Compression ratio calculator dome9/24/2023 So, instead of a well-timed expansion of gasses that begins burning when the spark plug fires as the piston nears TDC, and continue expanding as the piston begins to move back down, there’s a blast that hits the piston like a hammer blow while it is still on the upstroke. The biggest detrimental factor is “detonation.” This is when a fuel mixture ignites elsewhere in the combustion chamber other than at the spark plug when it fires. However, it gets a bit more complicated than that in practice, and there are more factors working against a high compression ratio than there are benefits. This is because the more a fuel mixture is compressed, the more complete it will burn and expand. Put simply: the higher the compression ratio, the more power will be produced for a given displacement, and for a given amount of fuel. Therefore, if the total volume of the cylinder is 100 cc, and the volume with the piston at top dead centre (TDC) is 10 cc, you have a mechanical compression ratio of 10:1. The compression ratio is the ratio between the total volume of a cylinder when the piston is at the bottom of its stroke and the volume of the combustion chamber when the piston is at the top of its stroke, including the head gasket thickness and the shape of the top of the piston. The more efficiently the fuel mixture burns, the harder the push on the piston will be, and the more power that will be produced.Īside from factors like cubic displacement, and intake and exhaust design, an engine’s compression ratio has a great effect on output and fuel efficiency. Power transfers to the rear wheel of a motorcycle by burning an air/fuel mixture in the combustion chamber of a cylinder, and the expansion of the burning gasses pushes on the piston, which turns the crankshaft that eventually propels you forward. If the heads are brand new, you can usually stop here unless you're blueprinting every variable, but if they're used heads and you suspect they've been worked, repeat this process on the remaining chambers.It can be a complicated process to determine a compression ratio. Once the chamber is full and free of air, check the burette for your chamber volume by reading the measurement at the bottom of the meniscus (or curvature of the fluid's surface due to surface tension). Fill the burette to the zero mark, position it over the head, and begin filling. Mineral spirits or alcohol are preferred, but even water would work here. Heavy grease is applied around the chamber, but not inside of it, before a block of acrylic (included with many kits, but can be easily made) is placed with its fill hole at the top of the chamber, ensuring air can escape while the chamber is filled. First, the heads are set with a slight uphill rake. Say a you've got a 700- to 800-inch engine with a 100cc chamber and you want to run 15 to 17:1, that's going to be easier because the chamber volume relative to the cylinder volume will be quite small, and consequently, you get a better piston design out of it."Ĭombustion Chamber Volume: While head manufacturers supply this number at purchase, say you bought a used set and you want to double-check if there's been any work done to them cc'ing the heads with a burette accomplishes this. On a decent-sized engine, 400-inch and up, you start getting close to flat pistons to get 10.5:1. The small engines, like a 300-inch motor, and put a big dome on it, that's not really good for flame travel. As you go larger, you start to revert more to a flat piston, which has the best of the flame travels of all of them. "The cool thing about big engines is piston design becomes way more practical. If you look at the work being done, if you have to fire the engine 10 degrees earlier on the compression stroke, the engine is now going to have to produce more work because you've started the burn earlier to achieve the same peak ATDC," he said. On the older, junk cylinder heads, you may have 45 degrees of total timing (approximately a 60-degree time frame). "And with a good cylinder head, you're looking at a 45-degree time frame (approximately 30 degrees of total timing) from the time you ignite it. Looking at the combustion chamber's efficiency, you can begin to look at compression as a function of the timing advance. He's looking for peak cylinder pressure at 15 degrees after top dead center (ATDC), where the most power is made. The reason for this, Duttweiler explained, is in the time it takes for the flame to propagate during the crankshaft's rotation.
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