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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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 Posted: Wed Aug 22, 2007 8:08 am Post subject: Reducing oil pressure at the turbo??? |
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I was reading this last night (in my newly acquired Corky Bell "Maximum Boost" book  ):
| Corky Bell wrote: | Oil Flow and Pressure Requirements
Too much oil pressure can create problems with turbos. It is possible to force oil past oil seals that are in perfect condition if oil pressure exceeds 65 to 70 psi at the turbo. If a particular engine creates more oil pressure than the seals can handle, it may be necessary to install a restrictor or bypass system to reduce pressure at the turbo.
Problems of oil pressure overpowering the seal are evident in a frequent if not quite constant smoking problem [ed. sounds like a lot of 931s I've seen and driven]. Anytime oil pressure exceeds the 65-70 psi range and smoking persists, a restrictor or bypass should be installed prior to any other changes.
These are good guidelines for virtually all turbos:
| Code: |
| min pressure (psi) | min flow (gal/min)
idle, hot | 5 | 0.1
max load | 25 | 0.5
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Excerpted from "Maximum Boost", Chapter 4: Turbocharger Lubrication, pp. 43-44. |
Now, by my calculations, my 931s produce in the neighborhood 5 bar (~72 psi) of oil pressure at startup and under load, which is far beyond the max load of 25 psi recommended by Mr. Bell. Is it possible that our cars would benefit from reduce oil pressure at the turbo as Corky describes? Anyone ever attempted anything like this? Or are they already so equipped, and I've overlooked it?
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Smoothie
Joined: 01 Jan 2003
Posts: 8032
Location: DE (the one near MD, PA, NJ)
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| Posted: Wed Aug 22, 2007 9:09 am Post subject: |
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I don't have this problem personally. I'd say if you do, you're either overdue for an oil change, are using a too-thin oil, turbo seals are actually bad, or your overall oil pressure is way high due to a faulty pressure relief.
I have no idea what was done by the factory - maybe someone with the parts apart can look to see if there's any type of restriction anywhere from where the turbo oiler originates at the oil cooler adapter to where it attaches to the top of the turbo. In addition, a pressure reading taken somehow right from the turbo oil feed line would be useful in answering this. 
BTW- For my last oil change, done just a few days ago, I went with Mobil-1 5W-40 Turbo Diesel Truck oil. -Had been using regular Mobil-1 10W30 previously. I'm thinking the "40" side of the spec will be slightly better for the old engine (and old turbo) in the heat..
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'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Aug 22, 2007 10:35 am Post subject: |
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My smoking problem (on the turbo, not personally  ) has in fact been alleviated somewhat by changing from Mobil-1 10W30 to Mobil-1 15W50. In fact, Mr. Bell advocates for it:
| Quote: | | In general, it is best to avoid wide-range, multiviscosity oils, as the materials added that create the multiviscosity capability are the same materials that cause the coling. Thus, 20W-50 is clearly better turbo oil than 10W-50. A straight viscosity is best of all... |
I'll have to go dig thru my disassembled wide body project bits to see if anything revealing is there...
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Smoothie
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Location: DE (the one near MD, PA, NJ)
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| Posted: Wed Aug 22, 2007 12:03 pm Post subject: |
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Didn't he say "best to avoid wide-range, multiviscosity..."?.. Then, you actually did the opposite of what he suggested because 15W-50 (50-15=35) covers a wider range than 10W-30 (30-10=20). -But he's talking about avoiding coking there, not specifically about leaking/burning, right?
For leaking/burning, I'm still thinking that the main problem (if not actual bad seals) is too-thin oil whether by design (low-viscosity oil) or from being worn-out (as in old oil that's been in need of changing).
Does he say anything about Mobil-1 or about fully synthetic oils in general and their affect on coke prevention?
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"..it's made in Germany. You know the Germans always make good stuff."
'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Aug 22, 2007 12:16 pm Post subject: |
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Hmmm. Yes, but there's seems to be some benefit in running heavier weight oil...don't think you can get Mobil-1 in 20W50...
He doesn't address Mobil-1 specifically, but regarding dino vs. synth, he does say:
| Quote: | | Synthetic lubes are manufactured fluids (not necessarily from oil) in which the basic structure of the lube is much more rigidly controlled than in standard hydrocarbon oils. The resultant product is a very consistent, stable fluid with uniform molecular structure, whose properties are highly predictable. Synthetics have clearly demonstrated their capability with respect to frictional losses, high-temperature stability, and basic toughness of the molecular structure. Mineral-based lubes are less expensive and more likely to coke. |
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Rasta Monsta
Joined: 12 Jul 2006
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| Posted: Wed Aug 22, 2007 12:44 pm Post subject: |
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The only way to know what is happening at the turbo is to install a pressure sender in the feed line. . .after all, it is likely that our pals in Stuttgart plumbed the system for appropriate pressure at the turbo, isn't it?
