Jayd3-you are starting to think on the issue-which is good-and more or less on the right track.......as I alluded to above in passing-there are multiple variables associated with plugs-firstly-when you replace a Cox head with a glowplug adapter you altered the head shape and increase the volume-so effectively lowering compression-how much-depends on the nature of the adapter and the plug-depending on whether it is a long short or medium reach plug-then how it seats in relation to the head surface-so you can see several variables there.....before we even look at the plug itself! Now the plug-ignoring 'reach' you have the following variables-all of which interact to a degree-but are independent:
Plug element-
(i)-size of wire,
(ii)-composition of wire,
(iii)-number of turns,
(iv)-shape of element [not all are a coil !]
(v)-relative location of coil within plug cavity-flush, protruding or recessed
Plug cavity-the free volume in the recess that the element sits in-
All of the above factors interact-and collectively determine the plug 'heat'-but there is no agreed standard internationally, so brand to brand there is enormous variation. So before you even start there are at least 7 variables-and you can add head shape to that as well! [by head shape I mean the internal profile-including any refinements such as squish bands-making 8-plus the head clearance-9. How much testing would you need to do to optimise something with 9 variables? To optimise would require an enormous amount of testing (and cost-if you were going to purchase a big range of plugs to test)-which is why no one has done it. At best people find a plug that 'works'.....in other words one that is not too bad-and live with an 'acceptable' power loss-which could be as much as 3000rpm in the worst case [most people would not be happy with this much and experiment a bit more]-if you could achieve a loss of only 1000rpm from the OEM Cox head by using a standard plug adaptor, -you would be doing very well indeed. [which is why people who can't take the rpm loss with a plug will use a Nelson head-and perhaps gain 1500rpm with careful shimming (and even here you have 3 plugs and 2 inserts available)
FWIW-my personal opinion is that the best combination using
a standard glowplug adaptor is likely to be using a globee style plug-as this minimises the 'dead' volume loss-by having none(!)-and using a thread locker on the plug threads. Merlin Plugs do the flat Globee style element plugs (#2016 and 2016B) in hot and medium heats. [there is also the possibility of using a turbo plug-but
AFAIK no one is currently offering these in a Cox 049 version. (I'm not sure if a Norvel turbo adaptor would fit....it might!)] Again personally I wouldn't touch a standard plug adaptor with a barge pole-I have an adequate number of Nelson heads for my 049s
I haven't touched on the fuel vs head aspects-as the story is still incomplete-and insufficient data exists to extrapolate-but where it is (in the .15 size)-20+ years ago F1C engines used glow heads derived from the Cox TD15 back in the mid 60s (ie before Rossi 15s were developed) when it was found the ST G15 went like the bees knees with a glowhead insert machined down from the TD or Spl 15 -these had a trumpet shaped head profile-and since then every F1C engine has used this style of head until Nelson plugs arrived. [ie the Rossi, AD OPS and Cox Conquest 15s and the various russian ones] The Nelson came out in the 80s IIRC...When the 'modern' generation of C/L combat motors arrived the so called 'double bubble' Nelson head was considered the best......except that combat fuel is 10% nitro, FAI F1C fuel is 0%-the trumpet heads don't work very well on 10%.....and the double bubble heads don't run properly on FAI.....and that's only the difference between 0-10% nitro-which most people would consider a fairly 'tame' fuel.........
ChrisM
'ffkiwi'