Quote:
Originally Posted by NinjaBraap
How long should each throttle run be?
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Up to target RPM: 9k, 11k, 13k with progressively larger throttle-openings. Use 3rd-gear and target throttle-opening. When you hit target RPM, stop accelerating and take break. Cruise around changing gears and RPMs. Then do next run, etc.
I've only got 50 engine-builds under me, nowhere nearly as much as the MotoTune guy in linked article. But have noticed exact same phenomenon with import autos and motos using harder break-in than traditional old-school methods.
Change in break-in routine has to do with more precise modern machinery. With old hot-rod V8s using severely out-of-round pistons & cylinders, tolerances were measured in thousandths of inch. Cylinder honing was very rough and deep. You need to use law of averages of many, many strokes to rock-tumble square pistons and bores into round shapes. And you have to be careful about removing too much material or you'll end up with huge piston-clearances and ring-gaps.
In contrast, tolerances on imported autos and motos are measured in
10-thousandths of a millimeter!. They're pretty much broken in and require just some high combustion-pressures to force rings against finely-honed bores to do the last bit of breaking-in.
Questions on your rebuild:
1. What was state of cross-hatching on cylinders? Did you ball-hone cylinders?
2. what was piston-to-cylinder clearance?
3. what ring-gap did you use?
This last part is important because it's difference between worn and new engine. If you have excessive gap with new rings, you've just created pre-worn engine. I'll usually install rings in progressively smaller and smaller gaps with multiple break-ins until I see shiny fretting from where ends butt up against each other when warmed up. Then replace rings with slightly larger gap for no fretting.. Might take 4 or 5 ring installations and tear-downs.
At that point, I consider leakdown to be "good" at 1-2% or less. My wife's Corolla with only 9.6:1 ratio gets 190-205psi on compression test. I think your'e leaving power on table here. As others said, if it runs smoothly, should be fine. You won't notice a difference of 2-3hp with seat-of-pants dyno anyway. But if this was competition engine, I'd do every last possible tweak to extract as much power as possible.