This 1937 Plymouth Sedan Traded Its Flathead for a Modern Fuel-Injected 350

Most 1937 Plymouth sedans left the factory with a flathead six that struggled to keep up with modern freeway traffic — this one didn’t stay that way. Somewhere along the line it traded that engine for GM’s Ramjet 350, a genuine fuel-injected crate V8 good for around 345 to 351 horsepower with real Vortec heads and a hydraulic roller cam. Bolted to a four-speed automatic, it turns a prewar sedan into a legitimate driver rather than a trailer queen. Here’s what makes this swap more than just another engine transplant.

1937 Plymouth Sedan Ramjet 350 CID V-8 4 Speed automatic

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Nobody expects a fendered 1937 Plymouth sedan to idle like a modern fuel-injected small-block — until you pop the hood and find GM’s Ramjet 350 crate engine sitting where a flathead six used to live. This isn’t a resto job chasing factory-correct numbers; it’s a builder’s statement that a prewar sedan can still get up and move when something built this century is doing the work. The Ramjet swap brings real port fuel injection, a hydraulic roller cam, and Vortec-style heads to a body that debuted the same year Amelia Earhart disappeared over the Pacific. Bolted to a four-speed automatic, it turns a car that once struggled to keep pace with modern traffic into something that can actually hang. What’s it like driving a car built for gravel roads with an engine built for the 21st century?

The Ramjet 350 Is a Real GM Crate Engine, Not a Frankenstein Build

GM Performance’s Ram Jet 350 is a genuine catalog crate engine, rated at roughly 345 to 351 horsepower and 396 lb-ft of torque, built around a cast-iron block with Vortec-style cylinder heads and a hydraulic roller camshaft. It comes with its own fully calibrated ECM and wiring harness, and the fuel-injected Ram Jet-style intake was designed specifically with swaps like this one in mind — classic looks up top, modern reliability underneath. That’s what separates a build like this from a garage-special carbureted swap: everything from the injection to the ignition timing is dialed in from the factory.

Why Prewar Sedans Make Unlikely Restomod Candidates

Dropping a modern crate V8 into a prewar Mopar sedan isn’t a new idea, but it never stops turning heads — there’s a real sleeper appeal to a car this old moving with this much confidence. Builders chasing this combination usually aren’t trying to win a concours judging sheet; they’re after a car that looks period-correct rolling into a show and then genuinely surprises people once it starts moving under its own power.

A Four-Speed Automatic That Actually Keeps Up

Pairing the Ramjet 350 with a modern four-speed automatic instead of a period-correct three-speed makes a real difference on the highway, keeping the engine in its power band without turning every on-ramp into an ordeal. The result is a nearly-90-year-old sedan that can actually hold its own in modern traffic — something the original flathead-powered version was never designed to do.

Plymouth’s Depression-Era Value Proposition

Plymouth was created specifically to give Chrysler a foothold in the low-price field, going head to head with Ford and Chevrolet at a time when affordability mattered more than almost anything else to American buyers. By 1937, the brand had built a reputation for offering solid engineering and a surprising amount of style for the money, which is exactly why sedans like this one found their way into so many driveways during one of the toughest economic stretches of the 20th century.

The Flathead Six That Powered This Car Originally

Before this car received its modern V8 heart, it would have left the factory with Plymouth’s dependable flathead inline-six, an engine prized more for its smoothness and durability than for outright power. That original six was never meant to impress with speed — it was built to run reliably for years on modest maintenance, which is a big part of why so many prewar Plymouths survived long enough to become popular resto-mod platforms decades later.

Why Builders Reach for a Small-Block Chevy

The Chevrolet 350 has become the default engine swap choice across almost every make and era of resto-mod, prewar Mopars included, simply because of how much aftermarket support exists for it. Parts are cheap, plentiful, and interchangeable, and generations of mechanics already know the engine inside and out, which makes a 350 swap dramatically easier and more affordable to execute — and to maintain afterward — than trying to build serious power out of a increasingly rare original flathead six.

Modern Fuel Injection Changes the Driving Experience

Swapping in a fuel-injected version of the 350 rather than a carbureted one solves a lot of the everyday annoyances that come with driving a resto-mod — no more fiddling with a choke on a cold morning, no flooding the carburetor on a hot restart, and noticeably better part-throttle drivability in traffic. For an owner who actually wants to drive a prewar sedan regularly rather than trailer it to shows, that difference in daily usability can matter as much as the horsepower gain itself.

Upgrades That Usually Come Along for the Ride

An engine swap of this scale rarely happens in isolation. Builders tackling a project like this typically upgrade the brakes to modern discs, stiffen up the front suspension, and rewire the entire car, since a 1937 chassis engineered around a modest flathead six was never designed to handle the power and speed a modern small-block V8 can deliver. Those unseen upgrades are often what separates a genuinely drivable resto-mod from one that just looks fast sitting still.

The Irony of a Mopar Body With a Chevy Heart

There’s a certain irony that doesn’t escape longtime Mopar loyalists: a Plymouth badge riding on a Chevrolet-sourced engine. But in the resto-mod world, that kind of cross-brand pragmatism is common practice rather than heresy — builders care more about reliability, parts availability, and driving experience than brand purity, and a 1937 Plymouth with a fuel-injected 350 represents exactly that trade-off, prioritizing a car that actually gets driven over one that stays strictly period-correct under the hood.

Value That Only Goes One Direction

A well-executed resto-mod like this tends to hold its appeal with a wider range of buyers than a strictly original, numbers-matching car — it draws interest from both classic car collectors and hot rod enthusiasts who want something they can actually drive hard without worrying about devaluing an irreplaceable original drivetrain. That crossover appeal has helped keep demand for quality prewar resto-mods strong even as the market for some other collector segments has cooled.

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