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Dirt Bike Mods & Maintenance
carb sizing formula revisited
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[QUOTE="jaguar, post: 1469233, member: 19340"] Redoing The Carb Sizing Formula It seems to me that the formula .8 x square root (engine cc * RPM/1000) came from car carburetor manufacturers and so I wanted to redo the formula based on the carb throat cross sectional area as a fraction of the intake volume of the engine. The intake volume depends on the area the piston displaces as it rises from transfers closing to TDC. (not so with a rotary valve engine.) A square bore engine intakes .76 of the engine size if the transfer duration is 130, and .79 of engine size if the duration is 120. If we just leave off the .8 as a multiplier in the formula then the square root of (cc x RPM/1000) gives 38.7mm carb diameter for a 125cc reving to 12,000 RPM. The KTM SX 125cc uses a 39mm carb and the Yamaha YZ 125cc uses a 38mm. Here is a more detailed version of the new formula: carb diameter = 20 x square root ((.00001 x volumetric efficiency x engine cc x top RPM) / 3.14) This formula depends on the static and not dynamic volumetric efficiency. Using it for the same engine I get a 38.1mm diameter for a VE of .76 and a 38.8mm diameter for a .79 volumetric efficiency. [/QUOTE]
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MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
carb sizing formula revisited
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