Z24i Engines Only
These engines have two spark plugs per cylinder. This arrangement allows the engine to burn large amounts of recirculated exhaust gases without affecting performance. In fact, the system works so well it improves gas mileage under most circumstances.
Both spark plugs fire simultaneously, which substantially shortens the time required to burn the air/fuel mixture when exhaust gases are not being recirculated. When gases are being recirculated, the dual spark plug system brings air/fuel ignition up to the level of a single plug system, without the burden of EGR gases.
The only adjustments necessary are the tune-up and maintenance procedures outlined in Chapers 1 and 2.