Rules for emphasizing by lengthening sounds


Sometimes, sounds are lengthened for emphasis. For example, see ["とっても versus
とても"](https://japanese.stackexchange.com/questions/808/%E3%81%A8%E3%81%A3%E3%81%A6%E3%82%82-versus-%E3%81%A8%E3%81%A6%E3%82%82).

What are the rules governing this process?

  * Are there restrictions on where lengthening can be inserted?  
とっても or とてえも or とても～?  
すんごい or すご～い or すごいー?

  * Is it predictable which sound is inserted?  
すんごい rather than す～ごい or すっごい?

I've given examples, but I'm hoping to learn if any general rules exist.




Maybe this is an overgeneralization, but I find this often has to do with
pitch accent. Example:

とても【LHH】

とっても【LLHH】

すごい【LHL】

すんごい【LLHL】

It seems as if inserted sounds, if they exist, all appear on low to high pitch
transitions. Obviously most words do not have these sounds inserted, and often
only very common interjections would use them.

As for why ん is used rather than a っ, try pronouncing すっごい. It's pretty
difficult, and since intervocalic "g" is allophonic with "ng" which sounds
like ん, it is natural for すっごい to turn into すんごい.

