The 400 was replaced for 1970 with the 455 cu in (7 l) Buick V8, used in the GS 455. The base model V8 produced 350 hp (260 kW) and 510 ft·lbf (690 N·m) at 2800 rpm. In the optional Stage1 trim it produced 360 hp (268 kW) and 510 ft·lbf (691 Nm) at a low 2800 rpm. 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).
The December 2004 issue of Musclecar Enthusiast" conducted an engine dynomometer 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.
The Stage 1 engine option consisted of a more aggressive camshaft, higher compression, unique cylinder heads with larger (2.13" intake and 1.755" exhaust) valves, a specially tuned 4 barrel Quadrajet carburetor, more aggressive ignition timing,5/8 inch oil pickup tube and a higher numerical final drive. The engine was available with either a firmer shifting Turbohydromatic 400 or a Muncie M-22 "rock crusher" 4 speed (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 under-sized 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 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.
There was also a rare Stage 2 option produced. This was a dealer-installed package (what was 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. If the car was ordered with the Stage 2 package, the parts were shipped in the trunk of the car. In 1970 the Stage 2 package included special D- Port exhaust port Stage 2 heads, matching Kustom brand headers, a radical cam, high compression forged pistons, Edelbrock B4B aluminum intake, Holley carburetor, and other equipment for racing. Few Stage 2s were ever used on the street and Buick only ever factory assembled 15 Stage 2 test units. One of which was a factory GSX 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 the 12 Stage 2 cars that were sold. One was campaigned by Kenne-Bell/Reynolds Buick and the other was known as the Jones-Benisek car. The Jones/Benisek car is known to have been delivered as a Stage 1 car with all the Stage 2 components in the trunk in GM boxes.
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.