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carb sizing formula revisited

jaguar

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Joined
Jul 29, 2000
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South America
#1
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.
 
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jaguar

Subscriber
Joined
Jul 29, 2000
Messages
1,384
Likes
37
Location
South America
#2
Now that I've researched even further I've found it's not as straight-forward as using a set variable formula. Looking at bike specs and figuring out each ones WOT max RPM carb velocity I have "seen the light" and now know that the engineers have been picking flow velocities to match the size engine and type powerband they want. More velocity atomizes the gas better and gives better mid range power so the highest velocities are found on enduro bikes. Lower velocities are found on MX bikes. And the bigger the engine is, the more the engineers favored a higher velocity. So for enduro bikes the typical range is 33-40 meters/second, and for MX bikes 17-33 m/sec. I'm going to try to make a spreadsheet that allows the user to select the variables so the formula will spit out the needed size. Then the user will know to try for something close to that size.
 
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