Poly 318 to 390 Stroker Conversion

The allure of increased displacement and power is a common desire among automotive enthusiasts. When it comes to classic Mopar vehicles, the venerable Poly 318 engine, often found in the 1963 A-block, presents an interesting platform for modification. Many owners wonder if this compact V8 can be transformed into a more potent stroker engine, specifically a 390 cubic inch variant. The answer is a resounding yes, but it requires careful planning, quality components, and a thorough understanding of the process. This article will delve into the feasibility of a Poly 318 to 390 stroker conversion, covering the essential steps, critical considerations for a quality rebuild, and important details regarding hardware like Grade 8 bolts.

Understanding the Poly 318 Engine

The Chrysler 'A' engine, commonly known as the Poly 318, is a distinctive V8 engine that powered many Mopar vehicles from the mid-1950s through the early 1960s. Its unique polyspherical combustion chamber design set it apart from contemporary engines. While often overshadowed by its more common wedge-head siblings, the Poly 318 is a robust and capable engine that can serve as an excellent foundation for a performance build. The information provided indicates that this conversion is indeed feasible, with detailed series available for rebuilding Poly 318 engines, including performance modifications and the building of stroker variants like the 390 or even a 402.

The Stroker Concept Explained

A 'stroker' engine is one where the stroke length of the crankshaft has been increased, thereby increasing the engine's displacement. This is typically achieved by using a crankshaft with a longer throw and often matching connecting rods and pistons. The increased stroke, combined with the existing bore, results in a larger cubic inch displacement, which generally translates to more torque and horsepower. For the Poly 318, achieving a 390 cubic inch displacement involves a specific combination of aftermarket crankshaft, connecting rods, and pistons designed to work within the Poly engine's architecture.

Can a Poly 318 be Rebuilt into a 390 Stroker?

Yes, a 1963 Poly A-block 318 core engine can be rebuilt into a 390 stroker. The process involves sourcing a crankshaft with a longer stroke that is compatible with the Poly 318 block. This is not a simple bolt-in affair; it requires specialized components and precise machining. The key to this conversion lies in the aftermarket availability of stroker kits specifically designed for the Mopar Poly engines. These kits typically include a new crankshaft, connecting rods, and pistons that work together to achieve the desired displacement increase. The information confirms the existence of detailed series covering this exact conversion, indicating that it's a well-established modification within the Mopar community.

Key Components for a 390 Stroker Conversion

To successfully convert a Poly 318 to a 390 stroker, you will need the following essential components:

1. Stroker Crankshaft

This is the heart of the stroker build. You'll need a crankshaft with a stroke length that, when combined with the Poly 318's bore and a suitable piston compression height, results in 390 cubic inches. These crankshafts are typically made from forged steel for increased strength and durability.

2. Connecting Rods

Matching connecting rods are crucial. These will need to be the correct length to work with the longer stroke crankshaft and the chosen pistons to achieve the desired compression ratio and deck height. High-quality H-beam or I-beam connecting rods are recommended for performance applications.

3. Pistons

Custom or shelf pistons designed for the stroker combination are necessary. These pistons will have a specific compression height to ensure the piston sits at the correct level in the cylinder bore at Top Dead Center (TDC). Piston material (cast, hypereutectic, or forged) will depend on the intended use and power levels.

4. Gaskets and Seals

A complete engine gasket set is essential for any rebuild. Ensure it's compatible with your Poly 318 block and the modifications you're making.

5. Bearings

New crankshaft and connecting rod bearings are a must for any engine rebuild. Proper bearing clearance is critical for oil flow and engine longevity.

6. Camshaft and Valvetrain

While not strictly part of the displacement increase, a camshaft upgrade is highly recommended to take advantage of the increased displacement and improve performance. This will likely involve matching lifters, pushrods, and possibly valves and valve springs.

