Indicated Mean Effective Pressure

You can't measure the horsepower and torque developed within the cylinders directly. However, using a device called an indicator, you can measure cylinder pressure during each of the four strokes-intake, compression, combustion, and exhaust-and, from them, you can find the indicated mean effective pressure, or mep, which is a form of output that occurs within the cylinders and is unaffected by friction and inertia. When the indicated mep is known, it's possible to calculate the indicated horsepower and torque within the cylinders.

An indicator is not something the average hot rodder is likely to have readily avaiable. Nonetheless, the serious peroformance enthusiast should be aware of the interrelationships of mep, hrosepower and torque. (It should be noted that the formulas involving these interrelationshps are eqaully valid for either brake or indicated figures.

Indicated Horsepower

There's a formula for calculating horsepower from mep that's favored by many engineering theorists becasue it involves a simple acronym, PLAN, that's easy to remember:

Horsepower = P x L x A x N/33,000

P stands for the mep in pounds per square inch or psi; L for the length of the stroke in feet; A for the top surface area of one piston in square inches; and N for the number of power strokes per minute. When these four factors are multiplied together, they show the total amount of torque the engine develops in one minute. That figure is then divided by 33,000- the number of pounds-feet per minute equal to one horsepower–to find the total horsepower.

Part of the appeal of PLAN is that it focuses on the aspects of engine design that ultimately determine horsepower. When you modify an engine to improve performance, you are really increasing P,L,A and/or N.

For example, when you rasie the compression ratio, P is increased. When an engine is bored or stroked, A or L is increased. When you regrind the crankshaft to get higher rpm, you increase N.

The formula is awkward in practical use, though because it requires separate calculationsto find L, A and N. To find L, the stroke in inches is divided by 12 or

L = stroke/12

To find A, divide pi by 4 then multiply it by the bore squared or:

A = pi/4 x bore(squareed)

And, to find N for a conventional four-stroke-cycle engine, divide the rpm by 2 and multiply the result by the number of cylinders, or:

N = rpm/2 x no. of cylinders

So our "simple" acronym evenutually leads to a much more complicated formula:

horsepower = mep x stroke x bore(squared) x pi x rpm x no. of cylinders/12 x 4 x 2 x 33,000

But look closely. Embedded in there are factors of the formula for displacement:

Displacement = pi/4 x bore(squared) x stroke x no. of cylinders

You can remove those and use the displacement in their instead, simplifying the raw formula to:

Horsepower = mep x displacement x rpm/ 12 x 2 x 33,000

And that, of course, can be reduced still further to:

Horsepower= mep x displacement x rpm/ 792,000

Example: Let's take as an example a 302 cubic-inch engine which has an indicated mep of 175 psi at 4200 rpms:

Horsepower = 175 x 302 x 4200/792,000

Working the equation through gives an answer of 280.265 indicated horsepower