From allpar.com:
The 400 engine was introduced in 1972 in standard and high performance versions. The 400, a “B” type engine, is a large (4.34) bore version of the 383B engine.
In 1978, the 360 and 400-cubic-inch V-8 engines gained dual concentric throttle return springs in addition to a torsion throttle spring. Exhaust-valve seats were induction-hardened on all engines to allow satisfactory use of lead-free fuels. In the hardening process, seats reach a temperature of 1700°F and are then allowed to air-cool. This hardened the valve seat surfaces to a depth of .05" to .08" which gives them greater resistance to wear than unhardened seats. The exhaust-valve stems were chrome-plated for increased resistance to wear.
The heat valve in the right exhaust manifold diverts hot gases to the floor of the intake manifold which helps to vaporize the fuel mixture when the engine is cold. This results in faster warm-up and improves driveability after a cold start. During warm-up, a thermostatic spring allows the heat valve to open to the exhaust pipe-so gas flow through the intake manifold crossover passage is decreased.
All 1978 Chrysler V-8 and 6-cylinder engines have an adaptor to receive a magnetic probe for timing the ignition magnetically. The adaptor is a little steel sleeve attached to the bracket that has the traditional timing marks-you can still set the ignition with a timing light. However, ignition timing was set magnetically on all engines at the factory for accuracy. The magnetic timing adaptor will also help those who have the equipment to time Lean-Burn engines magnetically.
The performance aspects of the 'B' and 'RB' engines faded from 1972 on until the last 'B-RB' engine was produced in August 1978, ending the history of Chrysler Corporation big-block engines.
