipe straight into engine oil-sump. It's also very neat with it's own little breather-hose

Remember, this was one of the first turbocharged engines and PAG engineers didn't quite know how to do it so they chose the safe route.

I don't see any reason why you couldn't vent this hose into oil-tank first as oil-tank itself is vented into airbox anyway.
This way, you will separate oil from gases, and gases will be inhaled by car later on.

My guess is that oil geats foamed/atomized when running at full blast and engine sucks it in.

If you are planning to race frequently, I suggest you to:
1. Connect turbo oil-sump breather hose to oil-tank breather-hose
2. Disconnect oil-tank breather hose from airbox alltogether and put it into small oil spearator (usually, a PET waterbottle will do)
3. Plug the remaining hole going into airbox.

This way you won't have a chance of inhaling the oil (it will just overflow from the bottle, unlikely) and your engine won't be inhaling oil-fumes (which is a good thing as it lowers the octane ratio and makes engine ping easier),

hobieboy · 08-10-2004, 06:37 PM

Hi Colin,
A couple of thoughts...
a) Make sure that you have that little ball (4mm I think) in the oil pressure switch where the turbo oil supply line is hooked up to. This ball is used to limit amount of oil going into the turbo

b) Maybe I misunstood the problem? I thought the sump tank is connected to the scavenge pump, hence oil is drained by the pump "pulling oil" out of the tank only? I am using a GHL tank - it does not have a breather hose... am I missing something?

cowtown · 08:51 PM

Quote:

Originally posted by beepbeep

If you are planning to race frequently, I suggest you to:
1. Connect turbo oil-sump breather hose to oil-tank breather-hose
2. Disconnect oil-tank breather hose from airbox alltogether and put it into small oil spearator (usually, a PET waterbottle will do)
3. Plug the remaining hole going into airbox.

Thanks Beep. This sounds like a good approach, and I'll try it. Too bad I won't know if it works until I get back on the track.

Patrick, thanks for the reply. I'm sure my ball bearing is in place. I'm not sure how your sump tank works without a vent - it seems to me the scavenge pump would cavitate without an air source, but I guess not. I have the older-style factory tank with a vent to the airbox.

I was in good company at the track. Too bad I was the only one with a puddle under my car!

dean · 08-11-2004, 04:48 AM

Colin,
How is your oil level? Is it too high? I am guessing that under braking the oil is goiing up the filler neck and finding its way into the air filter housing via the vent. Nothing to do with your scavange system.
Dean

beepbeep · 08-11-2004, 05:53 AM

Quote:

Originally posted by dean
Colin,
How is your oil level? Is it too high? I am guessing that under braking the oil is goiing up the filler neck and finding its way into the air filter housing via the vent. Nothing to do with your scavange system.
Dean

Check that first. People i know who race their 930's usually use slightly less oil, to prevent this from happening.

cowtown · 06:26 AM

I was running it pretty low on purpose - just above the Min line. After the "great flood" it was not even registering on the dipstick. Could have been the oil tank breather too, I guess. I still haven't been able to duplicate the problem.

john walker's workshop · 08-11-2004, 06:56 AM

try using a gasket with a smaller hole between the turbo and the oil feed inlet.

TonyG · 08-30-2004, 07:02 PM

john walker's workshop

Why would you want to use a smaller gasket? As an oil restrictor?

TonyG

iamchappy · iamchappy's Garage

I've found nitrous injector nozzles work good as turbo oil line restrictors.

TonyG · 08-30-2004, 07:42 PM

iamchappy

I've always used a allen plug tapped and threaded in, drilled/beveled, as an oil restrictor.

.050"

But... the super trick Innovative dual bearing cartrige turbos have it built in FWIW... which are the only turbo's I'd purchase these days. The oil requirements on these turbos, compared to a conventional turbo is almost trivial.

TonyG