Ensuring a Quality Engine Rebuild

A quality engine rebuild is paramount for the longevity and performance of your stroker engine. The provided information highlights a crucial aspect: cleanliness. An extremely clean assembly area is non-negotiable. Here's how to ensure a quality rebuild:

• Meticulous Cleanliness: Keep your assembly area as spotless as possible. Contamination from dirt, dust, or debris can lead to premature engine failure.
• Part Protection: Bag and cover the engine block, crankshaft, pistons, and all other internal components whenever you are not actively working on them. This prevents airborne contaminants from settling on precision surfaces.
• Separation of Processes: Crucially, separate machining and preparation procedures from the assembly area. Have your block bored, honed, and the crankshaft ground in a dedicated machine shop. Do not allow machined parts or metal shavings into your assembly space. This segregation prevents the introduction of machining debris into the clean assembly environment.
• Attention to Detail: Torque all fasteners to the manufacturer's specifications. Measure clearances meticulously (e.g., bearing clearances, piston-to-wall clearance, ring gap).
• Quality Components: Use high-quality parts from reputable manufacturers. Don't cut corners on critical components like bearings, rings, or gaskets.
• Proper Lubrication: Use assembly lube liberally on all bearing surfaces, camshaft lobes, lifters, and piston skirts during assembly.

What is a Grade 8 Bolt?

In the context of engine building, understanding fastener grades is important. The information mentions that factory bolts often have a tensile strength rating of 160,000 psi, which is the standard for Grade 8 bolts. While specific head bolts might have unique features like reduced shanks and shoulders for tighter tolerances, a quality Grade 8 bolt is considered a suitable and robust option for many engine applications, even if it's not the factory original specification. Grade 8 bolts are made from medium carbon alloy steel, quenched and tempered, making them significantly stronger than Grade 5 bolts. For critical applications like head bolts or main bearing cap bolts, using fasteners that meet or exceed the original strength specifications is advisable. If you are not using the factory bolts, ensuring you use high-quality Grade 8 fasteners is a wise choice for reliability.

Potential Challenges and Considerations

While the conversion is achievable, there are several factors to consider:

1. Block Machining

The Poly 318 block will need to be machined to accommodate the longer stroke crankshaft. This may involve ensuring sufficient clearance for the connecting rod bolts and the crankshaft counterweights. The block may also need to be bored and honed to match the new pistons.

2. Piston-to-Valve Clearance

With a longer stroke, the piston travels higher in the cylinder bore. It's crucial to ensure there is adequate clearance between the piston tops and the valves at all points in their travel to prevent catastrophic contact.

3. Deck Height

Achieving the correct deck height (the distance from the crankshaft centerline to the top of the cylinder block) is vital for setting the correct compression ratio and piston-to-head clearance. This is influenced by the crankshaft stroke, connecting rod length, and piston compression height.

4. Balancing

Increasing the displacement and using different internal components will alter the engine's rotating and reciprocating assembly balance. A professional engine balancing service is highly recommended to ensure smooth operation and prevent excessive vibration.

5. Oil Clearance and Lubrication

Ensure that all oil clearances are within specification for the new components. Proper oil flow and lubrication are critical, especially with the increased stresses of a stroker engine.

Performance Expectations

A 390 cubic inch Poly engine built from a 318 core will offer a significant increase in torque and horsepower over the stock 318. The larger displacement allows the engine to breathe more freely and generate more power, particularly at lower RPMs. When combined with appropriate camshaft selection, cylinder head porting, and a well-tuned carburetion and ignition system, the stroker Poly 318 can be a potent powerplant for a classic Mopar.

Frequently Asked Questions

Q1: Is a Poly 318 to 390 stroker conversion a common modification?

Yes, while not as common as wedge engine builds, stroking the Mopar Poly engines, including the 318, is a recognized modification within the Mopar enthusiast community, with specialized parts available.

Q2: What are the main benefits of stroking a Poly 318?

The primary benefits are increased torque and horsepower due to the larger displacement, leading to improved acceleration and overall engine performance.

Q3: What kind of power can I expect from a 390 stroker Poly 318?

With proper tuning and supporting modifications, a 390 stroker Poly 318 can easily produce 350-450 horsepower and significantly more torque than a stock 318, depending on the camshaft, compression ratio, and other components used.

Q4: Do I need special tools for this conversion?

Standard engine building tools are required, along with precision measuring instruments like micrometers and bore gauges. Professional machining services are also essential.

Q5: Can I use a different Mopar V8 crankshaft?

While some Mopar crankshafts are physically similar, a stroker kit specifically designed for the Poly 318 is highly recommended to ensure proper fitment, clearances, and performance.

Conclusion

Transforming a Poly 318 into a 390 stroker is a rewarding project that can breathe new life into a classic Mopar engine. By understanding the necessary components, adhering to strict rebuild procedures, and paying close attention to details like fastener quality and cleanliness, you can achieve a powerful and reliable engine. The availability of specialized stroker kits and detailed guides makes this conversion more accessible than ever for dedicated enthusiasts looking to maximize the potential of their Poly 318.

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