Modified Rules

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Minimum of a neck brace is required in all classes at any time you are on the track

  1. Hoosier F70
  2. Any 8” steel wheel
  3. Maximum tread width not to exceed 78” outside edge to outside edge of tire, zero tolerance.
  4. Minimum wheel base 108”
  1. Any 2 barrel or 4 barrel, no dual carb engines.
  2. No fuel injection or electric fuel pumps.
  3. Made for racing fuel cell is required and will remain within the confines of the original frame.  Must be mounted with minimum of two 2” straps.  Minimum of 4” in front of rear bumper mounted securely behind rear axle. No part of the fuel cell will be lower than the protective tubing. Must have check valve in place and fuel cap must be attached with tether or safety strap.
  4. Gasoline and Alcohol are the only approved fuels, no fuel enhancing products of any kind.
  5. If alcohol is used the letter “A” minimum 6” tall is required on the sail panel and Back panel in contrasting colors.
  1. Mufflers are mandatory, 100 decibel level max any car measured above maximum decibel will not compete until that cars decibel is reduced to or below maximum level.
  2. No part of any exhaust system will pass through the drivers compartment.
  1. Any American manufactured steel v-8 engine block
  2. Engine block must be mounted at least 72” forward of the center line of rear axle.
  3. No motor mount casting alterations.
  4. No dry sump systems allowed, no external oil tanks, single stage external pumps are allowed with use of oil pan on the engine.
  5. Minimum engine height of 11” from front center of crankshaft to the ground.
  6. No Magnetos.
  7. Aluminum heads are allowed.
  8. Engine offset must be kept within 2” of center line of front cross member.
  1. Automatic transmission, 3 speed, 4 speed are allowed. No 5 speed transmissions allowed. No in and out boxes allowed.
  2. All clutched must be inside scatter proof bell housing with a reach of 270 degrees around flex plate or flywheel.
  3. Automatic transmissions must remain in OEM stock case.
  4. Transmission must have one forward gear and reverse, car must start and move on its own.
  1. No anti-lock brake system of any kind.
  2. All four wheels must lock-up upon inspection.
  3. No brake shut-off devices allowed.
  4. Front to rear proportioning valve only.
  5. Brake rotors will not be lightened or drilled to reduce rotating mass, rotors may be modified to receive larger studs or change bolt pattern only.
  6. Calipers cannot be lightened and must remain O.E.M.
  7. No billet components, stock type rotor only.
  1. Any passenger car type rear end may be used, no aluminum allowed except lowering blocks, axle caps, hubs, drive plate and spool.
  2. Quick change rear ends are allowed, steel axles and axle tubes only.
  3. No cambered rear end, one piece drive flange only.
  4. Steel driveshaft’s only (no aluminum or composite materials).
  5. Driveshaft hoop is required and must be mounted no more than 6” back from front of driveshaft, must be constructed of .25” by 2” steel minimum.
  6. Driveshaft must be painted white.
  1. No weight jacks of any kind.
  2. No adjustment to chassis components within reach of driver anywhere on car.
  3. If using a bird-cage type rear end, Chassis must have a minimum 0.0625” steel plate behind the seat from the top bar to the bottom, including 3” beyond left and right sides of drivers seat.
  4. No rack and pinion steering system allowed, O.E.M. steering box will be mounted in original stock position, using O.E.M. bolt holes for type of frame being used.
  5. No aluminum “A” frames, hubs, calipers or spindles are allowed.
  6. The lower “A” frame cannot be adjusted and must remain in stock location using bolt holes, no bushings of any kind can be used at the lower “A” frame O.E.M. mounting points.
  7. If tubular lower “A” arms are used they must meet exact original dimension’s, mono/steel bushings allowed in place of rubber bushing.
  8. One shock per wheel.
  9. Aftermarket ball joints are allowed.
  10. No externally rate adjustable shocks allowed.
  11. Front coil overs are not allowed, Shocks cannot exceed $ 300.00 racers cost.
  12. 4” minimum spring diameter, all springs must be steel, no torsion bars, allowed in rear.
