Changing shim stack vs. changing glowplug heat range?
 

Thus far, the approach I've taken to tuning my nitro engines has been to always use medium glowplugs (except for one tiny .08 engine that requires a hot plug), and then raise the shim stack if I have to tune the carb so rich the engine is choking just to avoid overheating, or lower the shim stack if I can tune the carb to the point it leans-out on speed runs *without* overheating. I do this because it lets me keep just a single type of glowplug in my spare-parts bin.

This approach seems to work fine, but I'm curious, is there any scenario in which the ONLY solution is to install a hotter or colder glowplug? Likewise, are there any tradeoffs from raising or lowering the shim stack that are so significant I'd be better off changing the glowplug instead?

I suppose I should mention that I'm assuming spring/summer/autumn weather conditions, say, between 55-90°F. If I were going to run in the winter I'd use hot plugs regardless, to keep the ignition timing from becoming too delayed by the cold air. What I'm curious about is whether there's any *other* scenario in which the only solution is to install a hotter or colder glowplug, instead of changing the shim-stack

If you're having issues where changing the plug or shim stack to keep from over heating when leaned out, chances are you are already over lean. Shim stack should be adjusted to the nitro content in the fuel you use. What you are describing in your scenarios sounds more like an air leak than anything else. A hotter plug will let you run the engine richer and keep the performance. A colder plug will do the opposite, it will require a more lean setting to get it to clear out.

You still use the same wire and did change compression. While changing the plug keeps the compression and changes the wire

OK, in both ways you wiill change the ignition timing but not the heat correction.