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ECL-A heads were original equipment on 1955 272 Fords and 292 Mercurys. They have a lower static compression ratio (7.6) than the ECL-B/C heads (8.5) as found on 55 Tbirds and 4-barrel 292 Mercurys. Your ECL-A heads, although having the same valve sizes, must have larger combustion chambers, which would explain the difference in compression ratios.