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How to Calculate Cubic Feet per Minute (CFM) |
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Engine size (CID) x maximum RPM / 3456 = CFM This is calculated at 100% Volumetric Efficiency (VE) for Wide Open Throttle (WOT). However, most Street engines are only capable of achieving about 80% VE; a modified street engine can achieve about 90% VE and a fully modified race engine can achieve 95% or greater VE, so this will need to be factored into the calculation This formula is not new, but the carby size it calculates is generally always to small for best performance use. Dyno testing has shown me that I can fit a larger carb than the formula suggests and gain more horsepower. Spreadbore carbies where available, are the best choice to enjoy better fuel economy & have horsepower available when required. Square bore carbies were much cheaper and therefore used as an alternative. They were also easier to service, which made them more popular. However they just don't respond as well as a spreadbore in nearly all cases. For fuel economy I would use 80%VE, but remember the carb will be a little too small for the best performance output from the engine. For the best performance use 100%VE. You must balance performance with general driveability. This is best achieved using a spreadbore carb. If you can't get one in the size your after and are driving on the street then go down to the next size available. You may lose a little power but you will gain good low end response. As you can see, the formula is limited. I only ever used this formula for fitting a carby for economy and used the 80% VE to calculate this. In most cases the right size carb was not available so I generally had to go larger, also in most cases the engine does not have full power using the economy carb. I remember a limousine owner came up from the Gold Coast for me to fit a carby for him. He had a 390 CID engine that was very thirsty. He wanted to save fuel and asked for options. Being what it is I asked what RPM he normally goes to. He said no more than 4000. Using the formula @100% VE, I came up with 450 CFM. I fitted a Holley 450 CFM new generation spreadbore. The result was outstanding. The engine had very crisp throttle response and had loads of bottom end power but as I told him its runs out at higher RPM, which was not a problem for the type of driving he did. He rang me when he got back home and said the economy was way improved and he ordered another 4 carbs so he could convert the rest of his limo's. There are more choices available in carb sizes now so matching a carb to a specific need is much easier. Take a look at a few engines and what the formula recommends. The three columns on the right are what I recommend for specific purposes, nowadays I am sure there will be even better choices about. Colour codes: Green text = 2bbl carb Purple text = 4bbl square bore carb Red text = 4bbl spread bore carb
There is more to choosing a carburettor than just a formula. You don't drive around @ WOT while cruising do you. So when choosing a 4bbl carb the size of the primary venturi needs to be taken into account. Going to big will give poor low end response as the primary venturi will not be matched correctly to air flow, resulting in sluggish performance on pick-up. Going too small presents a problem with air flow through the venturi being to great. This will cause high air flow through the primary venturi at idle and this will result in a substantial pressure differential in the auxilary venturi. As we know from carb theory, fuel will discharge from the main discharge nozzle which will cause an over rich mixture at idle. The carb I fitted to the limo was not far from the border line of having too small a primary venturi. The next choice to make is whether to use vacuum operated secondaries or mechanical. I have seen so many people fit the wrong choice here it's not funny. The decision is simple and clear. Manual or automatic transmission, then take the other factors into consideration. There are some exceptions and you should talk to someone knowledgeable and who has experience with holly's. Vacuum secondary carburetors work best on: Mechanical secondary carburetors work best on: |