Originally Posted by Ledet
What do you think makes the difference in performance between the head plug and the adapter for the standard glow plug? Is it compression?
Boy, this discussion can really open a can of worms.
Compression certainly is a factor, but I believe the causes of performance differences are far more complex. The classic argument is tied to differences in compression and is based on how the plugs seal the combustion chamber. Three different sealing approaches are used. (1) The standard plug uses a compression washer located at the top of the plug thread to seal against a seat at the top of the head. (2) Turbo plug styles (Turbo and NELSON) seal at the bottom of the plug using a tapered plug face against a tapered seat in the head and (3) Flanged plug inserts seal using the head gasket (commonly a thin copper gasket) between the plug flange and a stepped shoulder on the cylinder.
Folks tend to point to the fact that standard plugs suffer compression loss due to the volume loss in the threads because the plug seals at the top. I don't think this is the cause of performance loss simply because if you coat the standard plug threads with pure castor before inserting the plug, any compressible volume due to thread mismatch is filled with a non-compressible oil, consequently, you won't lose compression due to differences in thread tolerances.
So, what causes the decreased performance? Compression probably does come into play, but it is more likely tied to filament hole size, the volume lost to combustion chamber shape and how far the standard plug threads into the adapter. I'm more inclined to believe the shape of the chamber is the primary contributing factor rather than compression. Turbo styles and flanged plugs have a very smooth internal head shape while standard plug adapters may or may not have a rough hole where the base of the standard plug terminates. The Galbreath head/NELSON plug combo probably runs well due to the design of the chamber rather than just compression differences. We also have more options in modifying compression with the turbo and flange designs -- if over compressed, we can add gaskets. For the standard plugs, if compression starts low, about the only means of raising it is to search for another plug vendor or to use a medium heat plug to reduce filament hole size.
Head design on larger engines is a science unto itself ---- for the 1cc engines, I believe a smoother chamber appears to be more important than compression in how well they run. That's ma story and ah'm stickin' to it. (Flame suit on, BTW