I was wondering if the '67 and earlier torque converters are more efficient than the '68-up converters. Among many other changes for the '68 model year, engine idle speeds were considerably increased, 200 rpm or so, to reduce exhaust emissions. Were the late torque converters modified to reduce creep when idling in gear, would this increased slippage / increased stall speed reduce overall coupling efficiency causing a gas milage penalty at cruising speeds, and are the stall speeds for these converters different indicating there was a change made, besides the external change to the hub dimensions where it fits into the crankshaft? If the early torque converters were more efficient, would it be worthwhile to build an engine for a late model car using an early crankshaft, early torque converter, and early front transmission pump assembly installed in a late transmission (to take advantage of the part-throttle kickdown feature of "68-up transmissions), assuming that one is building for efficiency not increased power doesn't need the increased power handling capacity of the late transmission's larger torque converter hub / front pump bushing?

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1964 Dart 225 1966 Valiant 225
1966 Dart 273 1966 Barracuda 273
1969 Dart 225
'64-170 engine '66-170 engine
(two) 198 crank/rod sets in custom-fitted wooden boxes

wooden motor scooter with hybrid-electric drive

If you really are that concerned with economy go for a lockup converter and transmission. Someone here can quote the stall speed for '67 and '68 to see if there's a difference.

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Joshua

No, stay with the 68 and later parts and use a lock-up tranny with the "wide ratio" low first gear. You can run a real tall rear end gear ratio with that set-up and still get decent "off the line" performance, and turn low engine RPMs at highway speeds.
DD