At 10:47 PM 5/21/02 -0600, Bullwinkle wrote:
>The modifications I think Denise is refering actually improve the pumps
>capacity or rate of delivery. As such, it may also raise the pressure as
>the flow rate is increased. I sent Denise the machining specifications.
Hey Blake,
Send me the machining spec's too if you would. I think I know what you're
talking about, but would like to be sure. The MG oil pump is a positive
displacement pump, so there's not much you can do to change the volume or
the flow rate. If it's worn out there's also not much you can do to help it.
I did have a problem in 1996 (sheesh, have I been on this list it been that
long) when my MGA lost oil pressure completely at about 7000 rpm because
the oil pump was cavitating. The real cause of the problem was a somewhat
inefficient design of the oil pickup pipe on the early MGA 1500 engines,
which was corrected later in production. But while I was investigating
this problem I found out a lot about MG oil pumps, and ran across one nifty
modification.
The MG oil pump has both inlet and outlet ports at the top of the
rotors. Oil is drawn through a drilling up the side of the pump to the top
where it enters the rotor through a pear shaped port cut into the top plate
of the pump, and is then pumped out through a similar port on the opposite
side of the rotor. The inlet port is relatively small, the oil passage
from the pickup screen to the inlet port is somewhat restrictive, and the
only pressure available to push the oil from the sump into the oil pump is
atmospheric pressure (14.7 psi at sea level). The oil pump will try to
move 54 quarts per minute (nearly 1 quart per second) at 7000 rpm
crankshaft speed, which is when it draws a vacuum bubble inside and just
stops pumping.
The nifty cure for this problem is to cut an additional intake port in the
bottom plate of the pump which mirrors the one in the top plate. I did the
first one with a milliing maching, but this can be done with a small hand
grinder if you're careful. It also helps to enlarge the oil passage hole
in the pickup arm on the bottom of the pump. With two inlet ports only
half of the oil (or maybe even a little less) needs to go to the top of the
pump and through the top port, while the other half is getting into the
rotors through the bottom port. As pressure drop increases exponentially
with flow velocity, cutting the flow through each intake port in half
reduces the pressure drop there considerably, and like magic no more
cavitation problem. After blowing the rod bearings out of my engine three
times in one month before this fix, I have since been running it up to 7500
rpm on occassion (lots of occasions actually) with no oil delivery problem.
Bottom line though is that this has nothing to do with increasing pump flow
or oil pressure at "normal" engine speeds.
A couple of years later I purchased a new oil pump (3-main engine) from
Moss Motors. When I removed the bottom of the pump to make this
modification I found that it already had the bottom inlet port machined (or
cast) in the aluminum bottom plate, which was a very pleasant
surprise. More recently I have purchased another oil pump from the same
source which did not have the bottom inlet port, so don't count on it.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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