1. Original Build Sheet/Dealer Invoice: This is the most reliable source. If the car has its original build sheet (a factory document detailing its specifications), it will clearly state the engine size (e.g., 400 CID, 350 CID, 455 CID). Similarly, a dealer invoice would show the original engine ordered.
2. Trim Tag (or Data Plate): Located usually on the driver's side doorjamb or under the hood, the trim tag contains a code that can sometimes *indirectly* help. The code indicates the options package which *might* imply a likely engine (e.g., certain performance packages were more likely to include larger engines). However, this is not foolproof as buyers could order options not normally paired with specific engines. You'll need a Pontiac decoder to interpret the trim tag's meaning.
3. Casting Numbers on the Engine Block: If the engine is original or a replacement of the same size, the engine block itself has casting numbers. These numbers can sometimes be cross-referenced with Pontiac engine identification guides or databases to determine the displacement (e.g., cubic inch displacement). Be aware that this only confirms the *current* engine, not necessarily the original one.
4. Previous Owner Information: Talking to previous owners, if possible, could yield information about the engine. This is unreliable as memories fade and people may not know the answer.
5. Vehicle History Reports (e.g., Carfax, AutoCheck): These reports might include some information about the vehicle's history, potentially mentioning engine swaps or replacements, although this isn't guaranteed.
Important Note: The vast majority of 1971 Pontiac Formulas were equipped with the 400 cubic inch (6.6 liter) V8. However, 350 CID and 455 CID engines were also available as options. Without documentation, definitively proving the original engine size is difficult. Any method other than finding the original build sheet should be considered circumstantial evidence.
