I want to get more power out of my lawnmower engine so I can improve its performance. There are a few key steps I need to take to rewire the engine properly and safely.
Understanding Engine Rewiring Basics
Before I start tearing into the engine, I need to understand some basics about small engines and rewiring.
How Small Engines Work
Small gasoline engines like those in lawnmowers have a simple design. Here's a quick overview:
- The carburetor mixes air and fuel to create an air-fuel mixture. This mixture enters the cylinder through the intake valve.
- The air-fuel mixture is compressed by the rising piston.
- The spark plug ignites the compressed mixture with an electric spark, causing combustion.
- The rapid expansion of gases from combustion forces the piston down, providing power to turn the crankshaft.
- The exhaust valve then opens to allow the exhaust gases to exit through the muffler.
So the key components involved in making power are the carburetor, cylinder, piston, valves, spark plug, and crankshaft.
Basic Engine Rewiring
The ignition system provides the spark to ignite the air-fuel mixture. This includes:
- The magneto, which generates electricity to power the spark plug.
- The ignition coil, which steps up the magneto's voltage to thousands of volts for the spark plug.
- The spark plug wire connecting the ignition coil to the spark plug.
To get more power, I may need to bypass or replace some ignition components. But I need to be careful not to exceed the coil's voltage capacity to avoid damaging it.
Evaluating My Engine
Before making any modifications, I should thoroughly evaluate my engine to decide the best approach.
Engine Specs and Condition
I need to look up my engine's specifications, including:
- Engine model and type: This will tell me the stock performance specs to compare against.
- Displacement: The size of the engine in cc's impacts how much power it can make.
- Ignition system: Whether it has points, electronic ignition, or a magneto.
I also need to assess the general condition of the engine. Problems like low compression or air leaks will hamper performance.
Current Performance
I should get baseline measurements of the engine's current performance, including:
- Horsepower and torque readings using a dynamometer. This will quantify any power gains after rewiring.
- Exhaust gas temperatures, measured with a probe in the muffler exhaust. Higher temps usually indicate more power.
- Acceleration tests with a tool like an inductive tachometer to measure RPMs. Faster acceleration points to more power.
Identifying Limiting Factors
I need to identify what's limiting the engine's power output before modifying the ignition. Likely limiting factors include:
- The carburetor: Too lean of an air-fuel mix can reduce power.
- Valve condition: Worn valves and seats leak compression.
- Muffler: A restrictive muffler causes backpressure limiting exhaust flow.
- Spark plug: An incorrect heat range plug can cause pre-ignition or fouling.
Upgrading these components may be required to fully optimize power.
Rewiring the Ignition System
Once I've addressed any issues found during evaluation, I can start rewiring the ignition system. There are a few options depending on the engine.
Installing an Aftermarket Ignition System
I can replace the stock ignition with a high-performance aftermarket system. Some options are:
- Capacitor discharge ignition (CDI): Uses a capacitor to produce a hotter, longer spark improving combustion. A significant power boost option.
- Electronic ignition kit: Replaces points with a magnetic pickup and ignition module for better timing and spark. A moderate power gain.
- Adjustable timing module: Allows tuning the ignition timing curve for maximum power. Requires some dyno testing.
Bolt-on kits make installation straightforward. I just match the wires from the old system.
Modifying the Stock Ignition
If the engine has points ignition, I can modify that system:
- Upgrade the coil and flywheel: Larger winding diameters produce more voltage for a stronger spark.
- Adjust the point gap: A tighter gap makes a snappier spark for better combustion.
- Modify the advance curve: Advancing timing at higher RPMs proportionally boosts power.
I have to be careful not to exceed the coil's limits with these mods. An aftermarket coil may be needed.
Bypassing Resistance
On engines with a magneto, power can be gained by bypassing internal resistance:
- Add solid-core wire and new spark plug terminals to bypass the wire's resistance.
- Install non-resistor spark plugs to eliminate that resistance in the circuit.
- Change the magneto grounding: Ground it directly to the engine instead of the chassis.
Lower resistance equals more voltage at the plug. But again, I need to watch I don't over-volt the coil.
Optimizing the Rewiring
To maximize the power gains from rewiring, I may need to perform some additional optimizations:
Tuning the Carburetor
With more power, I'll likely need to re-jet the carb for the optimal air/fuel mix:
- Move up 2-3 jet sizes at a time until maximum RPMs are reached.
- Be careful not to lean it out too much or risk burning up the engine.
- A wide-band O2 sensor can precisely dial in the mixture.
Retarding Ignition Timing
The extra combustion heat and pressure may require retarding the timing to prevent pre-ignition:
- Start by retarding 3-5° and testing performance.
- Check for pinking or knocking noises indicating pre-ignition.
- Excessive exhaust temperatures also signal timing is too advanced.
Lower Compression Ratio
Higher ignition voltages can cause pre-ignition on high compression engines:
- Install a thicker head gasket to reduce the compression ratio.
- Reduce ratio in increments of .5 until pre-ignition stops.
- May slightly reduce peak power, but makes the engine less prone to damage.
Testing and Troubleshooting
I need to thoroughly test my engine modifications and be prepared to troubleshoot any issues.
Performance Testing
I should re-test key performance metrics after the rewiring to validate the results:
- Run the dynamometer again and look for increased HP and torque readings.
- Check that exhaust temperatures have increased.
- Verify faster acceleration through RPMs with the inductive tach.
If I don't see gains, I may need to recheck my modifications or optimize further.
Troubleshooting Common Problems
Some issues that can crop up include:
- Pre-ignition: Caused by an overly-advanced timing curve or too much voltage at the plug.
- Fouled spark plugs: Can be caused by a rich fuel mixture or incorrect plug heat range.
- Loss of power at high RPMs: Indicates the limit of the coil's output voltage has been exceeded.
I may need to install instrumentation like cylinder head temp probes to further diagnose issues.
Conclusion
Rewiring a small engine like a lawnmower can definitely give it a nice boost in power. But I have to be strategic with my modifications and careful not to push components past their limits or else risk damaging the engine. Thorough testing is key to getting excellent performance results. With some smart optimizations to unlock the engine's potential, I can have my rewired mower really tearing up the neighborhood!