C5 Performance - Carburetor Calculator

Please answer the questions as accurately as possible. This form will help you determine the cfm requirements of your engine at various speeds. Based on experience the VE (volumetric efficiency) used to calculate CFM is set at 80% in our algorithm for best results on 4-8 cylinder tractor engines. John Deere twin cylinder engines require slightly different calculations so we have designed a widget just for them.

We suggest you measure your engine intake manifold vacuum under various loads. This will help you understand if a rated carb cfm will be accurate for your use (most are measured at 20.4" while actual engine vacuum can easily drop to near zero under heavy load). As load increases, intake vacuum decreases, which generally causes agricultural carburetors to faulter and "go lean".

The C5 carburetor size calculator will help you understand the "RANGE" of cfm needed from high rpm to low. Regardless of the reason, as rpm drops, so does the required cfm. Determining the low and high rpm range during a pull will help you choose the correct size carb, with particular focus on where your "peak torque" will be for the best match.

A carburetor that is too large for the application will not operate correctly at low rpm and with low manifold vacuum. It can cause erratic A/F readings and cause lean running during heavy loads.

A carburetor that is too small will work great at low rpm but limits horsepower at high rpm and may begin to exhibit A/F bounce as the signal starts to fail.

We hope this provides you with a better understanding of how a change in engine size or target rpm affects the intake/exhaust math.

We are creating an online quote that will be used to simplify orders. Until then, please contact us via

email (paul@c5ignitions.com)

call 920-403-0555

text 920-810-0946

weekdays 9am-6pm