The NHRA has strict guidelines for funny cars. Most of the rules relate to the engine. In short, the engines can only be V-8s at 90 degrees. (The most popular design is loosely based on the Chrysler 426 Hemi.) There can only be two valves per cylinder and superchargers are restricted to a basic Roots type—19 inch (483 mm) rotor case length with a width of 11.25 inches (286 mm). The engine displacement cannot exceed 500 cubic inches (~8.2 L). Only single camshafts are allowed. There are two common bore-stroke combinations: 4.1875 × 4.50 inches (106.4 × 114.3 mm) (called a 3/4 stroker) and 4.25 × 4.375 inches (108.0 × 111.1 mm) (called a 5/8 stroker). The 3/4 stroker is the most common combination used today and equals 496 CID (8.1 L).
Engine blocks are usually made out of aluminum billet. Crankshafts are made from steel forgings. Intake valves are titanium and of 2.40 inch (61 mm) diameter, while exhaust valves are 1.90 inch (48 mm) diameter made out of Inconel. Every funny car has ballistic blankets covering specific parts of the engine prone to explosion.
Funny car fuel systems are key to their immense power. Most systems will deliver from 500 to 600 psi (3 to 4 MPa) of fuel pressure at as much as 75 U.S. gallons per minute (62 imperial gallons/min; 5 L/s). During a single run (starting, burnout, backing up, staging, 1/4 mile) cars can burn as much as 15 U.S. gallons (12 imp gal/57 L) of fuel. The fuel mixture is usually 85–90% nitromethane with 10–15% methanol. The ratio of fuel to air can be as high as 1:1. Compression ratios vary from 6:1 to 7:1. The engines in funny cars commonly exhibit varying piston heights and ratios that are determined by the piston's proximity to the air intake. Funny cars have a fixed gear ratio of 3.20:1 and have a reversing gear,power is transmitted from engine to final drive through a multiple staged clutch which provides progressive incremental lockup as the run proceeds. The rate/degree of lockup is mechanically/pneumatically controlled and pre-set before each run according to various conditions, in particular track surface . Wheelbase is between 100 and 125 inches (2.54 and 3.18 m). The car must maintain a 3 inch (76 mm) ground clearance.
Horsepower claims vary widely—from 6,978–8,897—but are probably in the 8,000 BHP range. Supercharged, nitromethane-fueled motors of this type derive their extremely high speeds from their torque, which is estimated at about 7,000 ft·lbf (9,500 N·m). They routinely achieve a 6G acceleration from a standing start.
