Anatomy of a Pro Modified
Below is the anatomy of a typcial Pro Modified, however all cars vary.
Pro Mod cars are permitted to run either a nitrous assisted engine running on unleaded race fuel, or a supercharged or turbocharged engine running on methanol. The complete car including driver has to weigh a maximum of 2350lbs for cars with a nitrous engine or 2650lbs for a supercharged or turbocharged engine. Nitrous or blown motors cost approx £50,000 from a recognised engine builder. Connecting rods last from 25 runs on a blown car to 50 runs in a nitrous car and cost around £1000. Valve springs are consumable items and individual pistons and rings need changing on a regular basis. A blown car would change spark plugs either every run if they wanted to ‘read’ the plugs or every 2 or 3 runs if not.
Almost all Pro Mod cars have a data logger on board which monitors and records information from sensors all over the car. The data is used to make the fine clutch adjustments which are essential to get the car to run fast. Other sensors monitor exhaust gas temperatures from each cylinder, cylinder head temperatures, fuel flow, throttle position and rear suspension travel. A data logger would cost from £3000 (to monitor basics for tuning upwards.
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 down from speeds of up to 240mph. The bare chassis weighs only 200-350lbs (90-159kg). A chassis will range from about £7500.
Pro Mod body shells are highly modified replicas of original car bodies built in composite materials. The bodies may have been roof chopped, sectioned, stretched in the wheel base, scaled from the original or otherwise modified to give better aerodynamics and accommodate the huge rear tyres. The maximum front overhang is 4.5” (114cm). A rear wing is added to give down-force at the top end of the track. A central driving position is not permitted. Depending on what style, estimate £8000 upwards with many being lightweight carbon fibre.
Nitrous motors use an unleaded high octane race fuel or methanol, the blown and turbo charged cars use methanol. The fuel system on a nitrous car can flow up to 8.5 gallons per minute and the blown car fuel system can flow over 15 gallons per minute. In a blown car methanol costs around £1.70 per litre and each run would use up to 25 litres including the burnout. In a nitrous car unleaded race fuel costs around £6 per litre and each run would use approx 5 litres as well as 6lbs of nitrous oxide at £6 per pound.
The cubic capacity of nitrous engines is unrestricted and can range to over 900 cubic inches using either carburettors of EFI and muti-stage nitrous systems to make power. Supercharged combinations are allowed a maximum of 527 cubic inches and must use a 14/71 roots style supercharger running no more than 20% faster than crank speed. Turbo charged engines can run a maximum of 540 cubic inches with a maximum of two 91mm turbos. 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 are producing in the region of 3 horsepower per cubic inch and blown motors 5.31 horsepower per cubic inch so the difference in engine size, minimum car weight etc. are designed to redress this balance so that both combinations can be competitive.
Pro Mod cars can either use a multi disc clutch or a torque convertor drive unit (which uses a 'transbrake for launching). All cars use a clutchless style gearbox using either a planetary style design such as a Lenco or B&J or in the case of some nitrous cars a twin layshaft design such as a Liberty or G force. 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 transmission and clutch set up would cost in the region of £8000 and a titanium bellhousing to safely contain any clutch explosions would cost around £3500.
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 used the same suits, gloves and boots as Top Fuel drivers. Helmets and the Hans device as worn by Formula 1 drivers are now also mandatory in drag racing. A 5 point, 3” wide driver harness must be used in all cars. All cars have to use a shatterproof bell-housing to protect the driver in the event of a clutch failure, a ballistic blanket around the transmission and an engine ‘diaper’ to contain any spilt oil.
Cars are required to run with a damper and spring on each corner of the car. Double or triple adjustable coilover shock absorbers are commonly used as well as electronic control of the damping.
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 tyre. Rear tyre pressures are from 4.5psi and front tyres run up to 35 psi. The rear tyres are designed to ‘crinkle’ to assist the launch and will ‘grow’ by approx 4” in height during the burnout and at the top end of the track. A pair of rear wheels are in the region of £2800 and a pair of slicks about £650 – we’d need two or three pairs of slicks in a season.
The cars are equipped with after market 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 cars down from speeds of up to 240mph. 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.
Information & updates: Andy Robinson Race Cars. Photograph: Roger Gorringe.
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