The Ford 351M V8 Engine

The Ford 351M V8 engine, introduced in the mid-1970s, was designed as a robust and durable powerplant tailored for Ford’s trucks and large passenger vehicles. Part of the 335 engine family, the “M” stands for “Modified,” highlighting its adaptation from the 351 Cleveland to meet the stricter emissions standards of the era. Known for its strong low-end torque, the 351M was a workhorse engine, ideal for heavy-duty applications such as towing and hauling. Though not celebrated for high performance, it played a crucial role in Ford’s lineup during a transitional period in automotive history, providing reliable power in a challenging regulatory environment.

Ford 351M V8 Engine Specifications:
  • Engine Type: Naturally aspirated V8
  • Displacement: 351 cubic inches (5.8 liters)
  • Bore x Stroke: 4.00 in x 3.50 in (101.6 mm x 88.9 mm)
  • Compression Ratio: 8.0:1
  • Valvetrain: OHV (Overhead Valve), 2 valves per cylinder
  • Fuel System: 2-barrel carburetor (Motorcraft 2150)
  • Horsepower: 137 hp at 3,600 rpm
  • Torque: 262 lb-ft at 1,800 rpm
  • Block Material: Cast Iron
  • Head Material: Cast Iron
  • Cooling System: Water-cooled
  • Fuel Type: Regular unleaded gasoline
  • Ignition System: Distributor-based ignition
  • Oil Capacity: Approximately 5 quarts (4.7 liters)
  • Spark Plug Gap: 0.044 inches (1.1 mm)
Key Differences Between the 351M and Standard 351 Engines:
  1. Design Origins:
    • 351 Windsor (351W): The 351 Windsor is part of the Windsor engine family, which features a small-block design. It has a shorter deck height and is more compact than the 351M. The 351W was designed for both performance and versatility, making it popular in a wide range of Ford vehicles.
    • 351 Cleveland (351C): The 351 Cleveland belongs to Ford’s 335 engine family, sharing some design elements with the 351M. The Cleveland is known for its large, canted-valve heads, which offer better high-RPM performance.
  2. Deck Height and Block Design:
    • 351M: The 351M has a taller deck height (10.297 inches) compared to the 351 Windsor (9.500 inches). This allows for the use of longer connecting rods and a different intake manifold. The 351M block is wider and heavier than the Windsor, making it more suitable for trucks and large cars where durability is prioritized over weight savings.
    • 351 Windsor: The 351 Windsor has a shorter deck height, which results in a more compact and lighter engine. It’s favored in performance applications due to its lighter weight and higher revving capability.
  3. Cylinder Heads:
    • 351M: The 351M uses cylinder heads with large combustion chambers (typically around 77-78cc) to accommodate lower compression ratios. These heads are derived from the 351 Cleveland’s 2V heads, offering good low-end torque but limiting high-RPM performance.
    • 351 Windsor: The Windsor heads have smaller combustion chambers and valve sizes, which contribute to better airflow at higher RPMs and make it more suitable for performance applications.
  4. Intended Use:
    • 351M: The 351M was designed primarily for use in trucks and large passenger cars. Its focus was on durability, torque, and low-end power rather than high-revving performance. This made it well-suited for towing and heavy-duty applications.
    • 351 Windsor: The 351W was used in a variety of vehicles, including performance cars like the Mustang, where its lighter weight and higher-revving nature were advantageous.
  5. Carburetion and Fuel Delivery:
    • 351M: Typically equipped with a 2-barrel carburetor, focusing on efficiency and low-end torque rather than high power output.
    • 351 Windsor: Often equipped with either a 2-barrel or 4-barrel carburetor, with the 4-barrel versions providing better high-end performance.
Summary:

The Ford 351M V8 was a workhorse engine designed for trucks and larger vehicles where low-end torque and durability were more important than high-revving horsepower. It differs significantly from the 351 Windsor, which is more performance-oriented, and the 351 Cleveland, which is known for its high-RPM capability. While the 351M is heavier and less powerful than its counterparts, it was well-suited to the heavy-duty roles it was intended for.

Ford 351M History

The Ford 351M V8 engine, part of the 335 engine series, has a unique history that reflects Ford’s adaptation to changing market demands during the 1970s and early 1980s. Here’s an overview of its development, application, and eventual phase-out:

