049 Glow plug conversion perfected
No sweat Andrew, I kind of pulled a Homer too on the tuned pipe thread we had fun with. Your work on that has got me determined to make one.
I just got back from the flying field. OK. Stock Norvel plug with four washers under head, 16.5K on a 7 X 3.5 Cox prop. Idle, solid 4K minimum, though I fly at 5K.
The adapter I made is flat at the bottom and the threads are cut into the proper sized bore. Then, a 5/16"" round dremel bit is used, and spun till I get a combustion chamber cavity. I made several with the cavity at different volumes. All these adapters gave an average 2K loss of RPM no matter what plug. Best though, was a short reach #3 OS. The last one I made had a very small cavity, obviously too small but the idea was to install 5 to 6 washers and take one out at a time to max the required compression. Final result was two washers giving me15.5K. One washer more and 300 rpm loss. One washer less and same rpm loss. OK. This adapter was taken out and blue Loctite applied to the plug. The result was an extra 500 rpm, or total of 16K. A 500 rpm gain and only 500 less compared to the stock plug. Not bad.
Notable was that my minimum RPM with the adapter, went down to 2.5K. Rock solid idle, acceleration very good. The adapter with a conventional plug reduces top end AND bottom end.
Conclusion is, that the threads of a conventional plug DO interfere, somehow, with combustion and with power output. Nelson discovered this and invented a new type of plug.