3/12/2023 0 Comments Compression ratio calculator![]() Keep in mind that even seemingly small changes can have a direct effect on compression. JE Pistons keeps this information on record with each piston design. Your piston manufacturer should have this volume available when you need it. The piston’s overall dome volume needs to be calculated to reach an accurate compression ratio. Let’s say that that piston offers a slight dome, but also includes piston valve relief. However, most piston tops contain some combination of valve reliefs, a dish, or a dome configuration that constitute a given volume. If it were, the piston volume number would essentially be 0, or one that neither adds nor subtracts from the compression ratio. This is important, since rarely is the piston perfectly flat. We use an automotive cylinder head in the example below, but the same process can be done for single or multi-cylinder motorcycle engines.īe sure the combustion chamber is filled completely, with no air bubbles, to reach an accurate measurement. A square plexiglass lid sealed with grease with a small hole drilled in it to allow you to fill the chamber with rubbing alcohol mixed with a little food coloring works well as a measuring fluid. Chamber volume has a direct impact on compression ratio, so measuring this is important for accuracy. Most burettes are graduated in milliliters, and one milliliter is equal to one cubic centimeter (cc), so don’t let that throw you. The easiest way to measure this is with a graduated cylinder or burette. Next, we need to know the volume of the combustion chamber. With a 77mm bore and a 53.6mm stroke, this creates a cylinder with a volume of 249cc's. If you recall from high school geometry, Volume of Cylinder = Pi x radius squared x height (in this case, stroke). Our first step is to determine the volume of the cylinder based on bore and stroke. Pictured here is a single cylinder off a 250F motorcycle. For example, the area on top of the piston at TDC must take into account the individual volumes of the chamber, the top of the piston (the crown design), the head gasket, and the height of the piston above or below the deck of the cylinder.Ĭlick here to read about how to calculate compression and displacement for automotive engines.Ĭalculating the cylinder volume with Pi x radius squared x stroke gives you the volume of the cylinder only. Then, these smaller volumes can be added together to create the overall swept volume. This is a matter of breaking down a series of small areas that need to have their individual volumes calculated. It might be a good idea to review how compression is calculated. This is accomplished by better combustion space design, as well as superior air-fuel ratio control through electronic fuel injection (EFI). Production dirt bike engine compression ratios have increased over the years, to current ranges of around 12.5:1 to 13.5:1, while still capable of burning pump premium 91 / 93 octane gasoline. We'll use single-cylinder dirt bike engines as an example. Of course, higher compression also places a greater octane requirement on the fuel, so it’s important to keep that in mind. Compression ratio is the ratio between the swept volume of the cylinder with the piston at bottom dead center and the swept volume of the cylinder with the piston at top dead center.Ĭompression ratio is instrumental in making power, since it is this squeezing of the air-fuel mixture that enhances the combustion process and makes power.
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