Do you see what I'm trying to say about the 18mm plugs having more space around the center electrode for the A/F charge to swirl around? Since all of the related events are happening in an extremely short time, I'd think this would give a slight advantage.
HOWEVER, after I left the computer yesterday, it occurred to me that you biker guys probably don't ever run the recessed-gap plugs that outboard racers often use when we run more exotic fuels than gasoline. In Champion, these would be L87R or L84R, for example, the suffix designating the recessed side-electrode. This is what is in MY mind when the subject is racing spark plugs. But since nobody would run this sort of plug if he didn't have to, and since you don't have to when burning gasoline, I asked a question that is probably of no interest here. :facepalm:
Anyway, the combustion chamber shapes (and piston crown shapes) are more interesting. If you follow the car-racers' discussions of ports that impart swirl and tumble to the intake charge, you'll notice that (for them or us) the piston still has a long way to go, half a stroke, after the swirly, tumbling intake charge gets shut off. The question becomes, first, just how much is the increasingly-compressed mixture STILL swirling/tumbling as the piston approaches TDC and squashes the charge down into a tiny space? And second, how much and what kind of turbulence is imparted into the now-highly-compressed charge by the squishband? And third, can that be improved upon, and how?
My elderly brain is failing to recall an old, obscure bit of terminology trivia. I expect you all are aware of the experimental squishband grooves of the famous/infamous Mr. Sommender Singh in India. His ideas (and remember he started as a 2-stroke motorcycle racer) are a wonderful source of speculation, though only a few engine-builders with real credibility have tried the grooves . . . with varied results, of course. Anyway, as with a lot of new ideas, somewhat similar things had been tried long before Singh's grooves. The old term that I can't remember was something like, "fire-slots" or "fire-channels" or some-such, and I first saw them in the mid-late-Sixties when OMC (Outboard Marine Corp., maker of Evinrude and Johnson outboards) came out with their first loop-scavenged engine, a 55hp triple. The heads on that motor had wide squishbands, a curious (and cast, not machined) fez-shaped combustion pocket, and two of the "fire-slots" in opposite sides of the squishband and pointed at an angle to the centrally-located sparkplug to impart a last-instant swirl. Just what Singh wants to do.
The next iteration of the OMC triple soon followed, as the engine went in stages from 55 to 60 to 65 to 70hp. Boost ports were added (enormous boost ports, as big as the two tranfers), and the weird combustion pocket and fire-slots were replaced by a simpler, machined hemispherical combustion pocket. Maybe they learned something in the dyno cells, or maybe the new head was just simpler to turn out, with a lower scrap rate.
You know where this is going.:yawn: Have any of you done any good tests with Singh's grooves? My first reaction on hearing about them was skeptical, but then I thought, well, we think we want that final-instant turbulence from the squish, so why sneer at somebody's new idea on enhancing that effect? I'm going to try it myself sometime next spring. I have an obsolete, homebrewed racemotor that wouldn't be competitive today, but can serve as a fine dyno-mule. This is a '73 Yamaha 125cc 56X50 motocross engine with the cooling fins mostly milled off and water-jacketed, and turned on end to make an outboard. I have a couple of extra homemade heads, and can fiddle with grooves or whatever else. I wouldn't expect anything dramatic; it just seems like a fun thing to try.
What I'd really like to try is multi-plug heads. All the Top Fuel dragsters are using 3-plug heads, and if you fly you know that a little airplane engine always drops rpm when you switch from BOTH mags to either one singly. Again, there are undoubtedly more productive uses of one's time when searching for another couple of 2-stroke horsepower. But the weird stuff is FUN.
(You crazed Kiwi 2-stroke tinkerers really ought to think about outboard racing, where the rules are nearly wide-open, you can convert any piston motorcycle engine, and you can use any fuel concoction you want. Yowzah!!!)
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