"There are two basic ThermoQuad Carburetors: one with 1 3/8" primaries and 2 1/4" secondaries for the 318 and 340 (A) engines and a Holley comparison CFM of 650 CFM. The second one had 1 1/2" primaries and 2 1/4" secondaries with a Holley comparison CFM of 820. It's used with some 360s and all the 'B' engines. Actually, Carter called this one a 1000 CFM unit. Their airflow numbers were measured differently than Holley. Also, some ThermoQuads the secondary air-door limited, which also decreased
the secondary air-door limited, which also decreased the CFM."