I clean my exhaust and manifold when are warm with a little of methanol applied on a rag. Cleaning when warm makes wonders and removes all the dirt and grease without problems.
You can try with some Boiling water and vinegar. If don't work try with some Solution of water/ammoniac at 50% and let rest some hours. After this, clean with fresh water and all the dirt should be gone. Warning, depending of the maker of manifold, the Ammoniac can attack the finish.
[quote=MugenDrew;2684554]BATTERIES are for FLASH LIGHTS, gasoline is for cleanin parts, alcohol is for me to drink and well NITRO...everybody know thats for racing.[/quote] :D:D:D