an 11.0:1 compression ratio. Casting No. 540306 featured valves and was carried over from the previous model year, production stopped in March 1962 and then casting No. 544127 with larger valves entered production. Neither casting was equipped with an exhaust crossover. SD-455 Available only in the 1973 and 1974 Firebird Formula and Trans Am, the SD-455 consisted of a strengthened cylinder block that included 4-bolt main bearings and additional material in various locations for improved strength. Original plans called for a forged crankshaft, although actual production SD455s received nodular-iron crankshafts with minor enhancements. Forged rods and forged-aluminum pistons were specified, as were unique high-flow cylinder-heads. A camshaft with 301/313 degrees of advertised duration, net valve lift, and 76 degrees of valve overlap was specified for actual production engines in lieu of the significantly more aggressive Ram Air IV-spec camshaft that had originally been planned for the engine (initially rated at with that cam), but ultimately proved incapable of meeting the tightening emissions standards of the era with sufficient margin to satisfy Pontiac emission engineers given expected volume production variations. The very modest cam, combined with a low-compression ratio of 8.4 (advertised) and 7.9:1 actual resulted in SAE NET. The initial press cars that were given to the various enthusiast magazines (e.g. HOT ROD and CAR AND DRIVER) were fitted with the Ram Air IV-spec camshaft and functional hood scoops - a fact that has been confirmed by several Pontiac sources. Some production test cars ran considerably slower and yielded times in the 14.5 second/ range in showroom tune (uncited sources)- results that are quite consistent for a car with a curb weight of and the rated SAE NET figure that some sources suggest was "under-rated." However, in the June 1974 issue of SUPER STOCK AND DRAG ILLUSTRATED, a new 1974 Trans Am with the SD-455 motor ran 14.25 @ . This was a completely stock car on loan from a private owner for the test. Furthermore, this car had an automatic, air conditioning, a 3.08 axle and weighed . This test would tend to lend credence to the CAR AND DRIVER and HOT ROD tests of 1973 Super Duty cars with 3.42 gears, no air, and less weight as being representative of production specimens capable of mid to high 13-second passes at . Pontiac listed the rating at 4000 rpm for a motor that had a 5700 rpm redline on