* Compression Ratio: The most significant difference would likely be the compression ratio. 1970 350s, particularly those in performance-oriented vehicles, boasted higher compression ratios (often above 9:1, sometimes significantly higher depending on the specific model and options). By 1973, emission controls mandated lower compression ratios (typically in the 8:1 to 8.5:1 range) to run on lower-octane, leaded (and later unleaded) fuel. This directly impacted horsepower output.
* Cylinder Heads: While the basic small-block architecture remained the same, the cylinder heads themselves underwent changes. 1973 heads generally featured smaller combustion chambers, which contributed to the lower compression ratio and incorporated emission control provisions. They may also have different intake and exhaust port designs.
* Camshaft: The camshaft profile would be milder in the 1973 engine compared to a 1970 engine. Lower compression ratios and emission controls required less aggressive camshaft profiles for smoother, lower-emission operation. This also impacted horsepower.
* Emissions Equipment: 1973 engines would have more sophisticated emission control equipment. This would include things like:
* Exhaust Manifolds: Designed to better integrate with catalytic converters (if equipped, depending on the specific vehicle).
* EGR (Exhaust Gas Recirculation) System: Recirculated exhaust gases back into the intake to reduce combustion temperatures and NOx emissions. This system was becoming more common and refined by 1973.
* Air Injection System (AIR): Pumped air into the exhaust to help burn off unburnt hydrocarbons.
* Carburetion: The carburetor itself might be different, reflecting the changes in fuel delivery and emission requirements. 1973 carbs would be calibrated to work with the lower compression ratio and emission controls.
* Horsepower and Torque: As a result of all the above, the 1973 350 would produce significantly less horsepower and torque than its 1970 counterpart. The power difference could be substantial, easily 50 horsepower or more, depending on the specific engine options from each year.
In short: while fundamentally the same basic small-block V8, the 1973 350 is a detuned, emissions-controlled version compared to its 1970 predecessor. The 1970 engine, especially if it was an optioned performance engine, would likely be considerably more powerful. You'd need to check the specific engine codes on both engines to know precisely their specifications.
