00:00 How chamber volume affects the compression ratio
04:32 How to equalize chamber volumes
10:00 Benefits of polishing

What is up engine heads, today I'll show you how and why to equalize the volume of your combustion chambers. As you may know your engine's compression ratio is the relationship between the largest and smallest volume of your cylinder. The largest volume is achieved when your piston is at bottom dead center while the smallest volume is achieved when the piston is at top dead center.

This obviously means that your smallest volume practically equals the volume of your combustion chamber, which means that your compression ratio is strongly influenced by your combustion chamber volume.

When it comes to my build I have actually significantly modified my combustion chambers. My cylinder head is a 4AFE Toyota economy cylinder head which I'm planning to turbocharge. The valves were heavily shrouded so I had to remove a lot of material to enable better airflow. This meant that I increased the volume of my chambers significantly. Of course I was doing all of this modification by hand which dramatically increased the chances of the chambers having large volume inequalities. .

The first step is obviously to finalize the shape of your combustion chambers. The next step is getting a piece of thin transparent Plexiglas that is longer and wider than your cylinder head. Next we're going to install a set of dummy spark plugs and then we're going to overlay the Plexiglas over our chambers and mark the position right above the spark plug.

Now we're going to drill out the four holes that we marked. Now we're going to install all of our valves. We're going to smear the sealing surface on the back of each valve with vaseline or petroleum jelly. We're doing this because we need the valves to actually seal and not let water past them because we'll be later using water to measure the chamber volume.

Next we will need two syringes. A large and a small one. The large one must be larger than the volume of your combustion chamber. The small syringe must be able to measure 0.1 or one tenth of a millilitre.

Now we're going to smear a thin layer of vaseline around the outside of the combustion chamber. A little bit goes a long way. Wipe away any excess as there must not be any vaseline inside the chamber.

We'll first fill the chambers completely to see approximately how much volume they have. We will do this using only the large syringe as accuracy isn't paramount during this initial volume estimate.

Once we have done the initial estimate we will remove the plexiglass and restart the process. Because each chamber is around 35cc I will fill each chamber with exactly 34cc of water. This time around accuracy becomes paramount and your measurement will only be as good as your accuracy with the large syringe. Getting the exact amount of water into the syringe consistently requires a bit of patience but if you do it properly this process will have a pretty decent degree of accuracy.

Once you have the correct amount of water inside the syringe inject all of it evenly and carefully into the chamber. Now we're going to use the small syringe to fill the remaining volume of the chamber. As you can see here my last two chambers are the same but the first and second are not. This means that I must remove a bit more material from the first and second chamber to equalize them to my last two chambers. We will lift the plexiglass and evacuate the water from the chambers and get our air die grinder again.

Once all the chambers are equal you can finish things off by polishing the chambers. Obviously polishing removes an absolutely minimal amount of material that doesn't really affect the volume. Doing this before equalizing the volumes doesn't work because you'll ruin the polish each time you grind away material.

The logic behind the polishing is that it minimizes the chances of a hotspots but it also makes it harder for carbon to build up on the chamber surface. Less carbon buildup is always good because it ensures consistent performance and compression ratio and it also reduces the chances of a piece of carbon buildup falling off and acting as a hot spot for pre-ignition.

A special thank you to my patrons:
Daniel
Peter Della Flora
Daniel Morgan
William
Richard Caldwell
Pepe
Brian Durning
Brian Alvarez
Dave Westwood

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