Leave Carbon build up for higher compression or not?
 

Can carbon raise compression enough to raise mpg? Anyone notice gains after getting rid of carbon? Anyone notice a loss after switching to a carbon cleaning fuel like Shell?

Anything that takes up volume will increase compression to some extent.

The problem with carbon is, it tends to develop hot spots. Hot spots promote detonation, which will have an adverse effect on economy.

Carbon build up also disrupts flow within the chamber, and adds weight to moving parts, accelerating wear.

2¢

CJ

You don't want carbon build up. It'll eventually block up something that needs to be open and it'll cause problems.

Carbon is a bad thing regardless of what it does to compression.

Exactly.
Run a heavy water mist, or my favorite, ATF down the intake.
I used to put a nail hole in an ATF bottle cap and invert it down the carb of cars bought from little old ladies to clean the engine from their putting around.
Now I drive slower than they do.............
Warning, this blows a lot of nasty black gunk out of the tailpipe on older cars. I have a feeling it will trash a Catylitic (sp?) in no time. I suggest you disconnect the exhaust while doing this.
Just my $0.02

There is a certain amount of carbon build up on the top of the pistons that creates better combustion efficiency, purportedly. I've never experienced this, because I've never cared to test it.

The layer will be very thin and crystalline, though. It's not enough to even notice. In fact, it's the layer of carbon that most people work so hard to clean off when rebuilding an engine, when it doesn't, in fact, need to be cleaned.

You can use something like SeaFoam - but if the carbon build up is heat-tempered, it won't be taken off by a short-acting solvent or even water. Carbon forms a deposit that is basically hardened and non-porous, so short of being chipped away, not much will take it off quickly. The point of the water mist is to instantly cool the heated carbon build up, causing it to crack and shrink. The heat buildup and shock cause it to loosen, and it's ejected through the exhaust valve. This can plug up your catalytic converter, at best.

If you really want to clean your engine of carbon regularly, give it a good spirited acceleration onto the highway, which will raise cylinder pressures and combustion temps, causing the non-crystalline carbon to be blown from the cylinder.