Good oil pressure discussion

["Watry, Andrew (LNG-SFR)" <Andrew.Watry@domain.elided>]

Here's a good solid discussion of oil pressure and flow, copied from the MGA
Twin Cam discussion group I'm in.  Many of the members, including the one
who made this posting, are engineers (this guys works in gear theory) and
long-time Twin Cam racers, not something for the faint of heart.  Twin Cams
make Alfas seem like pieces o' cake by comparison.  Kind of reinforces what
Fred has said about how much oil pressure is enough, but goes into a lot
more detail on why.

Want to see the related post about the finer points of crossed versus
parallel helical gear mesh?

Andrew Watry
Berlina Register

>>The stiff-valve-spring theory -- I am not sure why this would cause 
abnormal oil pump drive wear, in and of itself.  Only possible thing I can 
imagine is that the springs cause camshaft deflection, throwing the gear 
mesh out of whack.  A little beam calculation would provide some answers.

I strongly suspect that the stiff valve springs are not the CAUSE of the 
problem -- correlation does not imply causation.  I suspect that, if one 
needs to run very stiff valve springs, it is probably because of a desire to

run very high RPM (big cam, big compression = very high power band).  I also

suspect these folks are probably still using the old Smokey Yunick rule of 
thumb of 10 psi for every 1000 RPM.  As I mentioned above, 8000 RPM x 80 psi

is probably much too much load on the poor little skew gears.

We have seen the same failures on T-series MG's in MG Vintage Racers -- 
years ago.  Billet crank, Carillos, 13:1 compression, 310 deg cam and all of

a sudden 7500 RPM is the target ... with at least 75 psi oil pressure.  
Nobody ever really solved the problem with XPAG engines, they just backed 
down to more realistic redlines & oil pressures.  So the problem is not 
unique to Twin Cams, or pushrod B-series.  I have heard that Alfa guys have 
had similar problems.

A word about oil pressure -- often misunderstood.  The old 10 psi for every 
1000 RPM is rubbish -- Smokey came up with that for big, lazy American V-8's

when 6000 RPM was a huge number.  Besides, most people don't understand oil 
pressure. The number you see on you gauge has nothing to do with the oil 
pressure in the bearings! (Well, very little).  A journal bearing generates 
it's own pressure, it acts like a pump & draws oil into the load zone and 
generates VERY high local pressures. You could not support the actual load 
of the con rod onthe crank with 60 or 80 psi!  So, all your oil pump is 
doingis generating enough flow & pressure to FEED the bearing -- not to 
support it.  And it doesn't take that much pressure to feed oil into the 
unloaded zone of a bearing.  Heavy-duty industrial equipment runs on 25 psi 
oil pressure -- at much, much higher RPMS than car engines.  I'm not saying 
25 psi is what you want for your Twin Cam, but you certainly don't need 80+ 
psi either.  My race pushrod motors are perfectly happy with 60 psi hot/race

conditions (rev to 7000 RPM).  I beleive 60 psi is sufficient, regardless of

RPM (realistic RPM for a three-bearing 4-cyl with 3.5" stroke, that is).

I think the bottom line is, we are asking an awful lot from a gear mesh that

is inherently very, very limited.  In a street car it's fine, pottering 
along at 3500 RPM with 50-60 psi oil pressure.  It "wears in".  It just 
doesn't have the capacity to do 7000 or 7500 RPM at 80 psi continuous use --

it "wears out" rapidly.<<
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