Unlocking the 351M/400: Cylinder Head Guide

The Ford 351M/400 engine, often an unsung hero in the Ford engine lineup, possesses significant untapped potential for performance enthusiasts. While it might not have the same widespread recognition as some of its contemporaries, with the right modifications, this engine can be transformed into a formidable powerhouse. This guide delves into the critical aspects of enhancing your 351M/400, with a particular focus on cylinder head choices and how they impact overall performance. We'll explore how to increase compression, the benefits of different cylinder head configurations, and other key upgrades that will allow your engine to truly 'scream'.

The Importance of Compression Ratio

One of the most impactful modifications you can make to your 351M/400 is to increase its compression ratio (CR). Stock 351M/400 engines typically came with compression ratios ranging from 8.0:1 to 8.4:1, with the exception of the 1971 400 which boasted a 9.1:1 CR. While these figures were conservative by modern standards, especially when compared to the muscle car era, they limit the engine's potential. Increasing the CR, generally advisable to stay around 9.5:1 for pump gasoline (92/93 octane), offers significant advantages:

• More Power: A higher compression ratio means the air-fuel mixture is compressed more tightly before ignition. This increased density leads to a more powerful explosion, translating directly into more horsepower. For instance, increasing the CR by one full point (e.g., from 8.5:1 to 9.5:1) can compress the charge by nearly 12%, significantly boosting energy output.
• Better Fuel Efficiency: In theory, a more efficient engine that produces more power for the same amount of fuel consumed will also be more fuel-efficient. By extracting more energy from each combustion cycle, the engine works less at any given RPM to achieve the same output, potentially improving mileage.

Methods to Increase Compression Ratio

There are three primary methods to raise the compression ratio of your 351M/400:

1. Shaving (Milling or Decking) Cylinder Heads

Cylinder heads can warp slightly over time. A standard procedure during a quality rebuild involves milling a small amount of material, typically around 0.010 inches, from the head's mating surface to ensure a perfect seal against the engine block. This milling process also has the effect of reducing the combustion chamber volume, thereby increasing the compression ratio. Milling 0.010" can increase the CR by approximately 2.3%. The maximum practical amount to mill from stock 351M/400 heads is around 0.025", which could increase the CR by about 5.75%.

2. Installing Different Cylinder Heads

The 351M/400 engines are part of Ford's 335 engine family. This means that cylinder heads from any engine within the 335 series are directly interchangeable. This opens up a range of options for cylinder heads with smaller combustion chamber volumes (CC), which will naturally increase the CR. Here's a look at some key options:

Analysis of Head Options:

• Australian 302 2V Heads: These are often considered the holy grail for 351M/400 builds due to their small 62.0cc combustion chambers. Bolting these on can yield CRs of 10.75:1 for a 400 or 10.12:1 for a 351M. However, they are exceedingly rare and command a high price.
• 351C 2V Heads: These offer a modest increase in CR. If milled by 0.025", they can provide a similar CR bump to milling stock heads by 0.010", making the swap less compelling unless other benefits are sought.
• 351C 4V Heads: While offering a decent CR increase, the larger intake and exhaust ports (2.19" intake vs. 2.04" for 2V) are optimized for very high RPMs and may sacrifice low-end torque, which is often crucial for truck applications.
• 351C CJ 4V Heads: These provide a substantial CR increase and are a good option for performance builds. However, their high-RPM focus might not be ideal for all 351M/400 applications, particularly those prioritizing low-end grunt.

Calculating Static Compression Ratio: The formula is: (Swept Volume / Clearance Volume) + 1 = CR. For a more user-friendly approach, a spreadsheet calculator is available (referencing the original article for its location).

3. Installing High Compression Pistons

Another method to increase CR is by fitting pistons with a smaller dish or even a flat-top design. Unfortunately, aftermarket options for 351M/400 are limited, often requiring custom pistons at a significant cost.

• Custom Pistons: Companies like J&E and Keith Black can manufacture custom pistons to your exact CR specifications, but expect to pay upwards of $700 and wait several weeks.
• 351M Specific: The TRW L2466F is a flat-top forged aluminum piston offering 8.6:1 CR in standard bore and 8.7:1 in 0.030" overbore for a 351M with stock heads.
• 400 Specific: The 1971 400 used a desirable flat-top piston. Badger Pistons by Dyna Gear and Ohio Piston & Pin both offer 9:1 CR flat-top pistons for the 400, providing a cost-effective way to boost compression. Combining these with head milling can easily achieve 9.5:1 CR or higher.

