Article by: Harley C
Copyright 2005. All rights reserved.
Disclaimer: This information is strictly for educational purposes. The author may not be held responsible for any damages caused by making use of the information presented.
When tuning a 4-stroke internal combustion engine, it’s important to understand how much theoretical pressure is being exerted on the top of the piston during the power stroke. This information is useful as a comparison tool between different engines, and provides a good indicator of what performance levels can be achieved from different setups.
During normal engine operation, when a piston’s compressed air/fuel mixture is ignited during the power stroke, the resultant combustion exerts pressure on the top of the piston. Since this pressure ultimately results in the movement of the piston itself, we can consider it as being correlated to the power output of the engine. The average (mean) theoretical piston-top pressure that would result in the measured power output of an engine is referred to as the “Mean Effective Pressure”, and is an important value to consider when analyzing an engine.
Mean Effective Pressure can be calculated using the following formula:
MEP = 150.8 x (Torque / CID)
CID is the cubic-inch displacement of the engine under scrutiny. If you are going to use this formula for an engine with a displacement measured in metric cubic centimeters, you have to multiply the cubic-centimeters displacement by 0.061 to get the corresponding cubic-inch displacement value.
As an example, let’s consider a typical Honda B21A1 engine. The B21A1 has a torque rating of 135 ft/lbs at 5000rpm, and a displacement of 2056cc. First, we need to convert the metric displacement of the B21A1 to a value in cubic inches.
2056cc x 0.061 = 125ci
Plugging this value into our formula, we get:
150.8 x (135/125) = 150.8 x 1.08 = 162.9 psi
Not too bad. Let’s try another engine, perhaps the Honda H22A1 which has a displacement of 2156cc and makes 158 ft/lbs of torque at 5500rpm. Again, we start by converting our displacement to cubic-inches.
2156cc x 0.061 = 131.5
Now using the formula, we get:
150.8 x (158/131.5) = 150.8 x 1.2 = 181 psi
With the MEP values of these two Honda engines at hand, we can safely predict that the H22A1 will have a high power output which in reality it does.
So how high can MEP values go? Well, in theory the limit would be very high, but for a naturally aspirated engine running pump gasoline it would be very difficult to achieve MEP values over about 200psi.
So now you know a little bit about Mean Effective Pressure. It’s important to remember that the values produced by the formula are for theoretical analysis only, and do not reflect the actual pressures inside an individual combustion chamber. However, MEP is still a very handy way to get a quick idea of the potential output of an engine.
More information on this topic can be read in the following books:
The Internal Combustion Engine in Theory and Practice: Vol. 1
The Internal Combustion Engine in Theory and Practice: Vol. 2