Torque Equation

[olddog]

Do you understand what Torque is?

If you attach a lever to a pivot point and apply a weight to the end of the lever, it will apply a rotational force to the pivot point.

Say the lever is 1 foot long and the weight is 100 lb.
The resulting torque is 1 ft * 100 lb = 1 ft-lb

If the lever is extended to 4 feet long: 4 ft * 100 lb = 400 ft-lb of torque

If we use the original 1 foot lever and increase the weight to 400 lb:
1 ft * 400 lb = 400 ft-lb

So a 4 ft lever with 100 lb weight and a 1 ft lever with 400 lb weight results in exactly the same about of torque.

There is no formula to convert cubic inches to torque. There are complex calculations that must be done that include bore, stroke, rod length, and cylinder pressure. The cylinder pressure is a huge set of equations that include compression ratio, amount of air, amount of fuel, type of fuel. You have to integrate these equations.

I think what you are wanting is a computer program like desk top dyno. They have done all this for you.


If you want my personal rule of thumb here it is. 1 lb-ft of torque per cubic inch was achievable with OHV mass produced engines in the 1950'S. This is easy to get. Any decent performance engine should make 1.25 lb-ft per cid. Normally aspirated 1.33 lb-ft per cid or more requires tuning ports and cams to supercharge the cylinder which requires expertise beyond the average hot rod guys.

In short shoot for 1.25 or so lb-ft of torque per cid. The head, intake, and cam combination will dictate what rpm range the engine can make that torque.