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Sorry, Greg, I can't name (or number) any specific NACA report addressing grooves. There are a LOT of those reports , mostly '30's--'40's vintage !!!! LOTS of fascinating info in them, such as most all one could ever want to know about water injection.
Too bad most of the seriously talented guys involved in that work went off into work on turbines--but, then again, turbines were the 'hot new thing' during that period, so, of course, they skimmed off the best talent !!
Such grooves are pretty common in big diesels--usually in the pistons, NOT the head. And, likewise, I think it would be wiser to put them in the piston on any spark engine, including an Alfa, if one wanted them on an Alfa.
Basically because it's less 'invasive'.
I suspect that the ones pictured were done in the head just so the builder could use off the shelf flat-top pistons without bothering (too lazy ??) to come up with a set-up/tooling to do them on his pistons.
The combustion chamber doesn't care in the slightest whether the grooves are on the piston crown or in the head. The point is to direct the squish turbulence in a consistent manner.
There are some modern (mostly Asian design) spark ignition engines that use various types of groove/texturing on the piston crown, there IS something to it. I've never tried such stuff on an Alfa engine, but may well do so soon.
I AM a FIRM believer in using 'squish' design pistons in Alfas, been doing THAT for about 35 years. The benefits of having squish in an Alfa Nord are striking.
Clearly, if no squish area, no point in grooves !!
Bear (also a Greg)
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