In the 1960s the popular 389 cu in (6.4 L) version, which had helped establish the Pontiac GTO as a premier muscle car, was cut in half to produce an unusual, high-torque inline four economy engine, the Trophy 4.

Unusual for a major automaker, Pontiac did not have the customary "small-block" and "big-block" engine families common to other GM divisions, Ford, and Chrysler.

When Robert Critchfield took over as general manager in 1952, he launched an ambitious plan to move Pontiac into the upscale, mid-range market occupied by Oldsmobile, and that demanded V8 power.

Federal emissions standards and the drive towards "corporate" engines shared among all GM divisions led to the progressive demise of the Pontiac V8 through the late 1970s.

The 1955-up Pontiac V8 that finally reached the public was an overhead valve engine with cast iron cylinder heads and block and wedge-shaped combustion chambers.

Pontiac differed from other GM Divisions and most other manufacturers in producing only a single sized V8 casting, rather than adding a larger big block to its line-up.

"Armasteel" was a trademark of pearlitic malleable iron developed by GM's Saginaw Metal Casting Operations around 1936, which was referred to as "locking ball" cast-iron, as opposed to the "flaking" type found in other engines.

The 326 subsequently became the optional V8 engine for Tempests, and later the Pontiac Firebird, through 1967 and maintained the 17 degree cylinder head valve angle for its entire production run.

The 301 Turbo package mandated air conditioning, the THM350 (sometimes referred to as the CBC350 in various literature) non-lockup automatic transmission (THM350C lockup in 1981 Trans Ams), and 3.08 rear axle gearing.

An HO version could be ordered in the GTO, as GM had lifted its restrictions on offering engines larger than 400 cu in (6.6 L) in mid-sized cars (resulting in the 454 (7.44) Chevrolet Chevelle, 455 (7.5) Buick Gran Sport, and 455 (7.5) Oldsmobile 442).

For 1971, Pontiac introduced another High Output (HO) version with standard internal parts, a reinforced block with four-bolt main bearing caps, and improved cylinder head design with 1⁄8-inch taller (3.2 mm) intake ports and special round exhaust ports for better breathing, yet still making just 335 hp (250 kW) gross (or 310 hp (231 kW) in the more accurate SAE Net system).

In addition to the more refined cylinder heads, block casting reinforcements in the lifter galley and main bearing oil pan rail area, it had forged connecting rods with larger 7⁄16-inch-diameter (11 mm) bolts.

The 455 was used through 1976 when it, as with many other large displacement engines, was discontinued as manufacturers moved to smaller, more efficient models, even in their full size car lines.

As Pontiac's 1967 performance brochure said, "You can add the Ram Air induction hood scoop and new high output cam and valve springs to the Quadra-Power 400 for better top end breathing."

But the "Ram Air" 400 also mandated steep 4.33:1 gears(or 3.90:1 in Firebird), making the Quadra-Power 400 (400 HO) the top practical street engine option for most drivers.

With some WS6 "Trans Am Special Performance Package" bundles in 1978, the W72 engine was incorporated with the WS6 option group and not listed on the dealer order invoice.

The 455 HO designation made its debut in 1970; Rated at 360 or 370 hp (268 or 276 kW) (depending on which vehicle it was installed into) and 500 lb⋅ft (678 N⋅m) of torque, it differed from the regular full sized car 455 by large valve heads with smaller combustion chambers, and a larger camshaft.

The "455 HO" moniker took on a whole new meaning with the introduction of the 1971 model year; Intended as a low compression progression from the previous years Ram Air IV engine, all 1971 455 HO engines used a heavy duty 4 bolt main block, round port cylinder heads (casting #197; with 8.4: compression), "Ram Air" style exhaust manifolds, and a two-part aluminum intake manifold.

However, the engine used in these Trans Ams was the same regular production 455 taken from the big body cars Pontiac was producing, and output 200 HP with a torque rating of 330 lb⋅ft at 2,000 rpm.

Upon reflection, many did not consider that it was the only large displacement engine still on offer for any performance car on the market, and reconsidered Pontiac's position between the rising CAFE emissions restrictions.

The main upgrade for the engine was the ESC, (Electronic Spark Control)[19] which provided slightly more power at 170 hp (127 kW)for the 1980–1981 model years.

The tri-power equipped XS-code 389 was shipped with the Ram Air pan in the trunk, and the dealer had to fit it and cut out the underside of the hood scoop to make it functional.

Hewing to GM's standing edict limiting engine size to 400 cu in for its midsize and smaller cars, the 360 hp (268 kW) (underrated),[citation needed] the 400 cubic inch Ram Air V-8 was the most powerful and advanced option available in the 1967 GTO and Firebird.

In addition, a shallower spherical-wedge combustion chamber moved the tuliped valve heads .040" closer to the piston at TDC, improving mixture draw considerably during the intake stroke.

The 303 had shorter connecting rods, smaller 2.5 in (64 mm) journals and a solid lifter version of the Ram Air IV camshaft.

[citation needed] The 421SD was available in 1961 as a dealer option or over the counter then in 62 and 63 from the factory, and was fitted with a list of internal modifications designed solely to withstand the abuse of drag racing.

Two different cylinder-head castings were used for the 1962 model year, both with a combustion chamber volume of 68 cc (4.15 cu in) to produce an 11.0:1 compression ratio.

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.

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.

Features: Most likely prompted by its development of the Pontiac OHC six, the GM division built three different experimental SOHC 421 CID V8 engines in the early 1960s.