Both the Nelson and the Turbo plugs "get the threads out of the combustion chamber"  which can greatly(1000 rpm) improve the engine power.  It eliminates one more variable in the engine operation.  How tight the plug fits is no longer an issue.

Another potentially big variable is the fit between the head and the cylinder.  If the head plugs into the cylinder the fit is critical.  A couple thousandths too loose can cost 500 rpm and change how the engine runs.  The proper fit is just enough clearance that the head will just fit into the cylinder when it is put in square.

If the head sits on top of the cylinder- some Supertigres, Magnum, ASP- make sure the gaskets are correctly sized.  They should be sized to the bore and centered on it so there isn't any small gap around the cylinder between it and the head.  The idea is to eliminate any crevices in the cylinder/head area.

Turbo plugs use an 8mm thread.  I saw a post on rcuniverse a few years back about fitting turbo plugs into a standard head.  One of the standard, double ended, center spotting drills is the right size and angles to drill out the threads and for the tapered area.  It leaves enough meat to then tap the hole 8mm for the turbo plug.  I forget the actual sizes so you'll have to look the up your self.  Best done on a small mill, but several folks were able to do it on a drill press.  Just take care to not reposition the head when switching between the drill and tap so everything stays centered.

But are available in a heat range suitable for stunt engines? I doubt. L

Are "turbo" glowplugs used in anything besides R/C car engines? I'd wonder if their definition of "hot" might be different than ours. Also not sure what makes a glowplug "hot", other than an idle bar. I suspect wire size, coil diameter and numer of turns would be determining factors, but would it be smaller wire or bigger? Bigger coils or smaller, etc? Most good glowplugs seem to use similar alloys.  Steve

Andrew:

You mean Nelson plugs, not Turbo plugs I assume.

I knew that Nelson plugs were popular in 1/2-A -- I assume that they make a big difference there because the swept volume is small to start with, so any plug that lets gas into the threads would have a proportionally greater effect.

What's the magic size you need, Ron? I could check around for you, while I still remember where the tool shops are! The funny thing about taps is that you can get anything you want, as long as you're willing to wait and also cough up the $.  Steve

the smaller the engine the better the turbo ,or nelson plug works .the benefit was first noticed when Carl Dodge made some special plugs using the glo bee 1/2A inserts  for his F2A 15,they made a big diff and nelson started making them  the originals had a horizontal wound coil with no hole therefor more comp for the same area it was a two piece plug  then he did the HD one piece plug that most use now .the main advantage the one piece has over a standard 1/2 32 plug is consistency. find an old adapter head for a TD .049 and put a standard plug in it and see the diff

the advantage of the turbo plug is that it is smaller and both Rossi and OS make make them in a complete heat range,but they are expensive

  Okay, it's tomorrow today. 
The tap is a M8 x 0.75 bottoming. Sounds expensive.
  For Tim: the thread I was refering to is:
http://www.rcuniverse.com/forum/1-2-1-8-airplanes-70/4533758-turbo-plugs-how.html#post4533758
 Kinda neat what you do with less than the basics, and since it seems you like to tinker........
   Later,
         Ron

I have done the mods to a std head for 1/2A engines and used Nelson plugs. For 1/2A engines like Cox and Norvel (plus the despised AP Wasp!). there is anything between 1000 to 2000rpm improvement. How much of this is due to the higher compression ratio and how much to other causes is debatable.
The other important thing is that the Nelson plugs seem to last forever. I have yet to blow one. So please take this into account when you are talking plug costs.
I am sure you will see an improvement in revs with a higher capacity engine, but the bigger it is the less effect you will measure. Quite why anyone would bother to do this for a stunt engine baffles me. For 1/2A one is always looking for more power (as long as it is consistent through the pattern). Speed and team race, well again probably well worth while, but stunt? Most modern engines have power to spare in the right airframe, so why bother? Maybe there is something I am missing?

Andrew.

Both the Nelson and turbo plugs get rid of the "threads in the combustion chamber" which mess up the combustion process.  Better combustion means more torque which you can use to run a somewhat larger prop, or save some fuel.  Whether or not the increased horsepower is enough to be useful in a big stunt motor is another question.  It certainly can be useful in smaller motors, but even the biggest model racing motors tend to use this type of plug.