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Fired the 340 for the first time last year, ran good. Oil pressure at idle is around 40-50psi, when the revs come up it's reading close to 80psi, that's off a gauge on the back of the block. I'm aware of the 10psi per 1000rpm rule of thumb but is there such a thing as too much oil pressure. Do I have a problem?

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you may have a problem, have you ran the motor enough to get the oil up to operating temps yet? If so what was the pressue then, hot idling and revved up above 2000 RPM with hot oil? Most Hi Po oil pumps bypass springs and valves are set at 80 lbs, not good to open that valve under WOT acceleration I have had to cut the springs down or change the springs on SB pumps to get them to have acceptable oil pressures such as you refferenced in the 10 lbs. per thousnad RPM rule Most good SB engine builders don't use the high pressure or high volume oil pumps any more for that reason, they have the high pressure springs in them

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An oil pump is a constant volume pump. That means it will pump a certain amount at a certain rpm regardless of the pressure it's pumping into. Well nearly regardless, close enough for discussion.

An engine looks like a controlled leak to the pump. Think of all the annular spaces around crankpins and main bearing journals, around lifters and some up into the valvetrain. Most of this stuff doesn't change much with engine speed. The things that control the pressure that the pump sees at its discharge are oil viscosity and flow.

As said before, the open spaces through which the oil squirts don't change much with rpm and for a given short time the viscosity doesn't change much either.

So the oil pressure varies with flow and flow is proportional to rpm. This means that oil pressure, over a short amount of time, is proportional to rpm.

So where does the pressure relief valve in the pump come in? It opens the bypass valve (controlled by that spring) WHENEVER the oil pressure exceeds a certain value. For example it's cold and the oil has a high viscosity, which tends to raise the oil pressure. You rev the engine so now on top of the viscosity you have higher rpm of the pump meaning more flow. That raises the pressure too. At some point your oil pressure goes above the setpoint of the relief valve and it opens, keeping your oil pressure below the maximum decided upon by the designers. It stays open until either the viscosity or rpm decrease and oil pressure goes below the setpoint.

Now let's warm up the oil and do another start. Oil viscosity is much lower, pump volume is the same, you rev the engine and the oil pressure goes up but not as much. The bypass stays shut. Now, if your pump is big relative to the leak, as you rev the engine up with warm oil at some point the pressure reaches the setpoint and the relief valve opens. So at this point your pump is pumping more oil than can leak through the engine so the relief is open and the engine is STILL getting as much oil as it can needs.

Now on to the subject of reducing the spring to reduce max oil pressure. Yes, that's what it does. Now when your oil pump pumps more oil than the engine can use, the relief opens BUT AT A LOWER PRESSURE! The relief valve STAYS OPEN as long as the pump is supplying more oil than can leak through the engine.


The idea that putting a smaller value spring in a pump will decrease the amount of time the relief is open is exactly backwards from reality. In the OP's question he stated that the oil pressure maxes out at 80psi. If he reduces the spring to one that gets the valve to open at 60 psi or above, it just means the bypass will open earlier and stay open longer.
In this situation, the ONLY WAY that the bypass can be made to not be open as much is to reduce the SIZE of the pump or the viscosity of the oil.

So what is a high pressure pump? A regular pump with a stiffer spring in the relief valve.

What is a high volume pump? A larger pump that fits in the same space as original. It can have a relief spring the same as a stock pump, thus will start relieving at the same pressure. But if your viscosity is low or your leaks are large it will maintain pressure longer than a high pressure pump.

Now lets put a high volume pump on an engine that doesn't need it. Oil viscosity is normal and leaks are minimal. The pump will build pressure quicker and the relief valve will open at lower rpm than the same valve on a stock pump. If your engine doesn't need the extra oil, the pressure relief valve will be open a lot. If the high volume pump has a high pressure spring in it it will open later and stay open until engine rpm or viscosity drop.

Now on to power. A pump takes power to operate. That power is proportional to both flow volume and pressure. Every pump is like this. Operating a pump at lower volume or lower pressure will decrease the power it eats.
If there are inefficiencies in the pump the wasted power is transferred to the oil as heat. There are always inefficiencies in pumps. Therefore, a pump that is too big or a pump that is operating into too high a pressure will take too much power and also heat the oil more than a smaller, lower pressure pump.
In the process of taking too much power the oil pump puts a larger load on the oil pump drive, sometimes causing failure, especially of stock drives. If the distributor is at the rear of the engine, it imparts a larger twist to the camshaft. This isn't good for valve timing. No matter how rock-solid a drive you have, the varying load of the oil pump retards the back end of the cam varying with engine rpm.

After you have read this three times you won't wonder why EngineMasters contestants use small pumps and low-viscosity oil.

One last thing: If you ae looking for max power or low ET, you need to experiment on a dyno to find out where the oil pressure must be at the end of the quarter mile to prevent engine failure and make max power. Then size the oil pump so the relief valve opens just as the car is in the lights. That will give you the least power wasted over the run.

End of lecture.

R.

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I'd say this subject has been covered quite well... Give the man Ice Cream...