Hey gearheads! Ever found yourself scratching your head trying to figure out the firing order on a modified 351 engine? You're definitely not alone. Getting the firing order right is absolutely crucial for your engine to run smoothly, efficiently, and without any major hiccups. In this comprehensive guide, we'll dive deep into the world of the 351 modified firing order. We'll cover everything from understanding what it is and why it matters, to providing you with clear diagrams and step-by-step instructions. So, buckle up and let's get started!

Understanding Firing Orders

Let's kick things off by understanding what a firing order actually is. Simply put, the firing order is the sequence in which the cylinders in your engine ignite. Each cylinder needs to fire at a precise moment to ensure the engine runs properly. If the firing order is off, you'll likely experience issues like rough idling, reduced power, and, in some cases, engine damage. Why is this so important? Well, the firing order is designed to balance the forces within the engine, reducing vibration and ensuring smooth operation. When you modify an engine, you might need to adjust the firing order to optimize performance based on the new components and configurations.

Why Modify the Firing Order?

Now, why would you want to modify the firing order in the first place? Great question! Modifying the firing order, particularly on a 351 engine, is a common practice when making significant performance upgrades. For instance, if you've installed a new camshaft, the stock firing order might no longer be optimal. Aftermarket camshafts are often designed to work with a different firing order to improve torque and horsepower.

Another reason to modify the firing order is to reduce stress on the crankshaft. The stock firing order can sometimes create uneven loads, which can lead to premature wear and tear. By changing the firing order, you can distribute these loads more evenly, prolonging the life of your engine. Furthermore, a modified firing order can sometimes help with exhaust scavenging, improving the engine's overall efficiency. So, while it might seem like a small change, modifying the firing order can have a significant impact on your engine's performance and longevity. Remember, always consult with an experienced mechanic or engine builder before making any changes to your engine's firing order.

Common 351 Firing Orders

Alright, let's dive into the specifics of the 351 firing orders. The 351 engine family, which includes the 351 Windsor, 351 Cleveland, and 351 Modified (also known as 351M), has a few different firing order options. The most common are the stock firing order and the modified firing order, which is often the same as a 302 HO (High Output) engine.

Stock 351 Firing Order

The stock firing order for the 351 Windsor and 351 Cleveland is 1-5-4-2-6-3-7-8. This is the firing order that the engine was originally designed to use, and it works well for most stock or mildly modified applications. If you're unsure which firing order your engine is currently using, this is a good place to start.

Modified 351 Firing Order (302 HO Firing Order)

The modified firing order, often used with aftermarket camshafts and performance upgrades, is 1-3-7-2-6-5-4-8. This firing order is the same as the one used in the 302 High Output (HO) engines, and it's a popular choice for those looking to improve their 351's performance. Using this firing order can help to improve torque and horsepower, as well as reduce stress on the crankshaft. When should you consider switching to this firing order? Generally, if you've installed a performance camshaft designed for the 302 HO firing order, or if you're looking to optimize your engine's performance for racing or high-performance street use, this is a good option. Always make sure to verify compatibility with your specific engine modifications.

Diagrams and Visual Aids

Visual aids can be incredibly helpful when working with firing orders. Here are some diagrams to help you visualize the stock and modified 351 firing orders:

Stock 351 Firing Order Diagram (1-5-4-2-6-3-7-8)

[Insert Stock 351 Firing Order Diagram Here]

In this diagram, you'll see the cylinders numbered and the order in which they fire. This visual representation can make it easier to understand the sequence and ensure you're connecting the spark plug wires correctly.

Modified 351 Firing Order Diagram (1-3-7-2-6-5-4-8)

[Insert Modified 351 Firing Order Diagram Here]

Similarly, this diagram illustrates the modified firing order. Notice the difference in the sequence compared to the stock firing order. This diagram is essential when rewiring your engine for the modified firing order.

Distributor Cap Layout

[Insert Distributor Cap Layout Diagram Here]

Understanding the distributor cap layout is also crucial. The distributor cap has terminals that correspond to each cylinder. Knowing which terminal corresponds to which cylinder in the firing order is essential for proper installation. Use this diagram to correctly connect the spark plug wires to the distributor cap.

