Pro Modified Cars are 400km/h doorslammers, running on petrol or methanol. They are the fastest class of drag cars with working doors; only rail Dragsters and Funny Cars are quicker.
Pro Mods are purpose-built race cars cloaked in replica street car bodies ranging in style from the 1930s to the present day.
Anatomy of a Pro Modified
Below is the anatomy of a typical Pro Mod, though cars do vary in detail.
Pro Mod cars are permitted either a nitrous oxide-injected petrol motor burning unleaded race fuel or a super- turbocharged or engine running on methanol. No minimum weight for nitrous-injected cars but 1204kg for super- and turbocharge cars. Connecting rods last from 25 runs on a blown car to 100 runs on a turbocharged car. Valve springs are consumable items and individual pistons and rings need changing on a regular basis. A blown car would have it’s spark plugs changed after every run if the tuner wanted to ’read’ the plugs or after every 2 or 3 runs if not.
Chassis are custom fabricated from 4130 chrome moly tubing according to strict rules. The maximum permitted wheelbase is 115″ (2.92m). Features of the chassis include twin chassis rails, strut-style front suspension, carbon or aluminium interior panelling, aluminium motor-mounting plates, coil-over rear suspension and 4-link or swing arm-mounted axles. The car has front and rear brakes but relies primarily on twin parachutes to slow it from speeds of up to 400km/h.
Pro Mod body shells are highly modified replicas of original car bodies built from composite materials. The bodies may have been roof- chopped, sectioned, stretched in the wheel base, scaled down from the original or otherwise modified to give better aerodynamics and accommodate the huge rear tyres. The maximum front overhang is 114cm. A rear wing is added to give down-force at the top end of the track. A central driving position is not permitted.
Nitrous motors use an unleaded high octane race fuel. The blown cars use methanol. The fuel system on a nitrous car can flow up to 35 litres per minute and the blown car fuel system can flow over 80 litres per minute. A blown car would use up to 25 litres of methanol including the burnout. A nitrous car would use approximately 5 litres unleaded race fuel, as well as 3kg of nitrous oxide.
Engine capacities are restricted to a maximum of 920 cubic inches (15 litres) for nitrous-assisted motors, 526 cubic inches (8.61 litres) for supercharged methanol engines and 540 cubic inches (8.85 litres) for turbocharged engines. In addition, the blown entries are restricted to a maximum valve size of 1.9″ exhaust and 2.4″ inlet and are restricted to 20% overdrive when using the 14-71 high helix blower. Nitrous motors produce in the region of 3 horsepower per cubic inch so the differences in engine size, minimum car weight, etc., are designed to redress this balance so that all combinations can be competitive.
Pro Mod cars use a multi-disc clutch or a billet converter. Nitrous cars can use a maximum of 4 discs and blown cars 3. Most cars use a planetary-design transmission, commonly the Lenco or B&J. Some cars have also started to experiment with a twin lay shaft transmission such as a Liberty . Nitrous cars generally use a 4-speed transmission and blown cars use a 3-speed. Blown cars are permitted a maximum rear axle gear ratio of 4.56:1. A titanium bell-housing constructed to contain clutch explosions.
6. Driver Safety Equipment
A fire-resistant suit including gloves and shoes must be worn by the drivers of nitrous cars. The drivers of blown cars use the same suits, gloves and boots as Top Fuel drivers. Helmets and a 360-degree neck brace must be worn by all drivers. The Hans device, as worn by Formula 1 drivers, is also permitted and is gaining in popularity. A 5- point, 3″-wide driver harness must always be fitted. All cars are equipped with a shatterproof bell-housing and a transmission blanket to protect the driver in the event of a clutch or transmission failure and an engine ’diaper’ to contain any spilled oil.
Cars are required to have production-style suspension systems. Most cars use purpose-built, McPherson-style struts with rod-ended control arms at the front and 4-link rear suspensions which allow an almost infinite range of adjustments to compensate for track conditions. Coil-over shock absorbers are used at the rear along with anti-roll bars.
8. Wheels and Tyres
Rear wheels are 16″ in diameter and 16″ wide and have to be ’beadlocks’, i.e. have clamping rings to secure the tyres or inner liner tube. Rear tyre pressures are from 4.5psi and front tyres run up to 34psi. The tyres are designed to ’crinkle’ to assist the launch and will ’grow’ in height by approximately 4″ during the burnout and at the top end of the track. Two or three pairs of slicks are needed in a season.
Pro Mods are equipped with aftermarket disc brakes on all four wheels with some cars using carbon fibre discs and pads. The brakes are used in conjunction with dual parachutes to slow the car from speeds of up to 400km/h. Brakes are generally single-pot on the front and 4-pot on the rear with the system designed to rear-bias through the large rear wheels and tyres.
Most Pro Mods carry an on-board data logger which monitors and records information from sensors located all over the car. Data is used, for example, to make the fine clutch adjustments essential to maintain the car at peak performance. Other sensors monitor exhaust gas temperatures, fuel flow, throttle position and rear suspension travel.