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Section 23 - MODIFIED TRACTORS (MOD)

Unless specifically outlined below in the class specific rules, the Tractor General Rules and the overall General rules outlined above will apply.

23.1 - General Rules

  1. All vehicles must be registered as a Modified Tractor to compete in the Modified Tractor class.

  2. Must have a minimum of one of the following combinations to compete at an OTTPA event:

    1. Two (2) supercharged V-8 engines.

    2. Turbocharged or supercharged industrial engine.

    3. A single turbine engine or combination of turbine engines with a minimum of 2850 horsepower.

  3. Modified Tractors will compete at a maximum weight of 7,400 lbs.

23.2 - Chassis

  1. Total length of the vehicle shall not exceed 14 feet from center of the rear axle to the furthest point forward on the vehicle.

  2. Minimum wheelbase is 138” inches.

  3. No engine sheet metal.

23.3 - Tires

  1. Tire size allowed is 30.5 x 32 maximum. See General Tractor Rules for dimensions.

  2. Tire size shall not exceed 31 inches in width.

23.4 - Engine Combinations

  1. Any engine combination not already mentioned in the rule book must be approved by the OTTPA Board.

  2. One (1) marine, or aircraft, or industrial type engine up to 2500 cubic inches with (12) cylinder limit. Two (2) compression stages maximum.

    1. Allow EFI on V-12 Allison and V-12 Packard.

    2. Rolls Royce Griffon limited to a maximum overdrive limit on the supercharger to the high-speed ratio of 7.7 to 1.

    3. Rolls Royce Griffon can have multiple turbos or the original supercharger, but not both.

  3. One (1) 60-degree Ford Tank V8 engine with multiple turbos

  4. One (1) 903 Cummins V8, maximum 650 CID with multiple turbochargers

  5. One (1) Chrysler Hemi automotive type engine with:

    1. Maximum of 550 cubic inches.

    2. Electronic fuel injection (EFI) is allowed.

    3. Multiple turbochargers two (2) compression stage maximum.

    4. Intercooler is allowed.

  6. Two (2) automotive type engines, with maximum 14-71 supercharger (no screw type), no overdrive limit, hemi type heads allowed. 650 cubic inch limit.

  7. Two (2) automotive type engines with maximum 2 turbos per engine, no Hemi, Pontiac, Oldsmobile, Big Chief or Big Duke style heads, 650 cubic inch limit per engine. 

    1. Turbos limited to 1 per bank, non-staged, 88mm compressor inducer & 103mm exhaust turbine exducer. Intake and exhaust wheels must protrude 1/8” into housings. 

    2. EFI is allowed.

  8. Two (2) 1710 CID Allison aircraft V12s, with a maximum overdrive limit on the stock Allison supercharger of Ø9.5” to a maximum speed ratio of 8.8:1. No turbochargers. EFI allowed.

  9. One (1) Allison aircraft V12 plus one (1) non-Hemi V8 automotive engine.

    1. The V8 can be a maximum of 650 cubic inches

    2. Maximum of a 14-71 roots supercharger, no overdrive limit.

    3. No turbos allowed on either motor.

    4. Maximum on the Allison supercharger is Ø9.5” and is limited to max overdrive limit of 8.8:1 with stock wheel and 8.1:1 if billet wheel.

  10. Three (3) automotive type engines with a maximum 8-71 supercharger limited to 43% overdrive, no Hemi, Pontiac, Oldsmobile, Big Chief or Big Duke style heads, 650 cubic inch limit.

  11. Three (3) automotive type engines with a maximum 2.300” intake valve, a maximum 8-71 supercharger limited to 60% overdrive, no Hemi, Pontiac, Oldsmobile, Big Chief or Big Duke style heads, 650 cubic inch limit.

  12. Three (3) automotive V8 Chevy LS engines with turbochargers. Maximum of 375 cubic inch each. No billet or canted valve cylinder heads.

    1. EFI is allowed.

    2. Turbos limited to 1 per bank, non-staged. Maximum turbo size 76 mm inducer compressor side and 76 mm exducer turbine side.

  13. Three (3) Big Block Ford engines with a maximum 2.350” intake valve in A-460 heads, limited to 60% overdrive with a maximum 8-71 supercharger, 650 cubic inch limit.

  14. Four (4) naturally aspirated automotive type V8 engines, any CID, any two (2) valves per head, carburetors, or fuel injection.

  15. All boosted automotive type V8 motors limited to a maximum 5” bore spacing.

  16. Turbine engine, or combinations of turbine engines, are allowed from the T53, T55 & JFTD families, see list.

    1. Combinations with a total military HP rating of 3750 or less are allowed water/methanol injection. Nothing allowed to be sprayed or injected in the airstream of the intake or turbine engines that total more than 3750 military horsepower.

    2. A combination consisting of qty two (2) turbine engines from the T55 family as long as the combined actual dyno HP is not more than 6800 horsepower total. This is to be set, and tamper proof lead sealed, on the Nebraska Gas Turbine dyno with an OTTPA tech official present. One (1) turn of the fuel screw is equal to 250 HP per turbine.

  17. All turbine engines need to have an over-speed safety device set to a maximum of 110% N2 RPM.

  18. OTTPA recognizes the following turbine engines for use in OTTPA competition, and their horsepower ratings are as follows: (No turbine engines other than those listed may be used.)

Recognized Turbine Engines
Lycoming Pratt & Whitney
T53 L1 = 825 hp JFTD12-4 = 3430 hp
T53 L3 = 920 hp JFTD12-5 = 4050 hp
T53 L5 = 960 hp JFTD12A = 4050 hp
T53 L7 = 1100 hp
T53 L11 = 1100 hp
T53 L13 = 1400 hp
T55 L1 = 1600 hp
T55 L5 = 2200 hp
T55 L7 = 2650 hp
T55 L7C = 2850 hp
T55 L9 = 3750 hp
T55 L11D = 3750 hp
T55 L712 = 3750 hp

23.5 - Transmissions / Drivelines

  1. All automatic transmissions must have six (6) strap approved blankets and have reverse lockout.

  2. All drivelines must be entirely enclosed in 5/16-inch steel or 3/8-inch aluminum and securely fastened to the frame.