2 Stroke Carburetor Sizing


Sizing is not about "flow resistance" but rather about matching all the other factors to have a combustion burn rate that gives the best power at the target RPM.


I found out that the formula manufacturers have used to figure out what carb diameter to use is not as simple as .8 x square root(cc x RPM/1000). They went by the flow velocity, compression ratio, cylinder diameter, and peak power RPM.



More velocity atomizes the fuel more which reduces the burn speed. More compression increases the burn speed. Also good to know is that high octane gas reduces the burn speed, and high squish velocity reduces it but I doubt manufacturers use that trick much because it also contributes to detonation. If you are wanting the best carb for racing and you can't get the program to show, by changing the cranking PSI, the same size you have now then you should buy the size indicated or just be content to know that a smaller size will be better for engine power at the beginning of the pipe powerband. Sometimes the crispness of the power exiting corners is just as important as the top RPM power.

Below is a screenshot of the Excel spreadsheet. To use it just enter the needed data into the light blue cells and the calculated results will display in the other cells. Best carb size for racing is shown at H11, and best carb size for street/trail is at G11




EXAMPLES:

Honda CR500: Honda equipped it with the same size carb as their 250. The calculator recommended a 40mm carb for trail/street and it uses a 38mm carb. I'm not sure if all CR500's had too high squish velocity but Honda used that trick instead of using a bigger carburetor. Unfortunately it often caused detonation. So if the squish velocity were reduced then it would need a bigger carb to get maximum power out of it. But with a 500cc that would probably bring it close to transmission destroying power.



Honda CR250: The calculator shows agreement with the carb size Honda equipped it with.


Honda CR125: The calculator shows a 33mm carb would be best. I know most all the 125's now use 38mm but I think that is more of a fad which the manufacturers have to cater to for sales. Tweaking the formula to show a bigger carb throws off the results for the other data sets I have. I believe tuners results more than a manufacturer that caters to the masses.



I went on some forums and asked for full data on peoples rides if they had experimented with different carb sizes for the highest power. Here are three examples:

2 cylinder 250cc for karting: I'm not sure which specific engine he has but karting engines are set to rev high and need big carbs. He selected the 42mm which my calculator is in agreement with if his cranking PSI is 182. (He said he didn't know the psi)



RD400 (2 cylinder): The calculator agrees with his selected 38mm if his psi is 209. I asked him about it since a stock RD400 is only around 109 and he said it had been bumped up and had a compression ratio more than 9.1:1 (192psi).


Honda NSR50: The calculator agrees with his selected 28mm if his psi is 20 less than what is typical for the stock compression ratio of that bike. 20 less is what I input when high octane gas is used because that slows down the burn and he did say he used 100 octane aviation gas.

Click here to watch my Youtube video on this subject.

Click here for a list of my 2 stroke calculators.

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