Your aim when modding the engine should be a nice flat torque output.
Typical stage 3 mods often include: Engine balancing, Adding or upgrading forced induction (turbo/supercharger), Sports gearbox, Competition cam, Internal engine upgrades (pistons/head/valves). Typical stage 2 mods often include: fuel pump upgrades, Fast road cam, Ported and polished head, Power/Sport clutch, high flow fuel injector.
Typical stage 1 mods often include: Suspension upgrade (drop 30-40mm), Panel air filter, Lighter flywheel, Remap, Alloy wheels, Sports exhaust.
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