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- WeiBe (1987 924S 2.5t) - 931 S3
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Aug 22, 2007 1:01 pm Post subject: |
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| Rasta Monsta wrote: | | after all, it is likely that our pals in Stuttgart plumbed the system for appropriate pressure at the turbo, isn't it? |
Judging by the problematic design of the oil drain system, I don't think that's an assumption I would make at this point. The turbo on the 931s in many respects was an afterthought, and I'm sure it's design had many compromises made in favor of cost and perception concerns.
| Quote: | | The only way to know what is happening at the turbo is to install a pressure sender in the feed line. |
Agreed...just wondering if anyone knew off the top of their head based on having done it before. If in fact there is no pressure restriction to the turbo, I'm just wondering why the system would have been designed near the limits (according to Mr. Bell) of acceptable pressure on the turbo seals, instead of installing what seems to be a relatively simple mod to reduce the pressure to something more tolerable like 25 psi. We all know that most of these turbos smoke unless they've been freshly rebuilt (in the case of my 941, it has NOT been freshly rebuilt)...so it would seem that that extra pressure on the seals would cause premature failure.
Or am I missing something????
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Rasta Monsta
Joined: 12 Jul 2006
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| Posted: Wed Aug 22, 2007 1:03 pm Post subject: |
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I haven't had it off for a while, but my memory is that the oil feed hole into the bearing hosing is much smaller than the feed line. Smoking turbos may also be due in part to the less precise balancing techniques than those available now, as well as the well documented cooling problems (coking after shutdown, no water assist, etc).
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- WeiBe (1987 924S 2.5t) - 931 S3
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Aug 22, 2007 1:09 pm Post subject: |
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No water assist I can buy. But according to Mr. Bell, coking doesn't occur after shutdown because the turbo isn't spinning and non-moving parts don't need lubrication. According to Mr. Bell, coking occurs for one reason and one reason only, and that is the breakdown and loss of cooling capability of the oil, primarily due to infrequent oil change intervals.
| Quote: | | oil feed hole into the bearing hosing is much smaller than the feed line |
I have a turbo and the oil lines off of the wide body project...gotta dig them out and do some measuring...gimme a minute or two...
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Rasta Monsta
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| Posted: Wed Aug 22, 2007 1:14 pm Post subject: |
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If he says that sitting in 1000 degrees doesn't cause oil to coke you should call him "Corky."
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ideola
Joined: 01 Oct 2004
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Location: Spring Lake MI
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| Posted: Wed Aug 22, 2007 1:22 pm Post subject: |
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| Quote: | | If he says that sitting in 1000 degrees doesn't cause oil to coke you should call him "Corky." |
Well, he doesn't say that, exactly. But re-read the second bullet point in his rebuttals against the claims of pre-luber manufacturers in this post (incidentally taken from the self-same chapter, just a page later, regarding turbocharger lubrication).
Regarding the orifice sizes, the orifice at the top of the oil filter housing where the feed line is plumbed in via the banjo bolt is ~12mm ID; the orifice on the turbo bearing housing is ~8mm ID. So there is some restriction...it would be interesting indeed to see what affect this has on the oil pressure that the turbo sees...hmmm...might just have to mount something on the wide body when it comes time to assemble...
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bass gt
Joined: 02 Dec 2004
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Location: Johannesburg for now!!
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| Posted: Wed Aug 22, 2007 10:59 pm Post subject: |
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The GT series of turbo's have an inbuilt oil restrictor in the oil inlet to the bearing cartridge.
Now, a word of warning, and i DO have major experience of this. If the oil drain line is not big enough, ie 20mm dia minimum, you will cause a build ou of oil in the turbo. This will manifest itself as a ton of smoke from the exhaust. Of course, you first think that the turbo is shot, bit not so. If the oil cannot get out , it will fill up the bearing section and force it way through to the inlet and exhaust sides. Not good.
I now run a 25mm diametre pipe from the turbo to the sump, which seems to work. Considering how low the turbo is in the engine, there is a very low head of gravity to allow the now non pressurised oil to return.
This is one of the resons i am looking into raising the turbo up 6-7".
Steve
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Aug 22, 2007 11:15 pm Post subject: |
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| bass gt wrote: | | Considering how low the turbo is in the engine, there is a very low head of gravity to allow the now non pressurised oil to return. |
Indeed, one of the aforementioned design flaws in the oil drain setup on the 931.
Steve, have you considered adding some sort of pump to the drain line? Corky Bell mentions the notion of an "oil-pressure-powered scavenge pump" where "the oil flow to the turbo is used to power a pump that in turn scavenges the oil sump." Sound like a novel approach...I wonder if that would be less work than figuring out how to raise the turbo by 6-7"...
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bass gt
Joined: 02 Dec 2004
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Location: Johannesburg for now!!
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| Posted: Wed Aug 22, 2007 11:50 pm Post subject: |
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Nah,
I've figured out the turbo relocation, and it offers far more benefits than just a higher oil head.
Things like-
seperate exhaust runners-not equal length though, so better turbo spool
True Garret T25 fiting, so tuning turbo's is a doddle
Easy location of turbo. Remove it in 15 minutes.
Direct feed into the Intercooler. Less than 6" of pipe between the two.
Ability to use a poppet wastegate, so much easier to tune for a 300-320 BHP engine, so less weight.
Of course, the oil drain head is much better.
So by mid October, i should have the new exhaust manifold ready. I willpost up some pics.
Steve
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Wed Aug 22, 2007 11:59 pm Post subject: |
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You are an industrious fellow, arent' you?! Sounds AWESOME, and I'm sure if it turns out like the rest of your mods, it's going to look absolutely drool-worthy  Can't wait for pix!
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