Pontiac 455 Engine: A Close Look at the GTO's Powerhouse

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Pontiac’s last big block engine helped define the muscle car era for the GTO and Firebird, but the powerhouse motor couldn’t keep the car company from fading.

The Pontiac 455 (7.4L) V8 engine was produced from 1970 - 1976 and was available for all full-sized Pontiac models. Pontiac offered a couple of variants - the HO in 1971 and SuperDuty in 1973, but in 1976, the engine was retired for good when customers opted for more fuel-efficient cars.

When GM rescinded the ban on higher displacement engines in 1970, Pontiac saw an opportunity to regain some of the muscle car glory it had been losing. Even though dealers like Royal Pontiac had been offering to modify GTOs and Firebirds with the “Bobcat” 428, Pontiac felt it needed a more robust motor than anything it had previously offered. As sister companies like Chevy, Buick, and Olds began promoting their sizeable V8 engines, Pontiac understood it could not be left behind. So, in that vein, they took the 428, increased the bore and stroke, and made it available for their full-sized lineup. Since car sales were beginning to show signs of weakness, company execs prayed that the new powerhouse might bring customers back. Instead, the development led to a performance war that couldn’t be won in the face of new emission controls and decreasing customer demand. Still, the Pontiac 455 is probably the most iconic V8 the company ever made, and while its time on earth was short (as far as motors go), it made a strong impact. Let’s look at the Pontiac big block to uncover why it might have been the best GTO engine ever.

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The Background Of The Pontiac 455 Engine

In 1970, GM finally lifted the ban on engine displacement that it had enacted almost a decade earlier. Each division responded by producing versions of large block motors and moved to install them in as many vehicles as possible. (Chevy introduced the 454 V8, Buick offered its 455 V8, and Olds named its version the Rocket V8). With more powerful offerings, GM customers had more choices than ever, and Pontiac could ill afford to continue to see its customers abandon their brand in favor of more luxurious Buicks and Oldsmobiles.

Pontiac had an advantage as an engine builder in making large journal engines, making their first one (421 ci)  in 1961. While Pontiac didn’t replace the 389 V8 they had been using for over a decade, it did offer the more robust engine as a dealer add-on. Eventually, Pontiac increased the bore to 4.12 in 1967, bringing the displacement to 427, offering the engine more prominently in their full-sized cars. The same year, Pontiac retired the 389 for a 400 V8 for the GTO and Firebird, which prompted dealers like Royal Pontiac to offer conversions to the big block 428 for customers who wanted more power.

The Features Of The Pontiac 455 Engine

The Pontiac 455 (7.45L) was a big-block eight-cylinder V-shaped engine with a cast iron block and cylinder heads. (The engine codes were stamped on the front block - left side, which is an excellent way of knowing the motor size your classic Pontiac has).

With a 4.15 bore and 4.21 stroke, the engine was an undersquare design, providing ample low-end torque and good acceleration. (In comparison, Oldsmobile achieved their displacement with a similar design (4.12 X 4.25), while both Chevy and Buick’s new V8s were oversquare units).

The 455 featured alloy cast iron heads, wedge-shaped to provide maximum squish, with a larger combustion chamber capacity (89.96 cc). As a large journal motor, the main bearing journals were the same diameter as the 428 and had four-bolt main caps for the first couple of years until they switched to two-bolt mains after 1973. The four-bolt caps provided excellent strength-to-weight ratios.

Pontiac designed the overhead valve motor with 16 valves, with a single forged camshaft inside the block. The Pontiac 455 used hydraulic lifters, ball-bearing hollow-steel pushrods, 8440 intake valves, and SAE 21-2M exhaust ports, with aluminized face and chrome-plated stems designed to reduce fatigue. The intake valves measure 2.11 inches, with slightly smaller exhaust valves of 1.77 inches. The Pontiac fashioned dual valve springs designed to help maintain the engine’s performance and durability.

