|06-21-2003, 12:45 AM||#1|
Join Date: Jul 2001
Location: Moreno Valley, CAVehicle:
Ball bearing vs plain bearing
I am not sure if posting this is a good idea because it may cause some panic. Due to the frequency of the discussion pertaining to to ball vs plain bearing turbos, I feel I should point out some of the differences between the two types beyond just the faster spool.
Plain bearing turbos need a lot of oil pressure at high rpms. The more oil you can get to it at higher rpms the better. If you run higher than the turbo's designed limits then you need to consider even higher oil pressure than you've been running. As an example, we had to run an oil pressure reading of 140psi@4000rpm when the oil temp reaches 170deg F with final oil temp around 220~240F. Garret gave us the turbo oil pressure we needed to run at choke flow.
Ball bearing turbos. High speed bearings require the oil to be fogged before it reaches the bearings (IE oil suspended in air). You can get the air to flow through the bearing already at high speed and the oil will just stick when it hits a surface. The air acts to cool the bearing and the oil lubricates it. Letting a stream of oil go through just doesn't work. The oil will stop at the bearing without going through it and the only time the oil will flow is when you get down to some idle speed. So, I hope no one has considered drilling that restrictor out.
Back to plain bearing turbos. The biggest problem I've encountered after satisfying oil pressure to the turbo is crankcase pressure. The oil from the turbo is almost always gravity driven from the turbo to the pan. If you develop positive pressure in the pan this will slow down the drain and keep the oil in the turbo longer than it should. The oil goes into break down and doesn't last as long. I don't quite know how to handle this in stock form and this is why I am sticking to stock boost pressures....those of you running higher pressures should monitor crankcase pressure as well. Measurement should be done in inH2OA or inHgA with A meaning absolute (this is for the finer graduations as psi is just too coarse). Guage pressure reads 0 at sea level and absolute guage pressure reads say 29.92 mmHg or 14.7 psi at sea level.
Once you start seeing positive pressure that will be your signal to keep checking the turbo for oil leaks. This will also be the time to check leak down. Don't change the turbo just change the seals...go to turbonetics or something with a seal and see if they can match it with something in inventory. Changing the turbo will be a waste of money until you are able to fix the positive crankcase pressure.
If you are running dry sump. Have a dedicated scavenge line for the turbo or if you have a massive scavenge pump then dump the oil in the pan. If you have a leak past the seals and oil is getting out you can be sure pressure is getting in and making your crankcase pressure go really positive.
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Last edited by thng; 06-21-2003 at 01:38 PM.
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