The 400 was replaced for 1970 with the Buick 455 cu in (7.5 L) V8 engine, used in the GS 455. The base model was rated at 350 bhp (355 PS; 261 kW) and 510 lb·ft (691 N·m; 71 kg·m) at 2800 rpm of torque. In the optional Stage 1 trim equipped with a single 4-barrel Rochester Quadrajet carburetor was rated at 360 bhp (365 PS; 268 kW) SAE gross at 4600 rpm and 510 lb·ft (691 N·m; 71 kg·m) at 2800 rpm of torque. As with all American engines produced prior to the 1972 model year, these were SAE gross ratings, which are generally significantly higher than SAE net ratings and are not indicative of what actual production engines produce in their "as installed" condition (with all engine accessories and full exhaust system in place). The fastest magazine test results from this period were obtained by MOTOR TREND Magazine, which managed to extract a 13.38 ET @ 105.5 MPH from their 3,810 pound GS Stage 1 coupe test car. Using Hale's Trap Speed formula, this result indicated actual "as installed" peak HP of approximately 360 SAE Net (ironically the same as its advertised Gross Figure, meaning this engine was very conservatively rated for that time period).
Dennis Manner, Buick Engine development engineer still recalls the production-line pulled engine dyno test results from 1970 where the lowest HP output of the 15 tested 455 Stage 1 engines was 376hp.
The December 2004 issue of Musclecar Enthusiast" conducted an engine dynamometer test of a freshly rebuilt and well documented 1970 455 Stage 1 (bored .040" over to 464 cubic inches and minus the power-robbing factory engine fan, air cleaner assembly and mufflers). In that condition and with factory timing and carburetor tuning, the engine produced a maximum of 360.9 HP. Optimal carburetor and ignition tuning yielded a peak HP reading of 381.7 HP - again with no engine fan, air cleaner or mufflers in place. While urban legend would have us believe that these engines made "420 HP from the factory," actual empirical results prove otherwise. Although another dynamometer test on a 1970 Skylark GS Stage I showed 471 SAE gross hp, which is more consistent with the engine producing about 360 SAE net hp.
The Stage 1 engine option used cylinder heads that, while using raw castings of the same pattern as all of the other Buick 455s sharing the same model year, were machined differently in order to accept larger valves (2.13" intake and 1.755" exhaust), and to produce smaller compression chambers for increased static compression ratio. The option also included a more aggressive camshaft, a specially tuned 4-barrel Quadrajet carburetor, more aggressive ignition timing,5/8 inch oil pickup tube and a higher numerical final drive, and was available with either Turbo-Hydramatic 400 3-speed automatic transmission, or a Muncie M-22 4-speed manual transmission (although it was later discovered, and confirmed, that one 1970 GS Stage 1 convertible was produced with a 3-speed manual and a standard 3.64:1 Positraction axle ratio). Stage 1 cars equipped with air conditioning received a 3.42 axle ratio.
While powerful in production form, the Buick 455 (including Stage 1) engines had problematic engine blocks. All used 2 bolt main bearing caps; the oiling system was undersized for high-rpm use (including Stage 1 engines) and thin walls in the lifter valleys promoted cracking. The magic of the Stage 1, it would seem, was primarily attributable to its advanced (for the period) cylinder heads, stunningly high torque and the relatively high mid-range horsepower they produced.
The relatively unknown, very expensive, and very rare 1970 GS 455 Stage 1 drew a great amount of attention and controversy in the muscle-car world when in the 1980s it was listed as faster than any of the Chrysler Hemi cars in the original "50 fastest muscle cars" list. This Hemi vs. Stage 1 controversy has prompted several contests to settle the issue; it remains an unsettled matter and has been a great boon to car magazine sales over the years.
Buick GS Stage 2.
There was also a rare “Stage 2” option produced. This was a dealer-installed package (known as a "dealer option"), first offered in 1969. It included a cam, headers, intake manifold, high compression forged pistons, hollow pushrods, and some calibration changes to the ignition and carburetor . In 1970(?) the Stage 2 package was designed for the 455 and included special round exhaust port Stage 2 heads, matching Kustom brand headers, a radical cam, high compression forged pistons, Edelbrock B4B aluminum intake, Holley carburetor #4781 850 cfm, and other equipment for racing. Few Stage 2s even exist and Buick only ever factory assembled 1 Stage 2 test unit, it was a factory GSX clone test mule with 4-speed manual transmission used for speed testing. That GSX test mule was equipped with 4.78 gearing and was driven on the streets and tracks on the West Coast. The Stage 2 package's existence was not made public until 1972 when the Stage 2 parts could be ordered in any combination. There is little documentation about any Stage 2 cars that were sold. Three are known to exist and all three of these cars were built as Stage 1 cars at the factory. One was owned by Kenne-Bell a factory backed Buick high performance specialist based in California . Sponsored by Reynolds Buick .This car was used as a test car in the development of the Stage 2 components in conjunction with Buick engineering . The second Stage 2 was known as the Jones-Benisek car. The Jones/Benisek car is known to have been delivered as a Stage 1 car .The Stage 2 hood and scoop was added later . The Stage 2 iron heads were purchased by the owner at a local Buick Dealer. The car had a very successful drag racing career with many wins and some world records also. The third Stage 2 is known as Wiley Coyote and as the Turner car. It was originally owned by Bob Thetford and campaigned as "Wiley Coyote" in the NHRA. Sponsored by Kenne-Bell and Dunn Buick in Oklahoma City, Oklahoma. Later it was owned by Jim Turner and was very successful in NMCA winning 5 National Records.
Output and sales for the assembly-line cars were down after 1970 largely due to reduced engine compression ratios and a change from gross to net horsepower ratings. In later years, air quality regulations further limited the power in part due to the addition of catalytic converters and single exhaust pipes. However, Stage 2 parts were available over the Buick parts counter although the Stage 2 heads were discontinued after about 75 sets were produced. The discontinuation was due to porosity problems with castings. The discontinuation of the Stage 2 also was due to the ever-tightening emission standards which resulted in lower performance.
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