what does cubic inches actually measure?
01-07-2009, 03:58 PM
#1
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what does cubic inches actually measure?
ive always wondered about this, what exactlly does cubic inches measure? the volume of the engien block? volume of the internal parts? are heads included in the measurement?
01-07-2009, 05:38 PM
#3
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To answer the OP's question seriously, an engine's displacement is the amount of air moved (displaced' in one complete revolution of said motor. For example, a 4.6L (281ci) has 8 cylinders, each with a bore of 3.54in and a stroke of 3.55in. The formula for the surface area of the piston is (pi)(radius^2), so with a radius of 1.775in, the surface area is 9.89 sq in. Multiply by the stroke (3.55in), and you end up with the total displaced volume of one cylinder, 35.14ci. Multiply by 8, and voila, 281ci.
Clear as mud? Good.
01-07-2009, 06:32 PM
#4
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i just wanna make sure i got this down, the surface area is only the area that is creating compression, not the side skirt? and does this "(pi)(radius^2)" translate to pie divided by radius squared?
01-07-2009, 07:28 PM
#5
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Location: Florida
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Please tell me you're being flippant...
To answer the OP's question seriously, an engine's displacement is the amount of air moved (displaced' in one complete revolution of said motor. For example, a 4.6L (281ci) has 8 cylinders, each with a bore of 3.54in and a stroke of 3.55in. The formula for the surface area of the piston is (pi)(radius^2), so with a radius of 1.775in, the surface area is 9.89 sq in. Multiply by the stroke (3.55in), and you end up with the total displaced volume of one cylinder, 35.14ci. Multiply by 8, and voila, 281ci.
Clear as mud? Good.
To answer the OP's question seriously, an engine's displacement is the amount of air moved (displaced' in one complete revolution of said motor. For example, a 4.6L (281ci) has 8 cylinders, each with a bore of 3.54in and a stroke of 3.55in. The formula for the surface area of the piston is (pi)(radius^2), so with a radius of 1.775in, the surface area is 9.89 sq in. Multiply by the stroke (3.55in), and you end up with the total displaced volume of one cylinder, 35.14ci. Multiply by 8, and voila, 281ci.
Clear as mud? Good.
01-07-2009, 10:55 PM
#6
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Join Date: Dec 2008
Location: WI
Posts: 172
What you're doing is measuring the volume of each cylinder with the piston at the very bottom of its stroke. The side skirt of the piston has nothing to do with it.
To measure the volume of a cylinder you must first figure out its area. The bore on the piston is 3.54 inches, so the radius of that area (the bore) is the 1.775 inches ripped camel referred to. Pi is 3.14159265 (it's actually a longer number than that, but this is plenty close). So, the area of the cylinder is 3.14159265 times 1.775 (the radius) squared, which equals 9.8978901. Round up to the last two decimal places and you get 9.90. Ripped camel got 9.89 because he didn't round up.
Multiply this 9.90 area times the stroke of the cylinder (3.55 inches) and you get 35.145 inches, or 35.15 if you prefer to round up. 35.145 multiplied by eight cylinders is 281.16 inches. If you rounded up to 35.15, you get 281.2. Either way, it's closer to 281 than it is to 282, so it's a 281 cubic inch motor.
To measure the volume of a cylinder you must first figure out its area. The bore on the piston is 3.54 inches, so the radius of that area (the bore) is the 1.775 inches ripped camel referred to. Pi is 3.14159265 (it's actually a longer number than that, but this is plenty close). So, the area of the cylinder is 3.14159265 times 1.775 (the radius) squared, which equals 9.8978901. Round up to the last two decimal places and you get 9.90. Ripped camel got 9.89 because he didn't round up.
Multiply this 9.90 area times the stroke of the cylinder (3.55 inches) and you get 35.145 inches, or 35.15 if you prefer to round up. 35.145 multiplied by eight cylinders is 281.16 inches. If you rounded up to 35.15, you get 281.2. Either way, it's closer to 281 than it is to 282, so it's a 281 cubic inch motor.
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