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How Does Spark Plug Gap Affect Engine Performance?

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Abstract

Do you know what a spark plug is? Have you ever replaced an old, dirty, or worn out spark plug in an engine? These small parts are crucial to the operation of internal combustion engines. In this science project, you will find out how "gapping" a spark plug affects engine performance. Can you get more power or better fuel economy out of your engine by changing the spark plug gap? Try this project and find out!

Summary

Areas of Science
Mechanical Engineering
Difficulty
Method
Scientific Method
Time Required
Average (6-10 days)
Prerequisites

Previous experience working with engines, or a mentor who can help, is recommended.

Material Availability

Access to a lawnmower or other gas-powered equipment (weed whacker etc.) is required.

Safety

Adult supervision is required when working with power equipment.

Credits
Ben Finio, PhD, Science Buddies
Science Buddies is committed to creating content authored by scientists and educators. Learn more about our process and how we use AI.

Objective

Find out how spark plug gap affects engine performance.

Introduction

Spark plugs are a critical part of an internal combustion engine. They produce an electrical spark that ignites the air-fuel mixture, which pushes on the piston as it expands. Watch this video for an overview of how spark plugs work, along with the rest of an engine's ignition system:

https://www.youtube.com/watch?v=742Ml4oD3s4

The spark in a spark plug jumps across the gap between the spark plug's two electrodes (Figure 1). The size of this gap can affect the spark's timing, or when exactly the spark occurs during the piston's stroke. This, in turn, can have an effect on engine performance. For example, a mistimed spark can result in an engine running "rough" (misfiring or experiencing engine knock etc.), or cause incomplete combustion, resulting in poorer emissions or fuel economy.

close up picture of a spark plug with a spark jumping the gap between electrodes Image Credit: Wikimedia Commons user Ralf Schumacher / Creative Commons Attribution 3.0 Unported license.
Figure 1. A spark jumping the gap between a spark plug's two electrodes. Image credit Wikimedia Commons user Ralf Schumacher, CC BY-SA 3.0.

The spark plug gap is something you can adjust yourself using a spark plug gapping tool. This tool allows you to precisely measure and set the gap distance between the two electrodes. In this science project, you will adjust the spark plug gap on a small engine (like a lawnmower) and find out how it affects engine performance.

Terms and Concepts

Questions

Bibliography

Materials and Equipment

Disclaimer: Science Buddies participates in affiliate programs with Home Science Tools®, Amazon.com, Carolina Biological, and Jameco Electronics. Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity, and keep our resources free for everyone. Our top priority is student learning. If you have any comments (positive or negative) related to purchases you've made for science projects from recommendations on our site, please let us know. Write to us at [email protected].

Experimental Procedure

Download PDF of Procedure
This project follows the Scientific Method. Review the steps before you begin.
  1. Familiarize yourself with the engine you are working with, including the number and location of the spark plugs and how to remove them. You may need to read the engine's manual or ask an adult for help.
  2. Remove the spark plugs and check each one for visible corrosion or carbon deposits. If the spark plugs are already very worn out, you should start the projects with new ones.
  3. Decide what dependent variable(s) you will measure for your project and how you will measure them. What you can measure will depend on your engine and the materials you have available. Here are some suggestions, but this is not an exhaustive list. Ask someone who has experience working with small engines if you need help or more ideas.
    1. Whether the engine is running "smooth" or "rough." You can detect this just by listening to the engine or operating the machine. Does it misfire or sound louder than usual? Is there a knocking or pinging sound?
    2. Ease of starting the engine. Do you have trouble starting the engine? How many times out of 10 does it start on the first try? (Note: doing a "cold start" on your very first trial may affect the results, so you may need to run the engine for a bit to warm it up before you start collecting data.)
    3. Power output. You can measure this qualitatively, for example, while mowing grass with a lawnmower. Does the lawnmower seem to have a more difficult time cutting grass? For self-propelled mowers, does it seem sluggish or like it has a hard time moving forward?
    4. Fuel economy. How long (or how far) will the engine run on a fixed amount of fuel? See this video for some ideas on measuring fuel economy of a lawnmower.
    5. Emissions. Emissions testing equipment for measuring exhaust gases is expensive, but you may be able to borrow it from someone or access it at an auto repair shop.
    6. Spark plug wear. For longer-term tests, you can examine wear and deposits on the spark plugs, but you will need to use new ones each time you change the gap and start a new trial.
  4. New spark plugs likely come with the gap pre-set by the manufacturer, but you should double check your engine's manual to see if it lists the correct gap distance. Watch this video to learn how to adjust the spark plug's gap, or read the instructions that came with your gapping tool. If needed, adjust the gap to the manufacturer-recommended setting. Otherwise, leave the gap as-is for now and proceed with your experiment.
    https://www.youtube.com/watch?v=IuW3svdqKZM
  5. Decide what other spark plug gaps you will test for your experiment. The gaps you decide to test may depend on your spark plugs and engine, but for example, you could test the manufacturer-recommended gap plus or minus 20 thousandths of an inch (so three different gaps total).
  6. Based on the independent variables and gaps you have decided to test, make a data table like Table 1 to record your results. This table is just an example. Your data table may need to be different depending on how you design your experiment.
Swipe left to see more
  Gap size
  0.040" 0.060" 0.080"
Trial 1 2 3 Avg 1 2 3 Avg 1 2 3 Avg
Starting                        
Smooth/rough                        
Fuel economy                        
Overall performance                        

