The carbon build up can happen on the cylinder head and on top of the pistons.
Does Carbon Alter Combustion in Other Ways?
Absolutely, in addition to causing poor combustion, excessive build up can also alter the vehicle’s compression
ratio. The carbon fills up spaces around the piston rings, head gasket, and spark plugs. This decreases the amount of space
that is available in the combustion chamber. This increases the compression, which can cause the engine to overheat, ping
(detonate), and also causes an increase in NOX emissions.
Check this link: http://www.bearriverconverters.com/data ... etter5.pdf
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