It goes without saying that whiskey cars had to be fast. Big V8 engines were the norm, and drivers would tune them to be even faster, with superchargers and turbochargers, boring and stroking out the engines to get every last horse pulling. Most moonshiners drove Fords with flathead V8s, though a popular modification was stuffing a Ford coupe with a monster Cadillac engine -- the same engine Cadillac used to power ambulances [source: Hot Rod Magazine]. Chevrolet 350 engines were also popular, because they were easier to modify than the Ford engines (that's one of the reasons you see a lot of classic Fords today, particularly hot rod Fords, with Chevy engines).
Unlike in NASCAR racing, where cars are ostensibly kept close to stock, whiskey cars were modified to give them every possible advantage over law enforcement. That includes some modifications that may not give the car more power or speed, but were sure to throw the cops off the trail.
Some drivers added switches to the dashboard that would cut the brake lights. When they were being chased, the police wouldn't know when they were braking -- so the cops could be lured into taking turns too fast, or running up on the rear of the whiskey car. Suspension modifications were also key. Not only did they help the car drive better, but beefy shocks kept the car from sagging when loaded. Instead of seeing a car obviously carrying a heavy cargo of liquor, police would just see a stock car going down the road.
There were other cargo modifications. Hidden panels in the trunk, seats and doors allowed illegal cargo to ride unseen, even if the car was pulled over and searched. But, law enforcement officers wised to those tricks, so outrunning them became even more important. That's where the skills of the drivers came into play.