Short versus Long Glow Plugs

[TGL]

Since a number of engines use a long glow plug (Traxxas for example) - what is the net effect of using a shorter glow plug in this type of engine and does it impact performance - and if so how/why?
How does it compare to changing the shim?

Installed - the shorter length glow plug won't reach the top of the compression dome - which to me means that it would be decreasing the compression ratio.
Does that recession (not reaching the top of the dome) impact fuel detonation since the fuel mixture has to be crammed up into that recessed area to reach the hot filament?

The difference between a long and a short plug is about 3/4+/- of a mm (.75mm).
Traxxas head shims come in three sizes .15mm, .25mm, and .35mm - so the net difference between the thickest and thinnest shim is .2mm.
I realize that the gap with the shim is spread out over the whole area of the cylinder versus being limited to the 1mm x .75mm hole in the top of the dome, but can anyone offer further information/perspective?

I've run a short plug in several engines (Traxxas and Picco for example) and while I haven't done a back to back (long to short) comparison I've been wondering about the net impact.

 

[StevoRevo]

I'm waiting on short glow plugs to come in the mail..
shld be here by thurs..
ill compare the long vs short and let u know cause I'm curious on this topic too..
Macguyver from this site said his Revo3.3 run better with the short plugs but dont have an explination as to why...
i hope some1 can explain the difference and the science behind it

 

[alien13]

I've broken in all my motors with the plugs that came in them, and switched to short plugs after the originals went bad, haven't had any issues, haven't seen much difference either.

 

[RC Addict]

We did some experimenting years ago on this subject in 1:1 rigs. The results were deceiving. While the longer plugs did provide a spark farther down in the combustion chamber there was little to no difference in actual fuel ignition nor in combustion. The major gains were achieved by going to hotter plugs. Once you reach the threshold though, the performance gains drop back off. You can go too hot...

I have seen the same hold true for RC and doubt that you will see much difference either.

 

[moe7404]

i did a LOT of experimentation on 1:1 small v8 with different plugs. i found out that a short plug was the same as turning the timming back, but by only 3-4 degrees. i tryed lots of different plugs did not find much different, except hot plugs caused detonation. so my idea is a short glow plug will drop the timming back, but iam not sure it would be much. just my idea,

 

[Scrogg]

You are correct. On RTR millls, using a short plug won't affect much. Once you get in to the high performance mills, using the wrong plug can make or break it. There is no good reason to run shorter plugs unles the manufactuer calls for it.

 

[moe7404]

double thumbs up to Scrogg.