As for when (what engine speed) it is practical to use the Colortune plugs,
I think I have a better answer.

From what I remember of reading their literature years (a couple decades?)
ago, Colortune can only be used at idle speed. At higher engine speeds the
glass insulator cannot withstand the vibrations created by the engine, thus
it can only be used at idle. There is a reason that spark plug manufacturers
use the materials that they use.

Cheers,
Bruce
motorbruce
motorbruce@hotmail.com

>From: Al Brase <albeeee@MCHSI.COM>
>Reply-To: Al Brase <albeeee@MCHSI.COM>
>To: vanagon@GERRY.VANAGON.COM
>Subject: Re: "enough" vs "too much" monitoring,              was: permanent
>fuel pressure gauge
>Date: Tue, 7 Mar 2006 12:45:48 -0500
>
>Funny, that cylinder monitoring thing was called "Colortune". I have one.
>It
>consisted of a spark plug with clear glass insulator which conducted the
>light of combustion out of the cylinder. An a small mirror to see it
>better.
>Not surprisingly, it is not ver much different than the flame color in a
>gas
>furnace. Blue-white is lean and yellow orange is rich.
>The only practical use of such a thing is to set the mixture at idle. It
>would be pretty diffficult to put one's face down by the spark plug while
>the engine was under full load.
>(Well, it was the 60's and it WAS made in Britain!) But still kinda neat.
>Al Brase