SBC oil pump & pressure question

[ian_uk1975]

I'm currently running a Melling Select 10550 HV (+25% vol) oil pump on my 383. I didn't really want an HV pump as I'm running tight-ish bearing clearances, but I was anxious about the stock Melling M55 pumps due to the problems some people seem to have with them. The stock volume Select series pumps weren't available from anywhere I could find in the UK at the time, so I took the 10550. With 10w30 oil, my hot idle oil pressure is 56psi @ 950rpm and 90psi @ 5,000rpm. When cold, it's easy to hit 100psi+ on the gauge. What I don't understand, is why my oil pressure can be getting so high, as my understanding is the oil pump should bypass oil once the bypass orifice has been exposed? I've tried to max-out the oil pressure to see if/when oil starts being bypassed, but I've briefly had it to about 110psi+ and the pressure showed no sign of abating. The pump came with a spare spring for slightly lower pressure, but I think even the installed spring should start bypassing oil @ 70psi, so why am I seeing so much more?

Should I change the spring for the lower pressure spring? Would this make any difference given the above?

Would I be better-off swapping the pump out for a stock volume pump? Logic tells me this wouldn't have much effect on peak oil pressure though.

Thanks.

[Storer]

It may be worth checking your pressure guage before you do anything else to ensure it is telling the truth.


Paul

[ian_uk1975]

It's a mechanical gauge, so unlikely to be giving a false reading. It's on my 'to-do' list to try another gauge, but I don't really see how a mechanical gauge could over-read though.

[738 driver]

Worth checking the instrument first.. Who built the motor originally, what spec pump was in the thinking then, etc .. Lots of tricks and variables in small block world which might affect oil pressure.. But once the indication is proven correct Id look at pressure relief next... then start thinking about pump selection fitting the build (any restrictors fitted, cam bearing alignment tricks etc etc)

[Steve_D]

After checking the gauge the next has to be the relif valve which would seems to be jammed shut. prove to yourself it is moving freely before thinking of replacing the spring.

Steve

[ian_uk1975]

Motor is newly built... has been dynoed and completed a few hundred road miles. Oil pump was new when fitted and the oil pressure has been this way since the motor was first fired-up, so I doubt anything could be sticking, but will check when I drop the oil pan (need to change that anyway for more ground clearance).

I suppose it could be possible to exceed the bypass pressure when the oil is cold, since the volume of oil being bypassed could exceed the flow capability of the bypass orifice, resulting in increased pressure? Not exactly sure how the bypass works though.

[738 driver]

... and the bypass route... I recall some aftermarket filters not coping with bypassed oil properly and causing issues.... pretty sure was on GM mills too.

[Steve_D]

ian_uk1975 said:

......Oil pump was new when fitted and the oil pressure has been this way since the motor was first fired-up, so I doubt anything could be sticking........

If the pump has a manufacturing fault it could have been sticking from day one.
Do you know what pressure the relief valve is supposed to work at?

Steve

[ian_uk1975]

Steve_D said:

If the pump has a manufacturing fault it could have been sticking from day one.
Do you know what pressure the relief valve is supposed to work at?

Steve

True, but very unlikely I would think. Relief spring is supposed to open the bypass at 70psi, but does that mean peak oil pressure should never go above 70psi regardless of bearing clearances, viscosity, etc? If the oil is viscous enough (like when it's cold), does my theory hold water in that the oil may not have sufficient flow rate to pass through the pump bypass quickly enough causing it to build pressure beyond the bypass activation pressure?

[738 driver]

Have you tried a different brand of filter... simple to eliminate ?

[ian_uk1975]

738 driver said:

Have you tried a different brand of filter... simple to eliminate ?

Yep... initially had a Fram HP4 and changed that to a K&N with the (very handy) hex bolt head end.

Think I'll #1 check oil pressure with another gauge I have laying around, #2 change the pump for a stock volume item when I change the pan/pickup and see if that makes any difference.

[MarkWebb]

I use the K&N filter and also have a similar problem except that I can see 100psi when cold but my warm pressures are much lower than yours! Don't think that the filter has much to do with it but if you try another make i would be interested in the results!

[Steve_D]

ian_uk1975 said:

.....Relief spring is supposed to open the bypass at 70psi, but does that mean peak oil pressure should never go above 70psi regardless of bearing clearances, viscosity, etc? If the oil is viscous enough (like when it's cold), does my theory hold water in that the oil may not have sufficient flow rate to pass through the pump bypass quickly enough causing it to build pressure beyond the bypass activation pressure?

The bypass should open at 70psi regardless of temp or viscosity. It would be a very poorley deigned pump if the bypass were too small.

Steve

[ian_uk1975]

Steve_D said:

The bypass should open at 70psi regardless of temp or viscosity. It would be a very poorley deigned pump if the bypass were too small.

Steve

Which begs the question how is it possible to have more pressure than the bypass pressure designed into the pump?

[738 driver]

only if the bypass route had a blockage...

Are you reading pressure straight off the pump or the top of the motor ?

[Steve_D]

ian_uk1975 said:

Steve_D said:

The bypass should open at 70psi regardless of temp or viscosity. It would be a very poorley deigned pump if the bypass were too small.

Steve

Which begs the question how is it possible to have more pressure than the bypass pressure designed into the pump?

That is why I surmised that the bypass valve is jammed closed.

Steve

[ian_uk1975]

Just wanted to resurrect this thread with what I found... there was nothing wrong with the bypass and, as I had suspected, the issue appears to be the bypass orifice cannot dump cold and/or thick oil quickly enough to prevent pressure rising above the spring pressure. This is apparently common with HV pumps. I've swapped the pump out for a standard volume pump.