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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →The Pontiac GTO used the 389 V-8 from 1964 through 1966, moved to the 400 in 1967, and added the 455 as an option in 1970. Their displacement labels point to real differences in bore and stroke, but they do not by themselves tell you which GTO was more powerful: carburetion, compression, camshaft, performance package, model year, and horsepower-rating method all matter.
Which GTO years used the 389, 400, and 455?
| Engine | GTO model years | Quick context |
|---|---|---|
| 389 | 1964–1966 | Early GTO engine, available with a four-barrel carburetor or Tri-Power. |
| 400 | From 1967; continued in later GTO years | Replaced the 389 in the 1967 GTO. The 400 range included distinct performance versions, including Ram Air models. |
| 455 | Added in 1970; offered in early-1970s GTOs | A larger-displacement option with a substantially longer stroke than the 389 or 400. |
The chronology and GTO-specific context are summarized in Web-Cars’ GTO engine history and the Pontiac engine reference at PontiacSafari.com.
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How do the engines differ mechanically?
The listed bore and stroke dimensions show how Pontiac arrived at the three displacements. The 400 is described as a bored-out 389: its bore is larger while the listed stroke remains 3.75 inches. The 455 has both a slightly larger bore and a much longer stroke.
| Engine | Bore × stroke | Main journal | What the dimensions indicate |
|---|---|---|---|
| 389 | 4.0625 × 3.75 in | 3.00 in | Baseline dimensions for the early GTO engine. |
| 400 | 4.120 × 3.75 in | 3.00 in | Larger bore than the 389, with the same listed stroke. |
| 455 | 4.150 × 4.21 in | 3.25 in | Much longer stroke and a larger listed main journal than the 389 and 400. |
These specifications come from PontiacSafari.com’s engine reference, published in 2007 and updated in April 2009: Pontiac V-8 engine specifications. It is a historical enthusiast reference, not a factory publication. Dimensions are useful for understanding the basic design, but they are not enough to determine whether a particular component interchanges.
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Why does the displacement alone not settle which one is more powerful?
Each displacement appeared in different GTO configurations. Carburetor arrangement and named performance packages changed the factory-rated output, and the ratings are tied to their specific model year and setup.
389: four-barrel and Tri-Power
Web-Cars reports a 1964 GTO 389 rated at 325 hp with a four-barrel carburetor and 348 hp with the Tri-Power option, which used three two-barrel carburetors. For 1965, it gives 335 hp for the single-carburetor version and 360 hp for Tri-Power. Those are period ratings for particular configurations, not one universal output for every 389: Web-Cars GTO engine history.
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400: several versions, including Ram Air
The GTO switched to the 400 in 1967, with Rochester Quadra-Jet carburetion described in the history. The 400 lineup included multiple output levels and performance options. Web-Cars reports 366 hp for the 1968 Ram Air II and 370 hp for the 1969 Ram Air IV. A 1970 GTO could also be ordered with a 455, but the history’s maximum listed output for that year belongs to a 400 performance version—not simply to the engine with the largest displacement.
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455: specify the year and version
Pontiac Motor Division’s reproduced 1970 material identifies a 360-hp, four-barrel 455 as GTO-only. The figure is a period factory rating for that named year and configuration. The source is a reproduction of Pontiac material hosted as an Over-Drive Magazine PDF: 1970 Pontiac GTO brochure.
How should you compare the horsepower figures?
Do not treat period horsepower ratings from different years as though they came from one identical measurement. Web-Cars flags a rating-method change around 1970 and presents both gross and net figures for the 455 HO. Gross and net ratings belong to different reporting contexts; a brochure number alone is not a controlled, apples-to-apples performance test. When quoting a figure, identify the model year, engine version, and rating context whenever the source provides it.
What matters if you are identifying or buying parts for a GTO?
For historical identification, start with the model year and engine configuration rather than assuming that the displacement tells the whole story. For a restoration or replacement part, verify the application against the exact year and drivetrain. A 2025 Ottawa Clutch catalog separates GTO clutch applications into 1964–66 389, 1967–70 400, and 1970 455 groups. That demonstrates distinct catalog applications; it does not establish universal compatibility or current stock. Check the model year, transmission, spline count, and seller’s application notes before ordering: Ottawa Clutch 2025 catalog.
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Which engine should you choose for comparison?
There is no sound universal winner based on the displacement badge alone. For a fair historical comparison, select a specific GTO model year and version, then compare its factory configuration and rating context. For a parts question, use the car’s year, engine, and transmission details to confirm fitment rather than relying on a broad 389-versus-400-versus-455 rule.
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