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High Oil Pressure for a 340 - What's Your Problem?

How Much Is Too Much?
Jon Grasher
, via CarCraft.com: I am working on a '69 Barracuda with a mildly reworked 340. I have had the engine reworked with a roller cam, Edelbrock RPM heads, and an Air-Gap intake topped with a Retrotech Stage II EFI. I've relocated the oil filter from the stock, impossible-to-change-without-spillage-onto-the-headers location to the fender. The engine has a Melling high-volume pump. When the engine is first started, the gauge reads one increment above the 80-psi limit. As the engine warms, the pressure drops to 70 psi at idle but pegs the gauge when the rpm comes up. Is this pressure too high? If so, what damage can occur? Changing the oil pump pressure relief spring would require pulling the engine to access the pump, which is not one of my wishes. But if it has to be . . . .

Jeff Smith: The short answer, Jon, is that high oil pressure is not necessarily bad for your engine, but it does place a greater load on the oil pump driveshaft and camshaft drive. It takes additional power to make greater pressure. We talked with veteran engine builder and NHRA drag racer Bob Lambeck who has a long history of competition engine building, which includes Mopar small-blocks. Lambeck says he would try a different, mechanical oil pressure gauge to ensure the readings are accurate. He has seen many oil pressure gauges go bad, although they generally read lower rather than higher, but anything's possible. Assuming the pressure is as high as it appears, Lambeck also says it's only a matter of time before that small oil pump driveshaft either rounds off or breaks in half from the greater load. Small-block Mopar engines need good oil pressure because, like 351 Cleveland and 429/460 Fords, they feed the mains only after pushing oil through the lifters. Lambeck says at normal engine temperature, idle oil pressure should be more like 30 to 35 psi, and peak rpm pressure will work fine between 60 and 70 psi.

You didn't mention the oil viscosity you are currently running. If there is 20W-50 oil in the pan, a quick way to trim oil pressure would be to try either 10W-30 or 5W-20 weight oil. This reduced viscosity will make a difference and may allow you to get by without having to tear apart the engine. If the current viscosity oil is 10W-30, then it's clear that the oil pump is working too hard. Assuming there is no restriction between the oil pump and where you tap into the engine for the gauge, the only way to repair this problem is to reduce the bypass spring load used in the pump to establish pressure. The best way to do this is to change the spring and then bench-test the pump assembly by driving the oil pump with an electric drill motor with the pump pickup submerged in oil and the outlet blocked so the bypass spring has to open. This way, you know the maximum cold idle oil pressure. This is a lot of work but the only way you'll be able to know for sure what the pressure will be in the engine.

Oil is Still Well
Chuck Habrack,
Raritan NJ: I just read your Oil Is Well answer in the Mar. '10 issue. I worked in a motor shop (Tony Feil Competition Engines, now closed) for 13 years building mostly big- and small-block Chevys. I agree with some of your answer. We would restrict oil to the top end on engines with a roller cam and rockers using either a shaft or stud mount. We also installed a breather between the lifter bosses on a small-block so only oil would return via front or rear and epoxy a plate to cover the large drain-backs above the cam on big-blocks. Most of our big-blocks and high-end small-blocks used an external Weaver spur gear pump. We would scavenge oil from the rear of the oil gallery and not let any oil drain back past the crank. On internal pump motors, we would do our best to keep return oil from hitting the crank. I fully agree that too little spring oiling is bad. Yet dumping excess oil on the springs has no advantage. On dry-sump motors, we used tubes mounted inside the valve covers with -3 lines and bleed jets to limit the oil on the springs. Shaft rockers with positive fulcrum feed used a spray hole. In short, with full roller engines (lifters and rockers), we would direct more oil to the crank and sump. In an endurance engine that turns 5,000 to 8,000 rpm, lube for the crank and rods becomes even more important. Another reason for positive valvespring oiling is to ensure cooling the springs on engines running negative crankcase pressure.

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