Depending on several factors like the bore size and the combustion chamber volume, a layer of carbon on a piston crown can raise the compression ratio a little. Rough or uneven buildup increases turbulence as the flame front travels across the combustion chamber, and if a glowing hot spot develops opposite the spark plug a second flame front can form, causing detonation when the two opposing flames meet in the middle.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!