In today’s video we’re talking about your engine’s compression ratio. First, we’ll explain the theory behind compression ratio, what it is, and how it influences your engines performance and efficiency. After that, we’ll dive into the practical side of things and look at how to calculate and modify your compression ratio, and finally we’ll talk about how to choose the best compression ratio for your application. So let’s get started. Now, when we say compression ratio, we’re actually referring to the static compression ratio of the engine, and that’s the ratio of the largest to smallest volume in your cylinder. In other words, it’s the ratio of the volume of the cylinder when the piston is at bottom dead center to the volume of the cylinder when the piston is at top dead center. Your compression ratio, as the name implies, determines how much the air-fuel mixture is squeezed and compressed inside your cylinder. The higher the compression ratio, the closer the air and fuel molecules come together, meaning we allow combustion to occur more effectively and quickly, ensuring the air-fuel mixture burns more thoroughly. A higher combustion ratio is generally achieved by reducing the size of the combustion chamber or by moving the piston closer to the combustion chamber. By doing this, we of course move the piston closer to the heart of the operation or the power source, allowing more of this energy to be transferred to the piston and converted into piston motion or mechanical energy. In other words, a higher combustion ratio can improve both power and efficiency. So the more the better, right? Well, as with all things, there is a reasonable limit, and you can in fact have too much of a good thing. Because a higher compression ratio contributes to more complete combustion of the air-fuel mixture, it also increases combustion temperatures. The more compressed the mixture is, the better it burns, and the better it burns, the hotter it burns. The advantage of this is of course higher power potential and higher efficiency, but the downside is that the engine will run hotter and have higher nitrogen oxide emissions. Higher combustion temperatures lead to more nitrogen oxide emissions, which is one of the main reasons why most modern diesel engines that have a EURO 6 emissions standard run on a lower average compression than their predecessors from a decade or two ago. But one of the main limiting factors when it comes to compression in spark ignition engines is of course detonation. When gases are compressed they get hot – air is of course a gas and if you compress it too much it can get hot enough to ignite the petrol before the expanding flame front created by the spark plug reaches it. This is called detonation. Obviously a higher compression ratio increases the chances of detonation and therefore limits the compression ratio that a petrol engine can have. This is especially true for forced induction engines that send already compressed air to the engine. Okay, that’s the basic theory. Now let’s move on to the practical side of things. So what determines your engine’s compression ratio? It’s actually seven things: 1. Your bore 2. Your stroke 3. The thickness of your compressed head gasket 4. The diameter of your head gasket 5. The distance between the top of the piston and the top of the block 6. The volume of your concave/convex piston 7. And the volume of your combustion chamber So how do you calculate it? Well, there are formulas, but the advent of the internet allows us to be lazy and just plug everything into free, readily available online compression ratio calculators. When it comes to changing our compression ratio, here’s a basic rundown. Bigger bore - higher compression ratio Longer stroke - increased compression ratio Thicker head gasket - reduces compression ratio Thinner head gasket - increases compression ratio Machining the block - increases compression ratio Machining the cylinder head - increases compression ratio Removing material from the combustion chamber - reduces compression ratio Convex pistons - increases compression ratio Concave pistons - reduces compression ratio #d4aenglish #d4a #cars #mechanics #compressionratio 00:00 What is compression ratio and how does it work? 04:10 How to calculate compression ratio 06:21 How to change it 09:19 Choosing the optimal one for your application.