Beneath the plain sheetmetal of this mid-size Plymouth sat one of the rarest engines Chrysler ever sold to the public: the 425-horsepower Street Hemi, built for NHRA stock classes and barely civilized for the street. Only 677 Belvederes were built with it in 1966, a year before the GTX name even existed to advertise what was under the hood. Buyers got forged internals, dual four-barrel carburetion, and a torque curve that demanded a reinforced rear axle. It’s the missing chapter most muscle car fans skip straight past.
The sixth generation Plymouth Belvedere is on Chrysler’s “B” mid-sized body platform.  This model could be powered with a light duty 273 cu in ((4.5 L) engine although there are four larger engine power options  on the table including the 426 cu in (7.0 L) Hemi V8  and these would be bolted to three speed automatic or a three speed standard transmission on a factory correct Belvedere.  The most powerful version of the Belvedere is an icon of the muscle car era and badged as the GTX .
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Only 677 examples of this mid-size Plymouth left the factory with the engine collectors chase hardest today — a lightweight, race-bred V8 that Chrysler barely tamed for public roads. It arrived a full year after the option first appeared on Chrysler’s B-body lineup, wedged into plain sheetmetal that gave away nothing about what sat under the hood. Buyers got a three-speed manual or TorqueFlite automatic and a rear axle that had to be reinforced just to survive the torque. Chrysler wasn’t chasing showroom glamour with this combination — it was chasing NHRA stock class records. The badge that would eventually make this exact combination famous hadn’t even been dreamed up yet. What was Plymouth’s mid-size lineup really building toward in 1966?
677 Cars, One Engine Nobody Expected to Tame
The 1966 Street Hemi made 425 horsepower at 4,000 rpm and 490 lb-ft of torque at 2,400 rpm, running dual four-barrel carburetors and forged internals lifted almost straight from Chrysler’s NASCAR and NHRA race engines. Only 677 Belvederes were built with it that year, part of just 2,730 total Street Hemi cars Chrysler built across the Belvedere, Satellite, Coronet, and Charger lineup for 1966.
The Year Before the GTX Name Existed
The GTX badge that would later define Plymouth’s mid-size muscle car wouldn’t debut until the 1967 model year. That means a 1966 Street Hemi Belvedere like this one is effectively a sleeper — the same engine combination that would earn 720 GTX orders a year later, wearing plain trim that gave no hint of what was underneath.
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