Step-by-Step Guide to Changing the Firing Order

Changing the firing order might seem daunting, but with the right tools and a systematic approach, it's a manageable task. Here’s a step-by-step guide to help you through the process:

Step 1: Preparation

Before you start, gather all the necessary tools and materials. You'll need:

• A socket set
• A wrench set
• Screwdrivers
• Spark plug wire separators
• A timing light
• New spark plug wires (if needed)
• The firing order diagrams (stock and modified)

Make sure your engine is cool before you begin to avoid burns. Disconnect the negative battery cable for safety.

Step 2: Identify the Current Firing Order

If you're not sure which firing order your engine is currently using, there are a couple of ways to find out:

• Check the Engine Specifications: Look up the original specifications for your engine. This will tell you the stock firing order.
• Inspect the Distributor Cap: Carefully examine the distributor cap and the way the spark plug wires are connected. Compare the arrangement to the stock and modified firing order diagrams to determine which one is currently in use.

Step 3: Disconnect the Spark Plug Wires

Once you've identified the current firing order, disconnect the spark plug wires from the distributor cap and the spark plugs. It's a good idea to label each wire as you remove it to avoid confusion later. You can use masking tape and a marker to label each wire with its corresponding cylinder number. This will make reassembly much easier.

Step 4: Re-Route the Spark Plug Wires

Now, re-route the spark plug wires according to the new firing order diagram. Start by connecting the wire for cylinder number one to the correct terminal on the distributor cap, then proceed with the remaining wires in the correct sequence. Make sure the wires are securely connected to both the distributor cap and the spark plugs. Use spark plug wire separators to keep the wires organized and prevent them from touching each other.

Step 5: Verify the Distributor Cap Position

Ensure the distributor cap is properly aligned. The distributor rotor should be pointing to the terminal for cylinder number one when the engine is at top dead center (TDC) on the compression stroke. If it's not, you may need to adjust the distributor position.

Step 6: Double-Check Your Work

Before you reconnect the battery and start the engine, double-check all your connections. Make sure each spark plug wire is securely connected to the correct terminal on the distributor cap and the correct spark plug. Verify that the wires are properly routed and not touching any hot engine components.

Step 7: Start the Engine and Check Timing

Reconnect the negative battery cable and start the engine. If the firing order is correct, the engine should start smoothly and run without any unusual noises or vibrations. Use a timing light to check the engine's timing and make any necessary adjustments. Proper timing is essential for optimal performance and fuel efficiency.

Troubleshooting

Even with careful planning and execution, you might encounter some issues when changing the firing order. Here are some common problems and how to troubleshoot them:

Engine Won't Start

If the engine won't start after changing the firing order, the most likely cause is an incorrect wiring configuration. Double-check the spark plug wires to ensure they are connected to the correct terminals on the distributor cap and the correct spark plugs. Also, make sure the distributor cap is properly aligned and the distributor rotor is pointing to the terminal for cylinder number one when the engine is at TDC on the compression stroke.

Rough Idling

Rough idling can also be a sign of an incorrect firing order. If the engine starts but idles roughly, re-examine the spark plug wire connections and make sure they match the firing order diagram. Another possible cause of rough idling is a vacuum leak. Check all vacuum hoses and connections to make sure they are tight and free from cracks or damage.

Reduced Power

If you notice a significant reduction in power after changing the firing order, it could be due to improper timing. Use a timing light to check the engine's timing and make any necessary adjustments. Also, make sure the spark plugs are in good condition and properly gapped. Worn or improperly gapped spark plugs can reduce engine power and fuel efficiency.

Conclusion

Changing the firing order on a 351 engine can be a rewarding upgrade, especially when combined with other performance modifications. By understanding the different firing orders, using the diagrams provided, and following the step-by-step guide, you can ensure your engine runs smoothly and efficiently. Remember to always double-check your work and troubleshoot any issues that may arise. With a little patience and attention to detail, you'll have your 351 engine running at its best! Happy wrenching, guys! And remember to always consult with a professional if you're unsure about any part of the process.