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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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Try different engine sizes and yours gets way out of whack without massive VE changes.

For instance, the original formula says a 300 with a VE of 0.8 needs a 36.7, 39.2, or 41.6 mm carb depending if you are spinning 7k, 8k, or 9k.

Same numbers in your formula spits out a unrealistic 46.2, 49.4, and 52.4 mm carb sizes for those rpms. VE need to goto about 0.5 to match the old way and give realistic numbers.

In the real world I've never seen a manufacturer NOT test a range of diameters and pick based off real results.

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I know the sizes look too big but it is correct for racing engines that consistently use the upper RPM range. For more moderate use you would want to use a smaller carb.

Your gripe about testing a range of sizes is a bit off. Yes we should test for different size engines but the new formula basically just gives a size 25% more than the old formula no matter what size engine you are testing.

It is assumed that the transfers of a race engine are maxed out with their size. Look at any good MX cylinder and you'll see for yourself. If you took an engine designed for moderate street use and ported/piped it to be a screamer then it would want a bigger carb but the limited transfer area would not be able to deliver all the intake charge from the crankcase into the cylinder at high RPM.

The smallest cross sectional area along any flow path is the "limiter". The carb diameter shouldn't be any more than the inner diameter of the intake manifold between it and the reed valve.

Also the flow area of the reeds should be measured. TorqSoft offers a reed petal deflection calculator for a $180 membership (ouch!): http://www.torqsoft.net/reed-petal-design.html I just do it the southern way and pull back on the reed with my fingernail very gently and measure how much it opens. The deflection times the flow area width gives you the total reed valve flow area. The carb flow area is (diameter/2)x(diameter/2)x3.14

Edited by jaguar57

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So the intention for the new formula is all out peak power, ridability be damned? Because a 50 mm carb on a 250 or 300 two stroke is not going to be very usable. If you are concentrating on just 125's though...

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The long version of the formula aims to have the same flow velocity thru the carb as racing 125s at top RPM. But like any formula it is theoretical. With most bikes the sizing of the intake tract already sets the limit for the carb size and so if you install a carb with a larger diameter than it then its larger size really is of no benefit. And also if the transfers aren't already racing size then forget it. So there are 3 different sizes here that have to be in harmony with each other.

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thanks for goading me to research more. It seems the carb area for the KTM SX250 is only 63% of what the formula recommends, and the KTM SX65 is only 76%.

So why would KTM maximize the carb area on the 125cc but not on the 65cc or 250cc?

Is the 125cc size optimal or excessive?

If we use 70% area of the complex formula then that is more or less this formula:

.88 x square root (engine cc * RPM/1000)

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OK here's the plus and the minus of it:

the smaller the carb the more pressure difference between crankcase and carb inlet. Basically you want as little pressure difference as possible to make as much use from the limited return diffuser wave (suction wave). More pressure difference equates to less flow.

but the larger the carb the slower the velocity thru the carb and past the jet and that limits the atomization of gasoline. The more gas is atomized the quicker it burns which usually translates to more power if the ignition timing is in harmony with it.

so a larger carb and intake tract does allow more intake flow up to a point. So now I want to find that point of balance.

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Looking at bike specs and figuring out each ones WOT max RPM carb flow velocity I have "seen the light" and now see 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 33-50 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.

Edited by jaguar57

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OK here is more or less what the manufacturers have done to determine carb sizing.

carbvelocity.gif

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