Auto Service World
Feature   September 1, 2007   by Jim Anderton, Technical Editor

Rod and MainMythology

Keeping engines lubed is simple why do oil pump myths persist?

Keeping moving parts lubricated is as basic a technology as it gets. Whether you’re driving an ox cart or a McLaren, preventing metal-to-metal contact with oil is conceptually simple. Inside modern automotive engines, however, bearing surfaces have to endure forces that are far beyond what their soft alloy surfaces could endure without serious lube technology. Delivering the oil needed to float crank and rod journals over soft bearing surfaces is the job of the oil pump, the subject of a surprising amount of mythology. A primary misconception is the relationship between volume and pressure.

A key factor to consider is that the pressure specification for an oil pump refers to the maximum pressure the unit will produce at the moment the bypass starts to open. Every oil gallery, bearing, lifter or any other internal engine part that’s lubed internally (not by splash) acts as a resistance to the pump’s output. This resistance is what determines the actual oil pressure measured in the engine.

According to Melling Engine Parts’ Technical Services Manager George Richmond, “If you don’t have enough resistance in the oiling system, you’ll never get into the bypass. Not enough resistance means poor pressure. It’s like putting your thumb over a garden hose. Volume is reduced, but pressure increases.” Richmond notes that a worn engine with excessive bearing clearances won’t repair itself by switching to a high volume pump.

High volume pumps are a popular way to upgrade oiling systems, especially when building “stroker” high-performance engines. For classic pushrod engines, the longer gears that make the bigger volume can usually be accommodated in the pan. For modern engines, however, it’s tough to get major increases in volume, says Richmond: “Gerotor types are the most common today. They’re over the crank pumps that spin at crankshaft speed, twice the RPM’s of pushrod in-the-pan pumps. The difficulty in getting higher volume is packaging. With cam drives, balance shafts and a compact front cover, improvements are limited to about 18 per cent.”

With limited upgrade options and difficult access, especially in front-wheel-drive applications, keeping bearings lubricated at the high RPM’s modern engines can generate will require careful attention to clearances. Can these smaller, harder-working engines hold up? While manufacturing clearances, including main and rod bearings and tighter, bearing materials are often harder, with less ability to imbed hard particles. If cleanliness was essential to pushrod V8s, it’s critical to modern overhead cam engines, both in the repair/rebuild process and in the engine lube. The same debris that can score journals can also destroy oil pumps, especially the newer gerotor types. If the debris is large enough it can jam the pump drive completely, with enough force to seriously overstress the pump drive. Richmond notes: “I’ve seen Ford gerotor pumps jammed with a tiny piece of valve seal. The shaft can be twisted like a candy cane. The number one factor, in engine rebuild or repair, is cleanliness, both in prep and assembly.”

All techs have encountered heavily coked and varnished engines, and most understand the importance of making sure that oil drain back isn’t restricted, but in the process of cleaning the junk out, do you keep it all out of the engine? Even a simple head gasket repair can result in hard particulates down in the oil pan if the technician isn’t careful. What about the pick up screen?

“Pick-up screens are not infallible,” says Richmond. “Many have a bypass built in for cold starts.”

Cold starts are always a major source of wear, and with much of Canada seeing sub-zero starting conditions for four or five months a year, oil temperatures won’t warm up fast enough to keep garbage away from oil pump and bearings.

What’s the bottom line? If you’re extensively modifying an engine for performance and expect higher RPM’s, defiantly go for a higher volume pump. If bearing clearances are worn beyond their service limit, however, don’t look to the oil pump as a quick fix for a dying engine. And if the top end is fragged, you need to do more than just change the oil and filter to keep that shrapnel from finishing pump and bearings, too.

Melling Engine Parts has technical advice on the Internet at:

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