The Ford 351 Modified, often referred to as the 351M, is a V8 engine that stirs both curiosity and debate among performance enthusiasts. While not as revered as its Cleveland or Windsor siblings, the 351M has its place in the world of classic trucks and muscle-era survivors. But just how much horsepower does a 351 Modified actually have?
Overview of the Ford 351 Modified Engine
The 351 Modified was introduced by Ford in 1975 as part of the 335 engine family. Designed primarily for full-size trucks and large sedans, it was produced through the early 1980s.
Unlike the 351 Windsor, the 351M shares its architecture with the 400 engine, using a tall-deck block. It came with low-compression cast pistons and large combustion chamber heads. The factory setup prioritized durability and torque over outright horsepower.
Factory Horsepower Ratings of the 351M
The 351M was developed during a time when automakers were transitioning from gross to net horsepower ratings. According to factory specifications, most 351M engines produced between 137 and 150 horsepower, with torque figures ranging from 260 to 280 lb-ft.
These ratings reflected the engine's purpose: to provide steady, low-end torque for trucks rather than high-revving power for performance cars. Emissions restrictions further limited power output during this era.
Factors That Limit the 351M's Stock Performance
The biggest limitations on the 351M come from its emissions-era design. The engine uses a low compression ratio of 8.0:1 and a two-barrel carburetor, both of which suppress power.
Its factory heads have restrictive ports and valves, while camshaft profiles are conservative. Combined with heavy cast components and inefficient exhaust manifolds, the 351M struggles to breathe freely in stock form.
Can the 351M Make More Horsepower? Yes — Here's How
If you're looking to make your 351M more powerful, several key modifications can significantly boost its horsepower. By upgrading specific components, you can unlock its performance potential and transform it from a stock engine into a more robust performer. Here are the main modifications to consider:
Install Headers and High-Flow Exhaust
Swapping out the stock exhaust manifolds for 351M headers and a high-flow exhaust system reduces backpressure and allows exhaust gases to exit more freely.
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Benefit: Better exhaust flow improves overall engine efficiency and increases horsepower.
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Expected Gains: Expect an increase of 15–30 horsepower.
Upgrade the Cylinder Heads
The stock cylinder heads on the 351M are quite restrictive. Replacing them with high-performance aftermarket heads, can drastically improve airflow and increase horsepower.
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Benefit: Improved airflow leads to better combustion and more power.
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Expected Gains: Upgrading heads can add 30–50 horsepower.
Install a Performance Camshaft
The factory camshaft is designed for efficiency, not performance. A high-performance camshaft can improve airflow by increasing valve lift and duration.
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Benefit: Better valve timing allows the engine to breathe more efficiently, boosting power.
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Expected Gains: This can add 40–60 horsepower.
Upgrade the Intake Manifold and Carburetor
Installing a high-flow intake manifold and a larger 4-barrel carburetor allows for better fuel and air delivery to the engine.
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Benefit: More efficient fuel-air mixture leads to improved power at higher RPMs.
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Expected Gains: This upgrade can add 30–50 horsepower.
Building a High-Performance 351M: What's Possible?
If you're aiming higher, a full rebuild can unlock serious potential. Forged pistons, ported heads, a stroker kit, and a custom-ground camshaft can transform the engine.
Performance shops and builders report builds achieving 350 to 400 horsepower, though this comes at a cost. Increasing compression, improving airflow, and tuning the ignition system are critical steps in achieving this level of output.
Is Modifying the 351M Worth It?
Whether the 351M is worth modifying depends on your goals. If you own a classic F-Series or a Ranchero and want to retain the original block, it makes sense. The 351M offers solid low-end torque and a unique powerplant character.
However, if your goal is maximum horsepower per dollar, alternatives like the 351 Windsor or 460 big-block may offer better value. The aftermarket support and ease of power production make them popular swap candidates.
Still, for enthusiasts seeking a challenge or wanting to preserve their Ford heritage, a well-built 351 Modified can be a satisfying project engine.
