Hobby Rules for the Nationals:
ENGINE SPECIFICATIONS
361 cubic inches (GM); 363 (Ford); 370 (Chrysler)
Maximum compression ratio is 9.0 to 1, no tolerance
No GM Bowtie, Ford SVO or Chrysler W components allowed
No porting, polishing or alterations of any kind to heads or intake, no beehive valve springs allowed. INTAKE: Must use unaltered OEM cast iron two barrel intake. No hi-rise or marine intake manifolds.
Compression ratio checked using Whistler and cubic inches checked using pump.
ELECTRONICS:
No ignition control boxes. OEM ignition only. Ford/Chrysler may use HEI distributor. No billet housings or crank triggers. All ignition rotors, caps, coils and modules must remain OEM-appearing. No alternators. No electronic traction control devices
EXHAUST:
OEM cast iron exhaust manifolds only. No center dump type manifolds. Exhaust must extend past firewall and turn towards ground. Must remain dual exhaust, no crossover or ‘Y’ pipes. No exhaust sensors.
FUEL SYSTEM:
Unaltered OEM two barrel carburetor for that engine, except: booster I.D. may be machined to 0.25 inch, venturi I.D. machined to 1.375 inch and throttle bore I.D machined to 1.6875 inch on Rochester carburetor. Must remove carburetor choke plate. No carburetor spacers. No top flow air cleaner housing, Gasoline only. Racing fuel allowed. No E85.
TIRES/WHEELS:
All four tires and wheels must be same size 14 inch or 15 inch passenger car tires only,  may be a maximum of seven (7) inches in width, unaltered, D.O.T.-stamped steel wheels with standard bead bump.
TRANSMISSION:
Manual: All forward and reverse gears must be operational.
Must be unaltered OEM three or four speed, with minimum 10.5 inch steel/organic single disc-type clutch and steel pressure plate assembly inside an explosion-proof steel bellhousing – minimum 270 degrees around top of clutch and flywheel area. No lightweight bellhousings. Steel unaltered flywheel only – 16 pound minimum.
Automatic: Must be unaltered OEM, with unaltered OEM pump, original bellhousing and minimum 10 inch diameter torque converter containing a minimum of three quarts of fluid. Torque converter must have a minimum 0.125 inch plug.
REAR SUSPENSION/REAR END/SPRINGS:
All components and mounts must be steel, unaltered, OEM, in OEM location and match frame. OEM rubber control arm bushings only.  No independent rear suspension. No sway bars, panhard bars, spring spacers, extensions, chains or cables.
One inch inspection hole in housing required. No lightened ring gears. Must be welded spider gears, or mini spool. No torque dividing differentials (no gun-drilled or titanium axles – axles must be steel). One steel spring per wheel only in OEM location. All coil springs must be minimum 4.5 inches O.D. and non-progressive. No spring rubbers allowed.