Methylcyclopentadienyl manganese tricarbonyl (MMT) is an organometallic compound blended into gasoline to raise its anti-knock index. It serves as a replacement for tetraethyl lead, preventing detonation in high-compression engines. While effective at boosting octane, its combustion leaves behind a reddish-orange manganese oxide ash that can coat spark plugs and eventually foul catalytic converters.
The plugs have a weird orange tint to them, probably from running octane booster with MMT.
If you pull a spark plug and see a rusty orange deposit on the ceramic insulator, it is a telltale sign that the fuel or octane booster contained MMT.
Related terms
Anti-knock index
The standard numerical rating of a fuel's resistance to premature ignition.
Knock
An abnormal and damaging combustion event where the air-fuel mixture self-ignites prematurely.
Detonation
An uncontrolled secondary explosion in the combustion chamber that causes severe engine damage.
Catalytic converter
An exhaust emission control device that reduces toxic gases and pollutants into less harmful compounds.