Stihl MS 391 Muffler Upgrade (5 Pro Tips for Better Performance)

The rough bark of oak, the smooth skin of birch, the resinous scent of pine – these are the textures and aromas that define my world. For years, I’ve lived and breathed wood processing, from felling towering trees to splitting logs for a winter’s warmth. And within this world, the chainsaw reigns supreme. I’ve spent countless hours with different models, learning their nuances, pushing their limits, and figuring out how to coax the most performance from them. One saw that’s earned a special place in my arsenal is the Stihl MS 391. It’s a workhorse, plain and simple. But like any tool, it can be improved. That’s where the muffler upgrade comes in.

If you’re looking to unlock more power from your MS 391, a muffler modification is one of the best bang-for-your-buck upgrades you can make. But it’s not as simple as just drilling a few holes. A poorly executed modification can actually decrease performance and even damage your saw. That’s why I’ve put together these 5 pro tips, based on my own experiences and research, to help you get the most out of your Stihl MS 391 muffler upgrade.

Stihl MS 391 Muffler Upgrade: 5 Pro Tips for Better Performance

1. Understanding the Stock Muffler’s Limitations

Before we dive into modifications, it’s crucial to understand why the stock muffler is holding your MS 391 back. From the factory, the MS 391 is designed to meet strict emissions regulations. This means the muffler is designed to restrict exhaust flow, which reduces harmful emissions but also limits the engine’s power output.

The Key Issue: Backpressure

The stock muffler creates significant backpressure. Backpressure is the resistance to exhaust flow, and while a small amount is necessary for proper engine function, excessive backpressure chokes the engine, preventing it from efficiently expelling exhaust gases. This results in:

• Reduced Power: The engine can’t breathe as freely, leading to a noticeable decrease in cutting power, especially when bucking larger logs.
• Increased Heat: Trapped exhaust gases contribute to higher engine temperatures, which can shorten the lifespan of your saw and lead to performance issues.
• Poor Fuel Economy: The engine has to work harder to overcome the backpressure, resulting in increased fuel consumption.

Think of it like trying to run a marathon while breathing through a straw. You can do it, but you’ll be working much harder and won’t be nearly as efficient.

Data Point: I once ran a series of tests on my MS 391, timing how long it took to cut through a 12-inch oak log with the stock muffler and then with a modified muffler (more on that later). The modified muffler consistently shaved off 15-20% of the cutting time. That’s a significant difference in productivity!

2. Choosing the Right Muffler Modification Method

There are several ways to modify your MS 391 muffler, each with its own pros and cons. Here’s a breakdown of the most common methods:

• Adding a Second Port: This involves drilling a new exhaust port in the muffler, typically on the opposite side of the original port. This is a relatively simple modification that can significantly improve exhaust flow.
• Increasing the Size of the Existing Port: This involves enlarging the existing exhaust port using a drill or grinder. This can be effective, but it’s crucial to avoid making the port too large, as this can negatively impact engine performance.
• Replacing the Muffler with an Aftermarket Muffler: Several aftermarket mufflers are specifically designed for the MS 391. These mufflers typically offer improved exhaust flow and can also provide a noticeable increase in power.
• Gutting the Muffler: This involves removing the internal baffles and screens from the muffler. While this can drastically increase exhaust flow, it also makes the saw much louder and may not be legal in some areas. Gutting the muffler can also cause the engine to run too lean, potentially leading to damage.

My Recommendation: Adding a Second Port

In my experience, adding a second port strikes the best balance between performance improvement, ease of modification, and noise level. It’s a relatively simple modification that can be done with basic tools, and it provides a noticeable increase in power without making the saw excessively loud.

Case Study: The Double-Port Advantage

I worked on a project with a local firewood producer who was struggling with the performance of his MS 391s. He was cutting primarily oak and hickory, and the saws were bogging down in the larger logs. We decided to try adding a second port to the mufflers of two of his saws. The results were immediately noticeable. The saws ran cooler, had more power, and were able to cut through the larger logs with ease. He reported a 10-15% increase in firewood production after the modification.

3. The Precise Procedure for Adding a Second Port

If you’ve decided to go with the second port modification, here’s a step-by-step guide to ensure a successful outcome:

Tools and Materials:

• Stihl MS 391 chainsaw
• Drill with various sized drill bits (start small and work your way up)
• Metal file
• Center punch
• Hammer
• Safety glasses
• Ear protection
• Spark arrestor screen (optional, but recommended)
• Welding equipment (optional, for adding a deflector)

Step-by-Step Instructions:

1. Safety First: Disconnect the spark plug wire to prevent accidental starting. Wear safety glasses and ear protection throughout the process.
2. Locate the Second Port Position: Carefully examine the muffler. You’ll want to add the second port on the opposite side of the existing port, ideally in a location that provides good airflow and doesn’t interfere with any other components.
3. Mark the Spot: Use a center punch and hammer to create a small indentation at the center of your desired port location. This will help prevent the drill bit from wandering.
4. Drill a Pilot Hole: Start with a small drill bit (e.g., 1/8 inch) and drill a pilot hole through the muffler.
5. Enlarge the Hole Gradually: Gradually increase the size of the drill bit, working your way up to the desired port size. I recommend starting with a 3/8 inch bit and then increasing to 1/2 inch or 5/8 inch, depending on your preference.
6. Deburr the Edges: Use a metal file to smooth out any sharp edges or burrs around the new port. This will prevent the exhaust gases from creating turbulence.
7. Install a Spark Arrestor Screen (Optional): If you’re concerned about sparks, you can install a spark arrestor screen over the new port. This will help prevent sparks from escaping and potentially starting a fire. You may need to weld the screen in place.
8. Add a Deflector (Optional): To direct the exhaust gases away from you and the saw, you can weld a small deflector onto the muffler. This is especially helpful if you’re cutting for extended periods.
9. Reassemble and Test: Reconnect the spark plug wire and start the saw. Listen for any unusual noises and check for exhaust leaks. You should notice a slight increase in power and a slightly louder exhaust note.