Table 1. Example data table.

  1. Some of the data you collect may be quantitative, for example, fuel economy in miles per gallon. Same data may be "yes/no," for example, whether the engine would start. Other data, like whether the engine is running smooth or rough, may be qualitative. You can take notes, but also rate the qualitative data using a numeric scale, for example a 0–5 scale where 0 indicates that the engine is running very roughly and 5 indicates smooth operation. Make sure you have decided how you will record data for each variable before you start your experiment.
  2. Collect data for your default gap size. The exact procedure you follow will depend on the variables you have decided to measure. For the example in Table 1, this would consist of:
    1. Making sure the fuel tank is empty.
    2. Putting a fixed amount of fuel in the tank.
    3. Starting the engine and recording whether it starts on the first try.
    4. Observing how the engine runs (rough or smooth) during operation.
    5. Timing how long the engine runs until it runs out of fuel (longer time means better fuel economy, even though this is not a direct measurement in miles per gallon).
  3. Repeat your data collection for at least two more trials with the same spark plug gap. Where applicable, calculate an average across the three trials.
  4. Remove and change the gap on all of the spark plugs. Repeat steps 8–9 for each of your remaining gap sizes.
  5. Analyze your data. How can you decide which spark plug gap leads to the best overall engine performance? This may be difficult to do when comparing different variables that all have different units or methods of measurement, and may have different levels of importance. One way to do this is to first convert all of your measurements to a 0–5 scale, where 0 represents "bad" (hard to start, rough operation, poor fuel economy etc.) and 5 represents "good." Once each variable is rated using the same scale, you can calculate an average, or overall performance, like this:

    Equation 1:

    However, some variables might be more important than others. You can assign a multiplier, or "weight," to each variable, and calculate a weighted average like this:

    Equation 2:

    The weights should be decimal values that add up to 1:

    Equation 3:

    That way, each weight represents the relative importance, or percentage importance, of that variable. For example, if variables 1 and 2 are equally important, but variable 3 is twice as important, you could assign weights of 0.25, 0.25, and 0.5 respectively.

    Can you assign a weight, or importance, to each variable that you measured, and then calculate a weighted average or overall performance metric for each spark plug gap? Which gap performs the best overall? Is it the manufacture-recommended gap, or something else?

icon scientific method

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Global Goals

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.

This project explores topics key to Responsible Consumption and Production: Ensure sustainable consumption and production patterns.

Variations

  • Spark plugs are available with electrodes made from different materials, like copper, platinum, and iridium, some of which are more expensive than others. Is a certain type of spark plug recommended for your engine? Are more expensive spark plugs "worth it"? Can you do an experiment to find out?
  • What other factors can affect engine performance? Can you conduct an experiment with a different independent variable?

Careers

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Automotive Technician
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There are so many different types of cars and trucks on the road: sports cars, sport utility vehicles (SUVs), pickup trucks, and even electric vehicles and hybrids. Have you ever wondered how they work? And, how to keep them running? How does the engine make a car's wheels turn? Why does pushing the brake pedal make a car stop? Automotive service technicians (also called mechanics) can pop open the hoods of these different types of cars and answer these questions. Automotive technicians know… Read more

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Cite This Page

General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Finio, Ben. "How Does Spark Plug Gap Affect Engine Performance?" Science Buddies, 15 Nov. 2024, https://www.sciencebuddies.org/science-fair-projects/project-ideas/ApMech_p059/mechanical-engineering/spark-plug-gap?from=Blog. Accessed 10 June 2026.

APA Style

Finio, B. (2024, November 15). How Does Spark Plug Gap Affect Engine Performance? Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/ApMech_p059/mechanical-engineering/spark-plug-gap?from=Blog


Last edit date: 2024-11-15

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