Camshaft and Valve Train Upgrades

The stock camshaft and timing chain in the 1978-79 351M/400 engines were designed to meet emissions standards and are generally considered inadequate for performance. Upgrading these components offers a significant improvement:

• Aftermarket Camshafts: High-performance camshafts from reputable manufacturers like Crane Cams and Comp Cams can dramatically improve engine response and power. Modern dual-profile or roller camshafts offer excellent benefits, though they can be pricey. Even a quality aftermarket camshaft kit will be a substantial upgrade over the stock setup.
• Recommended Cam: The Comp Cams 255 DEH Dual Energy cam is a good choice for a balance of performance and drivability, with a usable RPM range of 1000-5500 RPM.
• Complete Kits: It's highly recommended to purchase a matched valve train kit (e.g., Comp Cams K-kit P/N K32-206-3) that includes lifters, springs, retainers, and a dual-roller timing chain set. Do not mix and match components.
• Aggressive Option: The Comp Cams X-Treme Energy #268 camshaft, used in a Hot Rod Magazine build, yielded an impressive 382 HP and 456 ft-lbs of torque with a usable RPM range of 2000-5200 RPM.
• Roller Rocker Arms: Upgrading to roller rocker arms, such as the Comp Cams 1.72 ratio Magnum Roller rocker arms (P/N 1411-16), can provide an additional 15-30 horsepower. This upgrade requires converting to hardened pushrods, screw-in studs, and guide plates.

Intake and Carburetion

Pairing a performance camshaft and dual exhaust with the restrictive stock 2V carburetor is a common mistake. A proper intake and carburetion setup is crucial:

• Intake Manifolds: Dual-plane intake manifolds are recommended for optimal street performance. Options include the Edelbrock Performer 400 EGR (P/N 3771) or the Weiand Action +Plus (P/N 8010 or P/N 8013 for EGR compatibility).
• Carburetors: Selecting the correct carburetor size is vital. Over-carbing can hinder performance. A general guideline for CFM (Cubic Feet per Minute) can be calculated, but for high-performance 400 CID engines, a 750 CFM carburetor is often suitable. For 351M and milder 400 builds, a 600-650 CFM carburetor is typically sufficient. Popular choices include Edelbrock (e.g., 1406 600 CFM, 1411 750 CFM) and Holley (e.g., 80457S 600 CFM, 1850S 600 CFM, 80508S 750 CFM). Tuning may be necessary for optimal performance.

Adding More Cubic Inches

More displacement generally means more power. While the 351M/400 platform is substantial, there are ways to increase its cubic inch displacement (CID):

• Bore and Stroke: The 400 has a 4.0" bore and 4.0" stroke, while the 351M has a 4.0" bore and 3.5" stroke. The formula for calculating CID is: R² × Pi × H × number of cylinders = CID.
• Increasing Bore: For the 351M/400, the bore can typically be increased by a maximum of 0.040 inches. This can take a 351M to approximately 359 CID.
• Crankshaft Swap (351M to 400): A significant upgrade for 351M owners is to swap in a 400 crankshaft. This transforms the 351M into a 400 CID engine. Key considerations for this swap:
  • Pistons: You MUST use 400 pistons with a 400 crankshaft. Using 351M pistons will result in valve-to-piston contact.
  • Harmonic Balancer: It's best to use a 400 harmonic balancer. While a 351M balancer can be used, the crankshaft will need rebalancing, and timing marks will be inaccurate.
  • Connecting Rods: The 351M and 400 share the same connecting rods, simplifying the swap.
  • Cost: Be prepared for an additional labor charge from engine shops for this modification, typically around $100.

  With a 0.030" overbore and a 400 crank, a 351M can reach approximately 408 CID.

Conclusion

By carefully selecting and implementing upgrades focused on compression ratio, camshaft and valve train, intake and carburetion, and potentially increasing displacement, the Ford 351M/400 engine can be transformed into a formidable performer. These modifications, when done correctly, offer a cost-effective path to significant power gains, often outperforming more expensive 'crate motor' options. The example build discussed, costing around $3,400 for all parts, machining, and labor, demonstrates that achieving a truly potent engine is well within reach.

Frequently Asked Questions (FAQs)

Q1: Can I use 351 Cleveland heads on a 351M/400?Yes, all heads from the Ford 335 engine family, including 351 Cleveland heads, are interchangeable with the 351M/400.

Q2: What is the maximum compression ratio I can safely run on pump gas?For 92/93 octane pump gasoline, it's generally advisable to aim for a compression ratio around 9.5:1 to avoid detonation (pinging).

Q3: Is swapping a 400 crankshaft into a 351M worth the effort?Yes, this is a highly effective way to increase displacement and power. However, ensure you use the correct 400 pistons and consider the harmonic balancer and balancing requirements.

Q4: Which cylinder heads are best for a 351M/400 build?The Australian 302 2V closed-chamber heads offer the highest potential CR. If those are unavailable, consider 351C heads, paying attention to their port sizes and intended RPM range.

Q5: How much can I mill off my stock 351M/400 heads?The maximum practical amount to mill is typically 0.025 inches, which can increase the compression ratio by approximately 5.75%.

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