engine run in rationale

[Joe M.]

What is the reason for the recommendation that an new engine/ new cam be bought up to 2k rpm for 20 minutes?

What is that supposed to do?

Never did that way back when so what's the deal today?

Why wouldn't you just start it up and let it idle with an occassional slow increase to 1500 and back to idle?

As I recall reading, the factory never broke engines in at high idle for 20 minutes.

I remember the 1980 GM cars or was it just vettes had cam wear problems, but I assume that was due to metalurgy problems. When did the high speed break ins become in vogue? Why?

[Duke W.]

Re: engine run in rationale

The "cam breakin procedure" you refer too was NEVER used by GM for production engines. It was developed by the aftermarket back in the fifties/sixties for very aggressive dynamics racing cams that required high rate valve springs.

Microscopic examination of finished lobes and lifters will always reveal irregularities that are "ground away" in the first few minutes or hours of operation - sometimes referred to as "seating" the wear surfaces. A similar phenomenon occurs with piston rings, and ring and pinion gearsets are actually "seated" at the plant with a brief period of operation with an abrasive lubricant.

Though lobes/lifters are spash lubricated, hydrodynamic pressure should support the load as long as relative velocity is high enough to generate the required level of hydrodynamic pressure, and the higher the RPM the higher the relative velocity and hydrodynamic pressure. If insufficient hydrodynamic pressure is present to support the load at the lobe-lifter interface (due to low relative velocity or insufficient oil splash), not only will the microscopic irregularities wear away, but wear can rapidly continue through the relatively thin hardened surfaces, and from there a lobe/lifter can wear away - literally in minutes. A primary factor in the load at relatively low speed is valve spring force. At high engine speeds, inertia forces dominate.

It was found that if revs were not keep elevated well above typical idle speed for the first 20-30 minutes of operation on many of these aftermarket racing cams with high rate springs, the chance of infant lobe/lifter damage was much greater.

Though the above procedure is not necessary with OE cams and valvetrain components, it certainly does no harm. It will help ensure rapid "run-in" of the mating wear surfaces and long lobe-lifter life, and this initial 20-30 minutes of operation at 2000-2500 RPM is a good opportunity to check for oil leaks and any other abnormalities on a fresh engine.

Duke