In the mid-sixties, the Skylark Gran Sport was doing well. The mid-size sporty car was a hit on the streets, so in 1967, Buick shortened the name to GS400 and made it its own model.

So far, the Gran Sport looked just like any regular Skylark. In fact, it was only a couple of badges and a set of wheels that told them apart. The 1967 model, while still reasonably conservative for a muscle car, received some flashy treatment.

Buick redesigned the front and rear end. A red line with the GS badge decorated the centerline of the grille. The hood was equipped with a pair of faux scoops. Over at the rear, strong quarter panels and decently-sized C-pillars suggested a more muscular physique. The bumper and stop lights were redesigned with less chrome trim and sportier feel.

The Nailhead V8 had legendary reliability and was favorite to many performance enthusiasts in the fifties. But it had aged and was quite dirty, so by the mid-sixties, the writing was on the wall.

1967 Finally brought a modern big-block V8 to the Buick GS400

In 1967, Buick introduced its big-block engine family, featuring 400, 430 and 455 ci versions. It was an entirely new design, which offered better efficiency and lower emissions. The only similarity with the Nailhead was the bore center distance between the cylinders at 4.75 inches.

The new V8 featured a cylinder bore of 4.04 inches and a stroke of 3.90 inches. Cast aluminum pistons with 10.25:1 compression ratio, forged connecting rods and nodular cast iron crankshaft with 3.25-inch 2-bolt mains completed the rotating assembly.

The new heads were designed with larger valves and ports to allow for better airflow at high RPM. A 66 cc semi-wedge-shaped combustion chamber replaced the Pent-roof design in the Nailhead engine. The camshaft was specced similarly to other high-performance engines on the market – 298 / 315 degrees of duration and 0.421 / 0.449 inches of lift.

Topped with a 4-barrel Rochester Quadrajet carburetor, the 400 ci big-block produced 340 horsepower at 5,000 RPM and 440 lb-ft of torque at 3,200 RPM. This engine size was exclusive to the GS400.

To support the extra power, Buick used the stiffer boxed frame from the convertible model under every available body style. Additionally, stiffer springs were used front and back as well as a thicker anti-roll bar at the front axle.

Transmission choices now also included the 3-speed Super Turbine 400 automatic transmission with a variable pitch stator.

All around manual drum brakes were standard. The front end included aluminum drums with cast-iron liners for reduction of rotational inertia. Alternatively, you could get 4-piston 11-inch disc brakes for the front axle.

Performance jumped to 6.5 seconds for a 0-60 mph sprint and 15.20 seconds for a quarter-mile run. The Buick GS400 wasn’t the fastest car on the street, but it was pretty fast and it still offered a pleasant ride – something most of its contemporaries couldn’t brag about.

The GS400 was still the least popular muscle car of the A-body pack, but Buick was getting serious about building a performance offering.

1968 Buick GS400

All GM intermediates received fresh new bodies for 1968. The GS400 lost 3 inches in wheelbase, as all 2-door A-bodies did and 4.4 inches overall. The new design introduced the coke bottle style and emphasized on the muscle car proportions with a longer hood and sleeker, almost fastback rear end.

At the front, the GS400 sported a smoother, unified grille wrapped in thick chrome trim. The dual headlights were recessed deeper into the front end. The hood bulge disappeared and a much sleeker hood was introduced, featuring a cowl induction scoop.

The car sported a new caved-in rear end with a heavily sloped trunk. The stoplights were transformed into long strips positioned low within the bumper. A signature curve dressed the side body panels.

But even more interesting was a dealer-installed optional package called Stage 1. At the time, very few people knew about it and it practically didn’t exist in Buick literature. However, those knowledgeable GS400 buyers who wanted the ultimate performance in their muscle car were in for a treat.

Stage I – Prepare for liftoff

The Stage I package added a more aggressive camshaft with higher lift and more duration, upgraded valve springs, bigger valves, and a 10.5:1 compression ratio. The Quadrajet carburetor was modified and fed fresh air by functional hood scoops. A new distributor and high capacity fuel pump and a low-restriction dual exhaust were added.

A modified TH-400 transmission secured firm gear changes and shifted higher into the rev range. Power passed through a 3.64:1 rear gear ratio (for non-AC cars) and a positraction differential before meeting the wheels.

On top of this, the Stage I cars also came equipped with the Rallye heavy-duty suspension package, and power front disc brakes.

The result was just 10 horsepower more on paper (350 hp total), but at the drag strip, the car was a whole second quicker. The few Buick GS400s that were running the Stage 1 package in 1968 would crush the quarter-mile in 14.4 seconds.

In the following year, Buick added the Stage 1 option to its nomenclature and began upgrading GS400s from the factory. Some 1,500 customers ordered a Stage 1 1969 Buick GS400.

According to Dennis Manner – former Buick powertrain engineer directly involved in the development of the big block engine family, as well as the Stage upgrade:

“The objective, as we developed the Buick Stage 1 400/455-CID engines, was simply to provide maximum street performance in a vehicle sold from the showroom floor. We allowed no compromise of Buick features…There were no ifs, ands or buts about it – we optimized the Stage 1 package to move on the street.”

Original quote from Reynolds Buick GMC.

Stage II – Shoot for the moon

In 1969, dealers began offering the racer-focused Stage II upgrade.

It included a new Edelbrock intake manifold, an 850 cfm Holley carburetor, and Mickey Thompson exhaust headers. New heads were developed for the 1970 model raising compression to 11:1 which were backward compatible. A positraction rear end with a 4.78:1 final drive ratio was included.

In an interview for Hemmings, Dennis Manner said:

“The whole Stage 2 package was worth about a half-second over the Stage 1 and made 100hp more, so just under 500hp”.

A 1969 Buick GS400 with the Stage 2 upgrades would run the quarter mine in the high 13-second range.

And it wasn’t even running Buick’s biggest engine… Buick wasn’t done yet.

You are reading part 2/4 of The Story of The Luxury Muscle Car. Other articles in this series are available below:

1. The Story of The Luxury Muscle Car – Buick Gran Sport
2. 1697 Buick GS400 – Getting Serious About Performance
3. 1970 Buick GS455 Stage I – Faster Than a Hemi?
4. 1970 GSX – The Buick that Made Legends