* Engine: While the original movie makes reference to a 440 cubic-inch engine (a very common powerful option for the car), the actual engine used in filming is speculated to be highly modified, likely with increased displacement, forced induction (supercharger or turbocharger), performance enhancements to the heads, camshaft, and intake, possibly NOS (nitrous oxide system) for extra bursts of power, and upgraded internals to handle the increased stress.
* Transmission: The transmission was almost certainly upgraded to a high-performance unit capable of handling the increased power output. A modified automatic or a manual transmission with reinforced components would be expected.
* Suspension: A heavily modified suspension system with upgraded shocks, springs, and possibly aftermarket control arms would be present to handle the car's increased power and aggressive driving style. Lowering springs or coilovers would also be expected.
* Brakes: Upgraded brakes, likely disc brakes all around (possibly aftermarket performance brakes) were critical for handling the increased speeds and power.
* Body: The car featured a custom paint job (the distinctive black and orange), aftermarket wheels, and several other visual modifications, many of them probably added post-filming for promotional purposes. These visual changes would not have affected the car's performance significantly.
* Interior: Although less obvious, the interior likely had some upgrades for safety reasons during filming (perhaps a roll cage), though these may have been removed for specific shots.
In short, while the base car was a 1970 Dodge Charger R/T, almost everything under the hood and concerning the suspension and brakes would have been significantly upgraded for filming purposes to enable the stunts and the car's on-screen performance. The precise details of these modifications remain somewhat vague, however.