  13. No fabricated spindles, no made for racing spindles.
  14. Front suspension and steering must be steel, unaltered and in stock location.
  1. Factory productions 1970 and newer American passenger car parallel frames only
  2. Frame rails must be able to support roll cage on both sides ( full and complete)
  3. Driver’s seat and all driver controls must remain in the confines of the factory car frame.
  4. Frame and body will have a 4” minimum ground ride height with driver and equipment in the car (front cross members is the exception)
  5. Howe O.E.M. fabricated front clip will be allowed, with 60 lb. add on frame rail weight/mounted in designated area 30 lbs. per side
  1. Must consist of continuous hoops not less than 1.666” outside diameter and must have a wall thickness of at least 0.95”.
  2. Must be frame mounted in a minimum of 6 places, roll bar padding required on any bar entering or around the driver compartment.
  3. Must consist of a configuration of front and rear hoops connected by tubing on the sides or side hoops, drivers head must not protrude outside or above cage with helmet on strapped in drivers seat.
  4. Must have a minimum of one cross bar in the halo at the top of the roll cage, roll cage must be securely supported and braced, Driver must be able to exit through the top of the car.
  5. Low carbon mild steel tubing is recommended.
  6. Main cage can be no further forward than engine plate.
  7. Protection of feet is mandatory, a bar across the back of the engine with vertical bars and rub rails or similar protection.
  8. Brace bars forward of cage will not be higher that the stock hood height.
  9. A minimum of 3 driver side door bars, 1.5” od must be parallel with the ground as possible, and located perpendicular to the driver to provide maximum driver protection, side bars must be welded to the front and rear cage members.
  10. Driver side door bars must be filled, no openings exposing the driver will be allowed, from upright to upright and frame rail to top door bar.
  11. Bumpers will be used on both front and rear, bumpers will be mounted frame end to frame end of rails.
  1. Measured 4.75” high x 56” wide = 264 square inches.
  2. Measured 4.00” high x 66” wide = 264 square inches.
  3. Spoiler will be clear lexan only and must be centered and must not extend beyond bumper and quarter panel.
  4. Cars exceeding 4” body rake will reduce spoiler height proportionally for every additional fraction of an inch of body rake over 4 inches.
  1. 1970 or newer American compact passenger car only, no panel vans or station wagons, stock appearing front windshield and rear window supports. Painted roll bars are not an acceptable substitute. Half windshield may be used and must be flat with no wings mounted on roll cage. Must have minimum of three windshield bars in front of driver.
  2. Fire wall and floor boards are mandatory, it is recommended that a .09” plate be installed next to the driver’s right leg for added protection from the drive shaft. All complete floor boards and driver side firewall will be constructed of no less than .125” aluminum or 16 gauge/.060” steel.
  3. All body parts should be recognizable of factory production vehicle.
  4. Engine compartment will remain open. No side panels, hood panels may have a 4” drop and must be enclosed at the rear of hood.
  5. Maximum hood scoop height is 6”.
  6. Bodies will extend no further forward then the rear of the engine block.
  7. Rear of the body will be a solid panel measured at least 8” high and 90 degrees to the
    ground and flush with the rear quarters.
  8. No panel in front of the right door to engine compartment.
  9. No inner panels.
  10. Driver and passenger window openings must be at minimum 12”X 18”.
  11. No wings, vanes, or any other type of ground effects are allowed outside, inside or under the car.
  12. Body must be same width from front to rear and parallel to frame.
  13. Nose section of car 4” ride height.
  14. No radios or mirrors of any kind.


Plymouth Motor Speedway reserves the right to change, delete or add to rules throughout the racing season. The management reserves the right of interpretation of all rules and the right to inspect and reject any car and/or driver, crew member, etc. during the racing program.