The firing order remained consistent with other Pontiac engines available at the time (400) at 1-8-4-3-6-5-7-2. During its initial offering, the 455 produced 10.25:1 compression. Still, after emission controls were put in place in the early seventies, the engine compression was lowered to 7.6:1. Pontiac engineers lowered the ratios to meet new emission standards and allow the use of regular fuel. The standard carburetion for most 455 V8s was the Rochester Quadra jet, producing around 800 cfm of airflow. (Pontiac did use their forced induction Ram Air on the 400, which helped the 400 to produce slightly better power outputs). Pontiac mated the

Pontiac fashioned a couple of variants to the 455, with a High-Output engine with a reinforced block, more aggressive camshaft, and slightly taller intake ports. Initially, the 455 HO had D-shaped center exhaust ports. After revising the engine the following year, Pontiac decided to use round port cylinder heads incorporating Ram-inspired exhaust manifolds and an aluminum intake manifold. Even after the government instituted its “net” horsepower rating system, the 455 HO was still capable of 335 hp. As dynamic as the engine was, it was offered as the standard engine in the Pontiac Firebird Trans Am coupled with a 4bbl carburetor.

In 1973, Pontiac refined the 455 even further and labeled it the “Super-Duty” for use in the GTO, Firebird, and Gran Am models. The 455 SD used cast iron exhaust header manifolds with a provision for a dry sump oil pump system. Initially, we planned to use the Ram Air IV system but ended up settling for the 4 bbl when emissions testing didn’t provide reasonable enough numbers. When the government made Pontiac recertify its numbers, the compression ratio was lowered to 8.4:1 to allow the motor to comply with federal guidelines.

In 1975, Pontiac announced that the SD moniker was being dropped, which led everyone to believe that the old High-Output motor from the early seventies was about to be resurrected. Yet, when engineers further reduced the numbers to a wimpy 7.6:1, knowing that the engine was about to be discontinued, the lower output disappointed almost everyone.

The Power Of The Pontiac 455

Over the years of its production, the horsepower ratings varied slightly. Still, initially, the Pontiac sales brochure claims 360 hp for most models (370 hp for the G.P).  It was not unusual for automakers to “misreport” the power numbers for the public to keep insurance premiums down for potential owners.

Even though it was the eight-cylinder with the highest displacement, it wasn’t the most powerful. The 400 ci V8 (a small journal motor developed in 1967 to replace the long-enduring 389) competed alongside the larger 455. The ‘67 GTO produced 366 hp with Ram Air III (offered on the GTO and Judge) compared to the 360 hp on the 455. The smaller motor had the same power output (370 hp) as the highest-rated 455 when coupled with the Ram Air IV. The area where the 455 excelled was in low-end torque with over 500 lb-ft, significantly more than the 400 V8.

Regarding performance engines, the Pontiac 455 matched the power outputs of both the Olds and Buick 455s, but the best of the bunch was Chevrolet’s 454 V8. Although the LS engine power outputs varied between models, the LS6 in the Chevelle produced a whopping 450 hp, and even the LS5 in Chevy’s Corvette put out 390 hp.

The Demise Of The Pontiac 455

Part of Pontiac’s struggle stemmed from the fact that the company had announced the return of the High-Output motor in 1975. Early reviews led the public to believe that power numbers were returning (most Pontiac owners had anticipated that the company would return the High-Output motor from 1971-’72). When the revised engine was unveiled with much lower compression and less power, Pontiac owners felt betrayed and walked away. Sales slipped across all models except the Firebird Trans Am, which rose, surprisingly.

Eventually, Pontiac would drop the 455 V8 when federal regulations made it impossible to modify the engine enough to meet the new standards. Pontiac replaced the engine with a modified version of the 400 V8 (W72) to attempt to fill the performance gap the loss of the 455 created. (The W72 was only rated at 200 - 220 hp, so it did not have nearly the power of the larger V8 engine).