How To Polish A Combustion Chamber [A Complete Guide]

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Many car enthusiasts make it their personal mission to get their vehicles’ engines working at peak performance and efficiency. The truly dedicated will undertake even lengthy, complex tasks that add just a little to the engine’s power and responsiveness. One such job is polishing the combustion chambers in the vehicle’s engine. If you’re ready to put in some time and effort to get that little extra “oomph” from your car, you may be wondering what steps you should follow to polish the combustion chambers. We’ve done the research, and we have the answers for you!

Follow these steps to polish the combustion chambers in your vehicle’s engine:

  1. Measure and record the volume of each combustion chamber. 
  2. Remove excess material around the edges of the intake and exhaust valves.
  3. Smooth the transitions between the valve seats and the chamber walls.
  4. Again, measure and record the volume of each chamber. 
  5. Buff the chamber walls, removing any last bits of excess material, rough edges, and scratches.
  6. Measure and record the volume of each chamber.
  7. Polish the chamber walls.

In the remainder of this article, we’ll delve into each of these steps in greater detail, giving you a complete guide to polishing combustion chambers. We’ll also discuss the advantages and disadvantages of polishing your engine’s combustion chambers, and we’ll explain the difference between “polishing” and “cleaning” them. Keep reading to learn more!

Inside view of engine, close up detail of two pistons in cylinder with four valves,some gears aside, How To Polish A Combustion Chamber [A Complete Guide]

How To Polish Combustion Chambers

Polishing your vehicle’s combustion chambers is not a physically demanding task, but it does require time, patience, attention to detail, and a few specialized tools. This job is not something that the average driver will care to take on, as the increase in engine power and efficiency will be noticeable only to high-performance engine enthusiasts. 

The grinding and sanding required in this job will kick up a lot of fine metal dust. Make sure to wear appropriate protective gear, especially goggles and a respirator mask.

Click here to see this respirator mask on Amazon.

1. Measure And Record The Volume Of Each Combustion Chamber

One of the most important parts of the polishing process is making sure that all of the combustion chambers in your engine have the same volume, to within 0.25 CCs (cubic centimeters) of each other. This way, the compression ratio and the timing of the fuel/air ignition remain uniform across the chambers. So, you will measure the volume of each chamber several times throughout the polishing process. (This procedure is also called “CCing” the chambers.)

  • Clean the combustion chamber with a mild detergent, and wipe it dry with a rag or paper towel.
  • Waterproof the chamber by applying a thin bead of grease around the valve seats and around the top rim of the chamber.
  • In place of the current spark plug, put an old, expendable one that you can discard after the job is done.
  • Drill air holes into a clear, hard piece of plastic, and then place it over the top opening of the chamber (like a lid). 
  • Using a syringe marked in cubic centimeters, squirt water through one of the air holes into the chamber. Continue until the chamber is full of water, but not overflowing.
  • Record the total number of cubic centimeters of water you put into the chamber. This is the volume of the chamber.
  • Remove the plastic cover; drain and wipe the combustion chamber dry.
  • Repeat this process for each of your engine’s combustion chambers, recording the volume of each one.

Click here to see this cubic centimeter syringe on Amazon.  

2. Remove Excess Material Around The Edges Of The Intake And Exhaust Valves

In this step, you want to get rid of excess material that’s impeding airflow. The goal is NOT to make each combustion chamber as big as possible. On the contrary, enlarging a combustion chamber too much can throw off the compression ratio, actually decreasing efficiency. 

This step is often called “unshrouding the valves.”

  • Inspect the area around the intake and exhaust valves, especially the rims of the valve seats and the spots where they meet the walls of the combustion chamber. Look for any rough edges, discontinuities, and overlaps where excess material might be impeding airflow.
  • Mark these areas with a bright-colored paint pen.
  • Using a 1/4″ die grinder, grind off the excess material. 
  • Wipe away dust and grit with a clean rag.

Click here to see this 1/4″ die grinder on Amazon.

3. Smooth The Transitions Between The Valve Seats And The Walls Of The Combustion Chambers

Next, smooth the edges off the coarse grind that you completed in Step 2.

  • Using a Dremel or similar instrument with an 80-grit flap wheel, sand the areas that you ground in Step 2.
  • When all the cuts and chips from the grinding have been sanded off, switch to a 120-grit flap wheel and sand again until all the scratches are gone.

Click here to see this 80-grit flap wheel on Amazon.

