SBC Oil Pressure

A common error in building a small block Chevy engine is to use too much oil pressure. As is often the case with choosing camshafts, "More�s rule" occurs. You know- "If More is better then too much is just right!"

Sure, inadequate oil pressure will starve the engine for lubrication and cause premature failure, but conversely excessive oil pressure will cause problems as well.

Oil pumps don't create pressure. It moves oil, but until there is a restriction in the system, pressure can't build.

Bearing clearance, oil viscosity, temperature and the oil pump pressure relief spring all contribute to the observed oil pressure. The oil stays under pressure until it leaves the bearings. Then it whirls around in the tornado like vortex of air around the crankshaft. As the spinning crank smacks this cloud of oil droplets, it loses energy. This energy loss is called "windage".

Modern racing oil pan design has yielded significant horsepower gains by minimizing windage.

A good wet sump system with a scraper and windage tray may pick up 20-30 hp over a stock pan.

A dry sump system can gain as much as 65 hp over a stock type pan on a racing engine.

Another hp loss is the power needed to drive the pump. No mechanical device does work for free. According to Steve Morrison of Milodon a Big Block Chevy pump (5 bolt cover) on a Small Block wastes 12 hp.

Smokey Yunick says the small Block pump(4 bolt cover) uses 10 hp at 7,000 rpm.

He also reports that some Winston Cup engines run 50 psi for qualifying just to extract every last bit of power(Circle Track April '93)

In the late seventies and eighties, Chevrolet reportedly reduced oil pressure to try to squeeze more fuel economy out of 305�s.

Other reported problems are more strain on the drive system, and the speculation that mechanical pulses through the distributor drive increase spark "scatter". (see Melling Link below for comments on this)

How much is enough? The rule of thumb: "10 psi for every 1,000 rpm", was originated more than 25 years ago by Smokey Yunick. Does that mean that because you think your engine will run 7,000 rpm you need 70 psi?

No. Recommendations also depend on application.

Street engines 35-45 psi

Street HP 45-55 psi

Racing (only) 60 psi

Pro, turbocharged 70+ psi

In the 1984 Power Manual, Chevrolet recommends 65 � 80 psi for racing applications. This means purpose built, trailered racecars. Bob Mainetti of Canton Racing was quoted in Circle Track (April �96) as saying: " At 6,500, you want 50 to 60 pounds. At 7,500, 60 pounds is plenty of pressure."

What was thought true in the last century is changing in this one. Gary Penn from GM's Performance Parts division says (CT June '03) "At GM we are big fans of synthetic oil motor oil after break-in. The stuff has really improved over the years and allows you to get by with things you never thought possible." He goes on to say: "...We run only 3 qts of 0W-10 in the motor.... and run it with no oil pressure across the finish line at 7,500 rpm" Gary concludes with: "The advanced quality of the (synthetic) oils on the market means the old rule of thumb that you need 10 pounds of oil pressure per 1,000 rpm is not valid anymore."

Setting SBC oil pressure

Changing the oil pressure relief spring in the pump will adjust the maximum oil pressure. This requires removing the pump cover and carefully driving out the roll pin. Replace the spring, cup and roll pin exactly as removed. Spring pressure ratings are nominal hot pressures. If your bearing clearances are too loose, then your actual pressure will be lower. If you run a thicker viscosity oil then your pressures may be higher, especially when cold.

Most aftermarket high-pressure pumps you buy come with the Z-28 spring (#3848911) that produces 65-70 psi. This is too high for most engines. Not surprisingly, these pumps often come with a lower pressure spring in the box!

I�ve found that the best High Performance oil spring for sbc engines is the factory Corvette L-82 relief spring. The part number is: #10044435

This spring yields about 50-55 psi and is the same spring used in the high performance ZZ4 crate engine.

Cost is about $1 from the dealer and has to be the all time bargain factory HiPo part. While you're there, buy a magnetic drain plug under PN 23011420. This will trap metallic particles generated by normal wear and tear. Clean it off every oil change.

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Oil Volume

Oil pumps supply oil to the bearings. This oil is pumped into the clearance space between the crank and the bearing shell. The bearings ride on this thin layer of oil. As the crank rotates, the clearance is more at the side opposite to the force being exerted. This offset creates a hydrostatic "Wedge" of oil that forces the oil molecules between the two surfaces. The molecules act as ball bearings and minimizes friction. This wedge can support all the combustion forces except those caused by detonation.

As stated above, oil pressure is maintained by pumping more oil than can leak out past the bearings. Excessive bearing clearance increases the leak. Insufficient bearing clearance prevents oil from flowing properly. In addition to supporting the load, the oil film picks up heat as it flows past the journal. This oil film must be constantly sent to the pan to shed heat and replenished with cooler oil. Oil temperature increases dramatically when reduced below .0020". There is a 50% increase in temperature when bearing clearance is reduced to .0015"

Some builders erroneously think that increasing the bearing clearance and flooding the bearing with more oil volume will make the engine cooler running with more longevity. Load capacity is reduced when bearing clearances exceed .0025"

Oil flow rates increase when bearing clearance is increased. A change from .0025" to .0035" increases oil requirements 150% or 1.5 gallons per minute! Generally speaking, the bearing clearances are .002" on the rods and .0022" on the mains on a typical 350 Chevy.

So how much volume do you really need?

Fortunately the Chevrolet oiling system is very robust.

A stock volume pump is sufficient for almost any high performance street driven car. It�s one of the many reasons that the Chevy engine is so popular with hot rodders.

The only reason that you would need a high volume pump is if you have an external oil cooler, a remote oil filter or turbocharger that requires engine oil for lubrication. Common belief is a high volume pump can evacuate a stock oil pan, so think twice about getting one if you really don�t need it. Gary Cross from Melling Performance says that if you are turning 5,000 to 6,000 rpm with appropriately tight clearances you can run the stock volume pump. (Circle Track April �96)

In an article published in June 2003, Circle Track, 'Engine Oiling Tips From the Pros'; Kent Troutman from KT Engine Development says: " Oil is horsepower. Tighten up the clearances, close off any unnecessary oil feeds and use a lighter viscosity oil."

Since this was first published I've found that Melling disputes the common theory of spark scatter and HV pumps evacuating oil pans. See their tech page for more info.

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Last modified 04/18/2006

copyright 2000-2006 Paul C. Wright