Oil pumps: High Pressure Vs High Volume

[Dwight P.]

I have read several reports of too much oil pressure when an original pump is replaced with a "high pressure" pump. What effect will a "high volume" pump have on oil pressure?
I am considering replacement of my original 65 327/300 pump. The engine is original and has accumulated about 115,000miles.

Dwight

[Duke W.]

Re: Oil pumps: High Pressure Vs High Volume

Oil pumps are an example of a "constant volume" pump. The flow rate is basically a linear function of pump speed until you get to the point where the pump cavitates.

If you place a restriction in the output line (such as the cummulative bearing and rod side clearance and other internal restrictions in the oil system) the pump will continue to flow the same volume at any given speed, but pressure will rise. As speed increases, pressure rises further. The pump is more highly stressed and the amount of power required to drive it increases dramatically. Keep raising the speed and eventually something will break.

This is why oil pumps have pressure relief valves. When the valve opens, the excess flow is bypassed back to the inlet, but it still takes power to pump this excess oil.

Corvette road engines DO NOT NEED A HIGH VOLUME PUMP. Only racing engines that have loose clearances an need 80-90 psi at very high speed need high volume pumps.

Most SBs have a 40-45 psi release springs, but beginning in late '63 mechanical lifter engines got 55-60 psi relief springs, however, pump volume is the same - same pump, just a higher pressure relief spring.

The bottom line is use the same pump as OE. A "high volume" pump on a road engine will just increase parasitic power and heat up the oil.

The OE oil pressure gage is a good indication of OE pump pressure. If you have a 60 psi gage the OE pump has a 40-45 psi relief spring. If the gage is 80 psi the OE pump has a 55-60 psi relief spring.

The hot oil pressure spec for the 327/300 is 40-45 psi @ 2000, which is perfectly adequate.

Duke

[Joe L.]

Re: Oil pumps: High Pressure Vs High Volume

Dwight----

All high volume pumps for small block Chevrolets are also set for high pressure. So, if you use a high volume pump, you also get high pressure. I don't know how a standard pressure relief spring would work in a high volume pump. As I say, none are sold this way.

[Mark #28455]

HOTROD Bubba alert

DO NOT waste your money on a high volume pump! Real racing engines are doing the EXACT OPPOSITE! Remember Bill Jenkins of late 1960's super stock fame? He was revving the *^%^ out of his BB engines with a "low" volume modified standard pass car pump using STOCK bearing clearances. In addition, current NASCAR engine builders are doing everything they can to minimize HP wastage - they are using super thin oil for qualifying and are running as low pressure as possible. Ditto for current auto manufacturers - part of the push for roller cams and rockers was to get reliability with lower oil flow rates.

You will NOT improve your engine by increasing oil pressure/flow, but you will increase the load on your distributor gear, increase pulsations to the distributor shaft, and waste horsepower. Your call!

Mark

[Mark B.]

Re: Oil pumps: High Pressure Vs High Volume

Dwight, As Duke said, high volume oil pumps are not needed in a street driven engine. The majority of the oil in an engine is for cooling. It only takes about a cupful of oil to lubricate the entire engine. The rest of the oil is splashed around by the crankshaft to coat the internals and take away heat. In a new engine, the reason you want to break in the cam between 2000 and 2500 rpms is to splash coat the camshaft and all of the internals for lubrication. As long as you have oil coming out of the pushrods on the upper end of the engine, you should never have a problem with oiling.