John, your calculator failed to mention a couple of things. One is that
the oil in the bearing clearance gap is flowing out the sides of the
bearing and that the oil there is at a much much reduced pressure. So
although centrifugal force slings the oil outward it is not pressing
against a closed hole: the oil is escaping. Another thing is the phasing
of the oil flowing into the main bearing journal and on out to the rod
journal. Need to check to see if the oil for a short time has a direct
path from the oil gallery to the rod journal. It is necessary that the
oil escapes the bearings.it gets really hot from the shear forces and
from pushing the wedge of oil around between the two serfaces. I
believe tha the NASCAR folk use a smaller bearign clearance and a low
viscosity oil becaus eheat transfers to tha oil quicker than a heavier
oil. And at high rpm hea tis the enemy. It kills bearings. So low
viscosity oil, move a lot of heat, les shearing force, less bearing
generated heat, oil out to oil cooler all tend to longer bearing life.
Key here would be geting the clearance matched to weight of oil. Also
that low viscosity stuff is easier to pump so a free hp gain.
mayf
my 1/2 cents worh, lol.. and prolly not worh that...
John Burk wrote:
>My pocket calculator just told me some things I didn't know about getting oil
>from the main bearings to the rod bearings at higher rpm .
>
>Using the formula for centrifugal force .00034 x weight (lb) x radius (ft) x
>rpm squared
>
>The weight of motor oil .0318 # / ci ( 7.34 # / gal div. by 231 )
>
>The radius .115 ft of a BBC main bearing of . ( half of 2.75" div. by 12 )
>
>The radius .248 ft from the axis to the outside of the rod journal of a 427
>bbc (half of 3.76" + 1.1 div. by 12 )
>
>So at 7500 rpm it takes 70 psi to just push the oil to the center of the crank
>but centrifugal force adds 154 psi at the rod bearing . My 427 bbc dragster
>worked fine at 7500 rpm with 60 psi oil pressure so outward flow must have
>sucked the oil in to the center of the crank . I wonder if some combination of
>stroke , main journal dia , oil press and temp and rpm , rod bearing problems
>might not come from oil vaporizing at the crank axis , especially with fuel on
>the oil . Does anybody know the vapor pressure of motor oil at 200 deg .
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