Origins and Development:
  • 1970s Oil Crisis and Emissions Regulations: The 1970s were a challenging time for automakers, with the oil crisis and tightening emissions regulations forcing companies to rethink their engine designs. Ford needed engines that could meet these new standards while still providing the power and durability expected by consumers, especially in the truck and large car segments.
  • 335 Engine Series: The 351M was part of Ford’s 335 engine family, which included the 351 Cleveland (351C) and the 400. The “M” in 351M stands for “Modified,” as the engine was a variation of the 351 Cleveland, with changes made to meet emissions standards and to provide more low-end torque for heavy-duty applications.
Introduction and Features:
  • 1975: The 351M was introduced in 1975 as a replacement for the 351 Cleveland and to complement the 400 V8. The engine was designed to be more suitable for trucks and larger cars, offering a balance between power and emissions compliance.
  • Design Characteristics: The 351M featured a tall deck height of 10.297 inches, which allowed for the use of longer connecting rods. It shared its bore size (4.00 inches) with the 400, but had a shorter stroke of 3.50 inches. The engine was typically equipped with a 2-barrel carburetor and had a low compression ratio of around 8.0:1, which was necessary to meet the emissions standards of the time.
Applications:
  • Trucks and Large Cars: The 351M was primarily used in Ford’s F-Series trucks, Broncos, and large passenger cars like the Ford LTD, Mercury Marquis, and Ford Thunderbird. The engine was favored for its durability and ability to generate good low-end torque, making it well-suited for towing and hauling.
  • Low Power Output: Despite its size, the 351M was not a high-performance engine. It was designed for efficiency and compliance with emissions standards, producing around 137 horsepower and 262 lb-ft of torque, which was modest compared to earlier V8 engines.
Market Reception and Decline:
  • Mixed Reception: The 351M’s performance was often criticized as being underpowered, especially in comparison to the 351 Windsor and other V8 engines. However, it was appreciated for its durability and ability to handle heavy-duty tasks.
  • Early 1980s: By the early 1980s, the automotive market was shifting again, with further advancements in emissions control and the introduction of new, more efficient engines. The 351M, along with the 400, was becoming outdated as Ford moved towards smaller, more efficient engines that could better meet the needs of the evolving market.
Discontinuation:
  • 1982: The 351M was phased out after the 1982 model year. It was effectively replaced by the more efficient and versatile engines in Ford’s lineup, such as the 302 Windsor (5.0L V8) and the new line of fuel-injected engines that would come to dominate the 1980s and beyond.
Legacy:
  • Niche Role: The 351M remains a somewhat niche engine in Ford’s history, often overshadowed by the more famous Windsor and Cleveland engines. However, it still has a following among enthusiasts who appreciate its role in the history of Ford’s truck and large car production.
  • Collector Interest: While not as sought after as other Ford V8s, the 351M is still of interest to collectors and restorers, particularly those focused on preserving the authenticity of classic Ford trucks and large cars from the late 1970s and early 1980s.

In summary, the Ford 351M V8 was a product of its time, designed to navigate the challenges of emissions regulations while providing the durability needed for heavy-duty applications. Though it may not have the performance pedigree of other Ford V8s, it played an important role in Ford’s lineup during a transitional period in automotive history.

351M Transmission Pairings

The Ford 351M engine, used primarily in trucks and large passenger cars from the mid-1970s through the early 1980s, was paired with several transmissions that complemented its design for durability and low-end torque. Here are the main transmissions that were commonly used behind the 351M:

Manual Transmissions:
  1. Tremec T-18 (T98)
    • Type: 4-speed manual transmission
    • Application: Commonly used in Ford trucks, especially F-250 and F-350 models.
    • Notes: The T-18 is known for its ruggedness and has a very low “granny” first gear, making it ideal for heavy-duty applications and off-road use.
  2. Tremec (Ford) NP435
    • Type: 4-speed manual transmission
    • Application: Used in F-Series trucks and Broncos.
    • Notes: Similar to the T-18, the NP435 is a heavy-duty transmission with a low first gear, designed for durability and towing.
Automatic Transmissions:
  1. C6 Automatic Transmission
    • Type: 3-speed automatic transmission
    • Application: Widely used in F-Series trucks, Broncos, and large passenger cars.
    • Notes: The C6 is a highly durable transmission, known for its strength and ability to handle high torque loads, making it a perfect match for the 351M. It was often the transmission of choice for vehicles equipped with the 351M due to its reliability and simplicity.
  2. C4 Automatic Transmission
    • Type: 3-speed automatic transmission
    • Application: Used in lighter-duty applications, including some Ford trucks and passenger cars.
    • Notes: While not as robust as the C6, the C4 was occasionally paired with the 351M in less demanding applications. However, the C6 was more common behind the 351M due to its greater torque capacity.
Summary:

The Ford 351M was typically paired with robust, heavy-duty transmissions that could handle the engine’s torque and the demands of towing and hauling. The C6 automatic and T-18 or NP435 manual transmissions were the most common pairings, providing durability and reliability in Ford trucks and large cars during the engine’s production years.

351M Modification, Building, and Performance

The Ford 351M is generally not considered the best candidate for building into a high-performance engine. Here are some key reasons why:

1. Design Limitations:
  • Low Compression Ratio: The 351M was designed with a low compression ratio (around 8.0:1) to meet emissions standards, which limits its power potential. To achieve higher performance, extensive modifications to the heads and pistons would be required.
  • Heavy and Tall Deck: The 351M has a taller deck height and a heavier block compared to the 351 Windsor, making it less desirable for performance applications where weight and compactness are critical.
  • Cylinder Heads: The 351M typically uses cylinder heads with large combustion chambers and smaller valves, which are not ideal for high-revving performance. These heads restrict airflow, limiting the engine’s ability to produce significant horsepower without extensive porting and custom work.
2. Aftermarket Support:
  • Limited Performance Parts: While some performance parts are available, the 351M doesn’t enjoy the same level of aftermarket support as more popular engines like the 351 Windsor or 302. This can make building a performance 351M more challenging and expensive.
  • Alternative Choices: For those seeking to build a performance Ford V8, the 351 Windsor (351W) or the 351 Cleveland (351C) are better starting points. These engines have a broader range of aftermarket parts, are lighter, and have a better track record in performance applications.
3. Cost vs. Benefit:
  • Investment: Building a 351M into a high-performance engine would require significant investment in custom parts, machining, and tuning. Given the engine’s inherent limitations, the performance gains might not justify the cost compared to starting with a more performance-oriented engine.
Conclusion:

While the 351M can be mildly modified to improve its power output, it is not the best foundation for a high-performance build. If you’re committed to using a 351M, focusing on enhancing its torque for heavy-duty use or mild street performance might be more practical. For serious performance enthusiasts, the 351 Windsor or 351 Cleveland are better platforms to build on, offering greater potential and better support.

More information on the Ford 351M engine is available on Wikipedia.

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