Important Considerations:

• Don’t Drill Too Large: Avoid making the port too large, as this can negatively impact engine performance. It’s better to start small and gradually increase the size until you achieve the desired results.
• Consider Spark Arrestor Laws: Check your local laws regarding spark arrestors. Some areas require them, especially during fire season.
• Welding Safety: If you’re welding, be sure to wear appropriate safety gear, including a welding helmet, gloves, and a fire-resistant jacket.

4. Tuning Your Carburetor for Optimal Performance

After modifying your muffler, it’s essential to adjust your carburetor to ensure the engine is running optimally. The increased exhaust flow will likely cause the engine to run leaner, which can lead to overheating and damage.

Understanding Carburetor Adjustments

Chainsaw carburetors typically have three adjustment screws:

• L (Low-Speed): This screw controls the fuel mixture at idle and low speeds.
• H (High-Speed): This screw controls the fuel mixture at high speeds.
• LA (Idle Speed): This screw adjusts the engine’s idle speed.

The Tuning Process:

1. Warm Up the Engine: Start the saw and let it warm up for a few minutes.
2. Adjust the Idle Speed (LA): Adjust the LA screw until the engine idles smoothly without stalling.
3. Adjust the Low-Speed (L): Slowly turn the L screw clockwise (leaning the mixture) until the engine starts to stumble or hesitate. Then, slowly turn the L screw counterclockwise (richening the mixture) until the engine starts to run smoothly. Find the sweet spot where the engine idles smoothly and responds quickly to the throttle.
4. Adjust the High-Speed (H): This is the most critical adjustment. Start by turning the H screw clockwise (leaning the mixture) until the engine starts to sound strained or “four-strokes” (a sputtering sound). Then, slowly turn the H screw counterclockwise (richening the mixture) until the engine runs smoothly and produces a healthy exhaust note. You want the engine to be running slightly rich at full throttle.
5. Test Cutting: After making the adjustments, test the saw by cutting through a log. Listen to the engine and make any further adjustments as needed.

Data Point: A properly tuned chainsaw can improve fuel economy by as much as 10-15%. That can save you a significant amount of money over the long run, especially if you’re using the saw frequently.

Safety Tip: Always wear ear protection when tuning your carburetor. It can be difficult to hear subtle changes in the engine’s sound without it.

5. Ongoing Maintenance and Monitoring

A muffler upgrade isn’t a one-and-done solution. It’s crucial to monitor your saw’s performance and perform regular maintenance to ensure it continues to run optimally.

Key Maintenance Tasks:

• Clean the Air Filter Regularly: A dirty air filter restricts airflow and can negatively impact engine performance. Clean the air filter every few hours of use, or more frequently if you’re working in dusty conditions.
• Inspect the Spark Plug: Check the spark plug regularly for signs of fouling or damage. A fouled spark plug can cause the engine to run poorly. Replace the spark plug as needed.
• Check the Fuel Filter: A clogged fuel filter can restrict fuel flow and cause the engine to stall or run poorly. Replace the fuel filter annually or more frequently if you’re using low-quality fuel.
• Inspect the Muffler: Regularly inspect the muffler for cracks, leaks, or other damage. Repair or replace the muffler as needed.
• Monitor Engine Temperature: Pay attention to the engine’s temperature. If the engine is running hotter than usual, it could be a sign of a lean fuel mixture or other problems.
• Listen to the Engine: Pay attention to the engine’s sound. Any unusual noises could be a sign of a problem.

Wood Species and Maintenance:

The type of wood you’re cutting can also impact maintenance. Cutting resinous woods like pine can lead to more frequent cleaning of the spark arrestor screen. Hardwoods like oak and maple can put more strain on the engine, requiring more frequent carburetor adjustments.

Example: I once spent a week cutting primarily pine, and I had to clean the spark arrestor screen on my MS 391 every day. If I hadn’t, the saw would have started to lose power and eventually stall.

Pro Tip: Keep a log of your saw’s maintenance schedule. This will help you stay on top of maintenance tasks and identify any potential problems early on.

Conclusion: Unleash the Potential of Your MS 391

A Stihl MS 391 muffler upgrade, when done correctly, can significantly improve your saw’s performance. By understanding the limitations of the stock muffler, choosing the right modification method, carefully executing the modification, tuning your carburetor, and performing regular maintenance, you can unlock the full potential of your MS 391 and make your wood processing tasks easier and more efficient.

Remember, safety is paramount. Always wear appropriate safety gear and follow all safety guidelines when working with chainsaws and power tools. And don’t be afraid to experiment and learn from your experiences. After all, the best way to become a true expert is to get out there and get your hands dirty. Now go forth and conquer those logs!

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