What Size Carburetor Is Correct for My Engine: The Calculation Formula


When you are thinking about What size carburetor is correct for my engine ? know that there are some ways that you can use to choose the right carb for your engine. You won’t have to worry about the size or the arrangement when you will never make any change to the engine. However, when you are in the offroad industry or you simply like tweaking the engine, making changes to the engine and the carburetor is inevitable. So, what would you do about the whole arrangement?

The Most Common Misconceptions

The saying ‘bigger is (always) better’ doesn’t always apply to this world – and this condition. In general, most people think that bigger carb are related to more power – mostly because of the size that can accommodate more air and more fuel. Is it really true? Well, most people think it is so they will likely buy the biggest and baddest carb available.


What most people don’t know is that if the power is too much and excessive, it will choke the engine’s performance. You want to choose the right carb, because you want to have a proportional carb that fits to the volumetric potential of the engine (the breathing ability). The volumetric efficiency itself is the measurable value. When it is paired with the right carb, you will get the most efficient and the most powerful outcome from the engine.


So, how to answer the question of what size carburetor is correct for my engine? No worry, there is a calculating formula that you can try. You want to know the Cubic Feet a Minute (CFM) that is needed by the engine. You multiple RPM and cubic inches and Volumetric Efficiency and then divided by 3456. And that’s how you get the CFM.


If you choose the regular stock engine, the volumetric efficiency is set around 80% whereas the rebuilt street engines are set around 85%. What about the race engines? They have the range from 95% to 110%.


Let’s take an example of CID 355 engine x 5,500 maximum RPM will result in 1,952,500. Then multiple it with .85 resulting in 1,659,625. And then divide it with 3456, resulting in 480 CFM. With around 10% cushion, the carb with 500 CFM will be able to deliver impressive performance. If you want to try this method, you need to be truthful and honest about the engine RPM. It is always better to come with a carburetor smaller than you need than having an oversized carb that can actually make the bike’s ability poor in matter of performance and also drivability.


The Final Conclusion

As you can see, the bigger isn’t always the better – at least for this technical matter in the auto and bike industry. So, the answer to the question what size carburetor is correct for my engine depends on the condition and output of your bike.  Since you now know the formula to calculate everything, you can determine the numbers on your own.


It doesn’t hurt if you want to consult a pro if you want to know the exact calculation and numbers. If you consult an expert, at least, you can get the answer to the question what size carburetor is correct for my engine.

Leave A Reply