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5 math formulas car nuts can actually use

Math is far from the most popular subject for many car folks. Who wants to be scribbling when you could be driving? The reality is, though, that there are a few formulas that come in handy. You might not even realize you know some of them, simply because you rarely write them out. We won’t demand you show your work, but here are a few formulas that just might help you on your next project.


This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/11/27/5-math-formulas-car-nuts-can-actually-use

In the displacement calculation formula, the number 2 is an EXPONENT (squared) not a factor.

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@dsherman427 - Sharp eye! We are getting that repaired now.

For displacement.
Bore diameter X bore diameter x .785 x stroke.
.785 is a constant that works with any diameter for the area of a circle be it inches or miles.
Basic math.

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I also like this one which shows horsepower is derived from torque. Dynos cannot measure HP directly, just torque (i.e. the twisting force of the engine).

HP = (Torque X 5252) / RPM.

This also means that on any dyno run, the HP and Torque lines must cross at 5252 RPM. It’s a good sanity check.

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We already know these. How about a formula for find the CFM of a carb when the throttle bores and air horn opening are known?

The often quoted “carb size” formula plain doesn’t work. A SBC w/moderate cam, decent compression, street headers, al heads and dual plane intake will be at 100% volumetric efficiency. In drag race form will produce 115% on dyno.

For years I always heard "you can not over carb an engine,as the max the carb can produce is wide open throttle. Is that right or wrong? Jim

I have seen that fuses in production cars tend to be about twice the expected load. A 10 amp circuit would have a 20 amp fuse.

Carb flow rates are more dark science. Different carb manufacturers rate at different atmospheric pressure. Air entry is also important. Has choke horn been cut off ? Square corners remaining machined ?

Short answer, Call Holley !

If that were true, we’d all be running Dominators !

Rate of Speed equals Distance divided by Time

Rate in miles/hour
Time in hours
Distance in miles

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It is easy to over carburate. If the carb is designed to have a maximum flow significantly higher than the engines max airflow, it is difficult to tune the carb to provide the proper mixture under all conditions. Particularly when opening the throttle a large amount, you tend to get too much air and not enough fuel, resulting in hesitation.

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@reelysalty - I did note that the volumetric efficiency varies by engine setup and can be 95% or better, I am curious as to why you say the formula does not work?

The fuse should be sized to protect the WIRE SIZE. You need a 12AWG to run 25 Amps at 20 feet of length, as an example. You would put a 25A fuse on that wire so it would not overheat, melting the insulation and shorting out. This is the rule of thumb used by most builders.
They also use the smallest size wire possible to keep overall weight down.

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Please explain the 3456 divisor in the CFM calculation. I understand converting cubic inches to cubic feet, but wouldn’t that be divide by (12x12x12) 1728?

For the formula on Volumetric Eff. for estimated carb size. I believe it is cubic inches X designed Max running RPM X Engine Eff. as you stated. 80% would be .8 in the equation. But since you want your answer in Cubic Feet/Min you need to add a conversion for cubic inches to cubic feet. So 12 cubed is 1728, not 3456. So you would divide by 1728. Also years ago the number for stock volumetric eff. was closer to .6. .8 (80%) seems a little high even though todays engines are more eff., they are still a glorified air pump with a lot of losses in the mix. Thanks for the article!

A four stroke engine only draws in air once every two (2) revolutions. Hence you need to divide by 2.

Air is drawn in only during one revolution. The carb size is based on the air drawn into the engine.

Great article and even better comments !