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Tech Question

Ken Ward, Independence, MO, 1966 plymouth belvedere II 440

My 440 is bored 40 over, has 284duration 484lift hydro cam, also has deep sump oil pan with windage tray. I currently have Mobil 1 15W- 50W oil, but was told by a friend the oil is to thick for my motor (kinda like trying to run in water) so I’m now trying to figure out the best oil for my application. How about valvoline VR1 40W?? I don’t race the car but it sees spirited action on the streets when ever possible? Thanks for your help & suggestions. Ken

Ken, there are lots of variables that affect oil viscosity requirements.Stock, late-model engines, even going back to the late ’60s, have very tightbearing clearances – many times tighter then 0.001″! So a relatively thinoil works well, and build oil pressure immediately, even in cold weather.This is good.

Well-worn engines, or ones built with looser bearing clearances, requireheavier oil to maintain pressure at lower RPM. Especially street engines.

Race engines may have loose bearing clearances, but usually also have muchbigger oil pumps, dual pickups, etc. And they rarely see low RPM. So manyrace engine builders use very thin oils – to save the few HP lost to windagewith heavier oils.

For your street engine, we’ll assume typical hi-po street clearances(0.0015-0.002″) and a typical HV oil pump. The question then becomes: atwhat RPM does the system begin to regulate pressure? You’ve probably been itat least one car (yours?) where the oil pressure is maybe 20 PSI at hot idle(750 RPM), and increases to about 60 PSI at, say, 1200 RPM, then sees verylittle increase at higher revs. That 1200 is the RPM where full regulationoccurs. If you swap that car to thinner oil, the RPM needed to reach thereg. RPM would increase, all else being equal, and visa-versa.

The question then becomes: what is the lowest RPM that you actually placeloads on the engine? As long as it’s well above that regulation RPM, all iswell. But if it’s a 4-speed, tall-geared daily driver, and you frequentlylug the mill to, say, 1400 RPM, and the oil pressure is still low at thatRPM, I’d say you are asking for trouble.

So, in a nutshell: sure, go ahead, try lighter oil. But keep a close eye onthat gauge!

Rick

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