I just bought a new Silverwing scooter and the instructions (perfunctory) on break-in occasioned a question.
Why do they suggest you vary the rpm during the break-in period? Note, I’m curious about the rpm variance, not the different methods of break-in. I recognize that break-in is considerably less complex than on my first ox-cart but it seems to be a constant that rpm should be varied.
How come that is?
The standard response is that if you drove it at a constant rpm the valves would seat improperly. By varying the rpm you don’t run into this valve seating issue.
Someone who actually knows why this is so should chime in shortly…
Engineers have told me the reason is that the stroke of the piston is slightly dependent on the rpm because this influences how much the metal parts flex, and when breaking in a cylinder wall you don’t want the piston to keep stopping at exactly the same spot, because you’ll build up a bit of a lip just beyond that spot, and then if you run at a higher rpm at any future time you will collide with that lip and damage the sliding surfaces. So, you vary the rpm, and get more of a gradual transition than a discrete lip in the early wear pattern on the cylinder wall.
Never heard the thing about valves.
I hope when the knowledgeable person chimes in, this doesn’t turn out to be as dumb as toast.
People that build performance engines for a living have told me this same story.
In a properly built engine, there is not a lot of stretch in the piston, wrist pin, connecting rod and crankshaft, even at very high RPM. The difference in where the piston will slide on the cylinder wall may only vary .001" or so.
When breaking in a new engine, the part they are recommending is to seat the rings to the cylinder walls. During this time, a certain amount of metal from both is worn away and they match to each other better when this happens.
To help this to happen, the rings need to be loaded against the cylinder walls to help them match. The pressure from the combustion partially helps push the rings against the cylinder walls, and putting a load on the engine causes this pressure to be higher and force the rings harder against the wall. If the engine is run at a steady state, the pressure of the rings on the cylinder walls is not as great, the rings don’t conform as quickly and properly to the walls, and after some time, they all take their “set”, even though it may not be optimum. This will cause the engine to bypass more oil into the combustion chamber, and the engine will burn oil and smoke. Varying the RPM of the engine helps the rings and walls to more quickly mate to each other, and more thoroughly.
It is good to put a load on a new engine - cause it to pull, but high RPM is not recommended until there is some mileage on the engine.
Thanks for the response; makes sense. Next question: how do you put a load on a new engine with an auto tranny without high RPM? Not a snarky question; inquiring minds want to know.
To load the engine, just accelerate the engine fairly hard, as if you are getting onto the highway and traffic is coming up on you fast. Three quarters throttle for short bursts is fine. You just don’t want the engine to rev real high, so you back off the throttle or short-shift to the next gear. Actually, just letting off the throttle and letting the engine wind back down in gear to a slower speed helps also. If you have a tachometer, you just want to keep the engine from revving up close to the redline for the first couple of hundred miles or so. Any engine is under much more strain running at high RPM than it is under hard throttle.