looking for some sensible input and facts and not wild claims.
Good luck, you're gonna need it.
Drag race engines they tend to run clearances a little tighter.
Circuit style engines tend to run a little more open to allow for movement and heat.
Oil pressure, general rule is 10psi for every 1000rpm.
Have seen engines survive at consistant 7000rpm with 45-50psi tho.
Pressure tells you nothing conclusive. And the rules of thumb only apply to specific engines, the little six will generally have lower pressure at lower revs. But again, it means nothing. All that matters is that there is sufficient flow volume to cool and lubricate the bearings.
Let's say you are led to believe (through some braindead rule-of-thumb) your engine lacks oil pressure. To "correct" the problem you reduce the bearing clearances and perhaps run a thicker oil as well. Congratulations; you've just reduced the volume of oil flowing through the bearings and increased the bleed-off through places where it doesn't really matter. Your oil pressure gauge just convinced you to make your engine worse.
My 10 cents
Mains and big end .002"
Cam bearing are an issue, and an easy way to drop pressure
Lifter bores are also an issue to oil drop pressure
Yes, lifter bores can bleed off a hell of a lot of oil.
I usually aim for .002 on big ends and 002-003 on mains using 20w50 castrol. Std volume pump is fine with these clearances at any rpm , just do whatever is necessary to get oil down to the pickup and your engine will live a happy life .
Mick is spot on as usual. Keep the pickup covered and everything is fine.
Min clearances also mke them less forgiving during oil starvation periods.
With oil sloshing around it is easy for the pump to suck an air bubble, a 1mm bubble when going through a bearing can be squashed to look about 10mm in dia.
Yes some engine do run tight clearances but they also pre heat oil, coolant and the entire engine before even thinking about flicking the starter switch.
Keeping the pickup covered is the biggest single thing you can do to keep the engine alive. The trouble is even a very brief gulp of air takes a long time to purge. Oil pumps will pump oil very well but they are shit at pumping air. So when they swallow a little air that air just sits there in the pump until the downstream pressure has dropped right down to zero, allowing the air to exit the pump. This might take a while, and then of course you still have to push the air through the engine. So the bearings could have no real supply of oil for quite a while. This is why dry sump systems work so well - no matter how much the car is jerked around the engine has an uninterrupted supply of oil. And be aware that too much oil in the sump can be as bad as not enough - it can become aerated badly if the level is high. Incidentally, thin oils release entrapped air much quicker than thick oils.
If you're not losing oil to places it shouldn't be going - lifter bores or cam bearings for instance - then any half reasonable bearing clearance will be just fine.
Edited by oldjohnno, 17 April 2016 - 08:50 PM.
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