I used to drive around in a 1986 300ZX. Outside was totally stock and if you knew the car, it presented as a non-turbo. Condition was about 90%. Almost nothing about the exterior indicated any type of modifications.
There was an endless stream of ricers who would appear next to me at traffic lights revving their squirrel cages and jeering out the window. The car was a magnet for these guys.
What you couldn't tell was that engine was completely hand-rebuilt from a bare block, turboed, boosted, intake, exhaust (not flashy exhaust, just performance), aggressive clutch. Suspension was all redone with all urethane bushings, stiffer springs with a 1" drop, and 5-point adjustable shocks that could go from basically stock to ultra-tight. Brakes had been upgraded as well. The thing accelerated in any direction with a fierceness.
My favorite trick was to switch the ignition off while rolling, and just let the engine spin with no fuel or fire, powered by the car's momentum. At that point I would floor the throttle, which allowed the engine to pump enough air through the intake and out the exhaust that the turbo would actually spool up to about 1PSI even though nothing was being burned. Flip the ignition back on and the engine would kick in so hard that you might think you were just rear-ended by a bus!