The second generation debut for the 1970 model year was delayed until February 26, 1970, because of tooling and engineering problems; thus, its popular designation as a 1970½ model, while leftover 1969s were listed in early Pontiac literature without a model-year identification.
Replacing the "Coke bottle" styling was a more "swoopy" body style, with the top of the rear window line going almost straight down to the lip of the trunk lid—a look that was to epitomize F-body styling for the longest period during the Firebird's lifetime. The new design was initially characterized with a large C-pillar, until 1975 when the rear window was enlarged.
There were two Ram Air 400 cu in (6.6 L) engines for 1970: the 335 hp (250 kW) Ram Air III (366 hp (273 kW) in GTO) and the 345 hp (257 kW) Ram Air IV (370 hp (280 kW) in GTO) that were carried over from 1969. The difference between the GTO and Firebird engines was the secondary carburetor linkage which prevented the rear barrels from opening. Bending the linkage to allow full carburator operation resulted in identical engines.
A distinctive, slant-nose facelift occurred in 1977, redone somewhat in 1979. From 1977 to 1981, the Firebird used four square headlamps, while the Camaro continued to retain the two round headlights that had previously been shared by both Second Generation designs. Curb weights rose dramatically in the 1973 model year due to the implementation of 5 mph (8.0 km/h) telescoping bumpers and various other crash and safety related structural enhancements; SD455 Trans Ams weighed in at 3,850 lb (1,750 kg).
The 455 engine available in the second generation Firebird Trans Am was arguably the last high-performance engine of the original muscle car generation. The 455 cu in (7.5 L) engine first made its appearance in 1971 as the 455-HO. In 1973 and 1974, a special version of the 455, called the SD-455, was offered.
The SD-455 consisted of a strengthened cylinder block that included 4-bolt main bearings and added 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 1967 GTO Ram Air camshaft with 301/313 degrees of advertised duration, 0.407 inch 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 style cam that had originally been planned for the engine (initially rated at 310 hp (230 kW) with that cam), but proved incapable of meeting the tightening emissions standards of the era.
This cam, combined with a low compression ratio of 8.4 (advertised) and 7.9:1 actual resulted in 290 SAE net horsepower. Production test cars yielded 1/4 mile times in the 14.5 second/98 MPH range in showroom tune – results consistent for a car with a curb weight of 3,850 pounds and the rated 290 SAE net horsepower figure some sources suggest was "under-rated," High Performance Pontiac magazine dyno-tested an SD and gave it 371 SAE net rating.
source: Mecum Auctions, Wikipedia, 2012
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