I've run the Nelson plugs with the Kustom Kraftsmanship and Galbreath head/adaptors for years with good results on fast reed engines and TD's. I think the Nelson plug magic is due to the good heat transfer characteristics it has, not necessarily because of any compression loss in other setups. This is due to the tapered bottom or "nose" of the plug that seals against a matching tapered lip at the bottom of the threaded hole when tightened. The heat path is far more direct than with plugs that seal with a copper gasket at the top of the threads. My guess is that Henry came up with the system from racing engines which use a glow head "button" with the head clamping it down.
The KK/Nelson heads when properly shimmed were usually good for at least a 1000 rpm and less needle sensitivity to boot, plus lower operating costs. The only problem I had was in trying to use them on my detuned throttled sport TD's. The top end was better as usual but the Nelson plug was (I think) colder which required a richer mixture setting and caused a rich low end and sluggish transition.
The other plug I want to experiment with is the Lite Machines plug which is the same (I think) as the old Glo Bee plugs. They have a "button" style insert with a flat spiral element. They used to work very well way back when on fast engines but I haven't tried one lately. Al Clark reports VERY good results with this plug on his D/Bird TD....an even better top end but still retained good low speed. Gotta try one!
