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Here are some good starting points when considering oil line size. The three most popular line sizes are -3AN, -4AN, and -6AN lines. Also keep in mind that the oil needs to flow through at least two fittings.
3AN (line) I.D. is about .145″ or 3.68mm
4AN (line) I.D. is about .187″ or 4.75mm
6AN (line) I.D. is about .375″ or 9.5mm
This is just the measurement I took by putting a drill bit into the lines. Actual specs will very from brand to brand.
So those are the line sizes, not the orifice going through the fitting. The fitting that presses into the line has a much smaller inner diameter.
For example my Porsche runs 6 bar of oil pressure. So we are at almost 90psi. I am running a S256 Borg Warner turbo. I know it will fill the turbo up with more oil than can properly drain out in time (overcoming the seals, not blowing them out). So instead of using a -4AN line with a restrictor, I can use a 3AN line. This should keep adequate flow to the turbocharger. A lot of small and mid frame turbos can get away with a -3AN line when the oil pressure is high.
Now, Running a larger frame turbo for racing and what not. You need a lot of oil flow for cooling. This would be s400’s HT3B’s, S500, HX50’s, Gt40’s. These are big boys! I would not use ANYTHING under a -4AN line. And if you can use a -6AN, use it! Even the G forces can cause loss of oil flow to these turbochargers. I can go on all day with this stuff. And there is much more I could cover. I don’t want to go into detail overload because everyone has a different set-up.
Well most turbo smoke problems are oil line issues. The feed being to big over flowing the turbo. Or the return line being to small. A -8an return is to small. You can get away with a -10an. But, you really need a 12an. The problem with the 944/951 return line is the location. It feeds into the bottom of the oil pan. So you have to lift the oil in the pan to get the turbo to drain properly. This is why ALL of the 951 turbos have duel piston rings on the turbine seal. They didn't have gapless rings back in the 80's. So they off set the two the ring gaps.
The problem with the 944/951 return line is the location. It feeds into the bottom of the oil pan. So you have to lift the oil in the pan to get the turbo to drain properly
Have you not noticed that the oil drain is vented?
Maybe someone can give me some help here. I recently bought a 951 with a Lindsey Super 75 turbo. It's a .60ar on the compressor side and a 8cm hot side. It was blowing smoke like a bandit out the exhaust.
ive since had it pulled and sent it to Evergreen Turbo in Florida for a rebuild. They tell me these aftermarket turbos do not have a dual seal setup and blowout quite often. They are going to put in a dual seal journal bearing to replace the old one.
what size oil lines should I be running for the supply and return to not blow out the seals? I think the turbo is using the stock lines at the moment. No -AN fittings.
Yes, the after market turbos do sometimes need some " seal help". Gapless rings and duel rings are the best way to go. If you decide to go to a AN fitting, Run a -3AN. The -4AN still has alot of flow. You can put a plate on it as I show in the pic. The stock steel line flows about the same as a -4an. But if you want to stick with the stock line don't put to small of a restrictor on it. Start out at about .150in and step it down to .090in. Once you get below .090in the flow is dramatically decreased. Once the smoking stops, your at the right size.
You also have a non water jacketed bearing hsg in the pic. So the only thing cooling that turbo is OIL. The seals are not really getting blown out. It's the oil coming in to fast. The inside cavity fills and actually contacts the seals. The seals are not there to hold in the oil. They are there to keep exhaust and boost pressure out of the crank case. FYI there is NO OIL PRESSURE ON THE SEALS. Glad to help out!!
Question, if you dont pick the correct size restrictor and end up getting oil forced past the rings and smoke, does that mean the rings are damaged and the turbo will smoke forever; even if the over oiling issue is corrected? They're steel rings, so I figure as soon as you correct the over oiling, the problem would go away with no lasting damage. I'm using -4AN oil feed for my holset but it has a restrictor in line, .035" IIRC.
Good question! If the turbo smokes from to much oil pressure, it does not blow out the seals. If you correct the problem the smoke will go away. That goes for the return line also. But, the oil does get into the turbine heat shield. That can smoke for a little wile. It will burn away though.. A .035 restrictor is to small for the HX35. You are going to burn it up eventually. The bronze bearings will break down from oil not cooling the shaft enough. A .035 is something you would run a ball-bearing turbo on.
Here are 2 hx35 shafts. The top one is oil starvation. Bluing for the shaft over heating.
The bottom one is from a restrictor. You can see the bearing bronze breaking down. That is the gold ring you see. Metal transfer.
You can just drill out your stock restricted -4an fitting. You will probably be fine with a 7/64 drill bit. That's about .109in same as using a -8AN fitting. Try a regular -4an first. If its not smoking your fine. Your not going to blow anything with a 4AN.
I have been running a -3AN line on one of my 951 for about 3 years now. Turbo has plenty of oil. But, it depends on your set-up.
11-19-2016, 09:54 PM
Turbocharger Oil Smoke problems