4. Measure And Record The Volume Of Each Combustion Chamber

At this point, you should again measure and record the volume of each combustion chamber. It’s important that all of the chambers’ volumes are within 0.25 CCs of each other at the end of this step. If they are not, grind/sand additional material from any chamber whose volume is not within 0.25 CCs of the largest chamber. 

5. Buff The Walls Of The Combustion Chambers

When all of the combustion chambers are sanded and their volumes equalized, it’s time to buff them to remove any tiny imperfections that might remain to impede airflow.

  • Using a 180-grit buff, go over the entire surface of the combustion chamber at least twice, in perpendicular directions.
  • Repeat this process, using a 280-grit buff.
  • Repeat again, using a 320-grit buff. When done, the walls of the combustion chambers should appear shiny and free of even the tiniest scratches.

Click here to see 320-grit abrasive buffs on Amazon.

6. Measure And Record The Volume Of Each Combustion Chamber

One last time, measure and record the volume of each combustion chamber. If any of them are more than 0.25 CC off from the largest-volume chamber, buff them again until they meet the 0.25 CC criterion.

7. Polish The Walls Of The Combustion Chambers

The final step is to polish the walls of the combustion chambers to protect that smooth finish!

  • Spread a thin coat of white rouge polishing compound over the entire surface of each combustion chamber. 
  • Polish with a felt polishing wheel until all the white rouge is removed. The surface should be shiny and free of any marks or scratches. 

The following video demonstrates the entire process described above:

Should I Polish My Combustion Chambers?

If wringing the highest performance from your vehicle’s engine is very important to you, polishing your combustion chambers is probably worth the effort. There are two major benefits to polishing the chambers, and a couple of potential drawbacks.

Benefits

  • Combustion chambers get very hot, and casting flash can create hot spots on the chamber walls. This can cause the fuel/air mixture to ignite prematurely, decreasing engine power.
  • Polishing the chamber makes the surface smooth, which decreases the likelihood that carbon will stick to the chamber walls. Carbon buildup leads to a variety of problems, ranging from minor decreases in power to catastrophic engine failure. 

Drawbacks

  • The process of polishing the combustion chambers removes substantial material from the walls and valve seats. Removing too much material — thus increasing the volume of the combustion chamber — can change the compression ratio, leading to an overall decrease in power and efficiency. 
  • In order to ensure that the compression ratio remains optimal, you may have to mill the surface of the cylinder head. This is expensive and time-consuming.

What Does Polishing A Cylinder Head Do?

You can polish your engine’s cylinder head in much the same manner as the combustion chambers. Polishing the head removes all the accumulated oil, dirt, and rust buildup, increasing airflow and efficiency.  

How Much Does It Cost To Polish Heads?

You can polish your cylinder head yourself for just the cost of cleaner, sanding/buffing wheels, and several hours of labor. A quick survey of mechanics’ websites across the country shows prices averaging about $600 for a 4-cylinder engine, $750-$800 for a 6-cylinder, and $800-$1,000 for an 8-cylinder engine. 

What Is Combustion Cleaning?

automobile cylinder block and car piston view

Cleaning combustion chambers is a different process from polishing them. It involves using chemical cleaners to remove carbon deposits from the chambers and valves. Cleaning the combustion chambers does not require you to tear apart your engine, or even to have any major mechanical skills.

As fuel burns, it produces sooty carbon, which can build up on the backside of the intake valve, the crown of the piston, and the walls of the combustion chamber. Additionally — particularly in older vehicles — oil can leak past the pistons and valves and into the combustion chamber, where it partially burns, creating more buildup.

Such buildup blocks airflow, leading to lower gas mileage, increased emissions, less power, knocking/pinging, and, in severe cases, stumbling and hesitation. Fortunately, you can easily clean these deposits using readily available chemical cleaning kits like the one shown below. 

Click here to see this combustion chamber cleaner on Amazon.

How Often Should The Combustion Chamber Be Cleaned?

Driving conditions play a major role in determining how often you should clean the combustion chambers. Stop-and-start driving, low speeds, and frequent idling result in more carbon buildup than highway driving. If most of your mileage is at highway speeds, you should clean your combustion chambers every 30,000 miles; if you drive mostly in the city, you may want to clean them every 15,000 miles. 

In Closing

Inside view of engine, close up detail of two pistons in cylinder with four valves,some gears aside, How To Polish A Combustion Chamber [A Complete Guide]

If you’re a muscle car aficionado or just a gearhead who wants to wring every last bit of power out of your car’s engine, polishing the combustion chambers can be a great way to go. Grab your tools, set aside 10-12 hours of your time, and grab that competitive edge! 

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