The C4 was the small light duty automatic transmission. Units were strengthened in 1970 and are considerably stronger than those built before 1970. The C4 can be rebuilt to very tough standards and it is very light, making it popular with small block drag racers. Lots of aftermarket performance kits are available for the C4.
The C6 was ford's heavy duty unit and was most often found behind big blocks, or high performace small blocks. The C6 is considerably larger and heavier than the C4 but it is pretty tough from the factory and can be made virtually bullet proof.
Both units are aluminum case transmissions as opposed to the FMXwhich was a medium duty cast iron case transmission.
That pretty much exhausts my C4/C6 knowledge.
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