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Original Message
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| Bearing Clearances vs Oil Pressure |
| By John - 08/06/2002 8:45:46 PM; IP 142.177.79.148 |
The oil pressures I mentioned in a previous response to this thread are for 0.0025 bearing clearances. Changing oil weight from 10W30 to 20W50 doesn't do much for oil pressure...a few psi at idle for example. But changing bearing clearances does. My 1966 Ford factory manual for these engines specifies:
40-55 psi @ 2000 rpm for the 427 (hot); and,
45-70 psi @ 2000 rpm for the 428 PI (hot); and,
35-55 psi @ 2000 rpm for all other FE's (hot)
The bearing clearances specified are wide:
Mains at 0.0005 to 0.0031" in total for the various FE's and 0.0008 tp 0.0032" for the rod bearings for various FE's.
There is no mention of what weight oil is used for the oil pressure spec. I suspect that the wide oil pressure range correlates with the wide bearing clearances.
I will admit, I can't imagine a clearance of 0.0005" and the engine staying together.
David Vizard writes in his S-A Design book "How to Build Horsepower, Vol. 1":
"Increasing bearing clearances beyond necessary and raising oil pressure by modifying the oil pump relief spring are common "backyard" hot-rod tricks. It remains uncertain what most enthusiasts expect to gain from these modifications, except that they must be driven by the old proverb "A little bit is good, more is better". In the real world, this practise should be avoided. In most cases, abnormally high oil pressure (above 45 to 50 psi) [He's talking about Chev's here I suspect] increases pump drag, absorbs engine power, increases the chance of oil entering the combustion chambers and does not provide improved bearing lubrication or extend component life".
My own thoughts are this...all engines need about the same oil flow at the bearings. This requires a fairly consistent oil pressure with respect to typical clearances. The only reason Ford FE's require higher than the 10 psi/1000 rpm rule is that the pressure is measured way forward of the typically small diameter oil galleries. 40 psi measured on an FE may very likely give the same pressure and flow at the bearings as a Chev measuring only 20 psi.
Anyway, I agree with Dave Vizard's final comment about the best oil prssure is that minimized with consistent reliable operation. I hear that these days, more and more racers are using stock bearing clearances with 10W30 motor oil and have no problems. Many writers to this forum have stated that they don't even believe in HV pumps...I can only assume that for them, they are using bearing clearances much less than the highly touted 0.0025" "blueprinting" figure. |
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