Hmmm.... Never had to change the percentage of nitro for any of my engines to run fine in colder weather. Guess I'm just lucky.

Yep, this is an urban myth. The idea behind it is only for ABx engines and goes like this. When the engine gets a bit worn the piston to liner clearance gets larger. Because the liner in this type of engine has a taper the assumption is made that less nitro gives a lower engine temp so the liner doesn't expand as much at the top so the piston then fits better to regain compression.

But the fact is that IF there's any difference in running temps between say 5% and 10% nitro then the clearances might change on the order of millionths of an inch. The thing that actually gives the final seal against compression is the oil film between the liner and piston. ABx type engines really aren't as critical as some people think.

Since the oil film is a little thicker than 1/2 thousands, I think change in size, due to heat, is much more than a millionth of an inch. I don't have my inside micrometers, so it is a little hard for me to check the exact size and changes. But, it made sense when I read it. I haven't actually had a chance to try it!

Have you actually tried it? If you haven't, dismissing it as an "urban myth" may be a little premature

Some people will never admit they are wrong, lol. When they realize they have been speaking from their rearside they change the subject,lol. .


Wings

O.K.
Here goes.... I have always UPPED the nitro in my really worn oldies, though I don't have any long-hair sci-en-tif-ic study to back it up, they start easier. Can't claim a power increase,- like I said these are pretty whooped by this time. Most were abc, plain bearing, .25-.40 size. Ever ran a couple on 25% car fuel, and they went from almost ready to die to lively engines for several dozen more flights. I have only "broken" one engine, and it was a .19 Veco, and that was due to running LOWER nitro. It had been running on my standard mix of 15% with 4 oz. of castor, and 2 drops of Armor-all. We ran out, and a guy had some 0% in his car, so we filled it up with that, and though I can't give you a real sci-en-tif-ic explanation as to what happened, it broke the CONNECTING ROD right about in the middle. (too cool, started to seize, and fatigued the rod?)

Jetts