I'd Like to Play with the Question a Bit

Here's a quick overview of Manifold Pressure, with a lot of poetic license. See what "you'al" think of these thoughts...

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One formula for horsepower is: HP = T x N / 5252 T = torque in feet x pounds N = speed in revolutions / minute Ref: http://www.iprocessmart.com/techsmart/formulas.htm

Torque is hard to measure in an aircraft, but if you assume that you would make full rated power at maximum rated RPM, at standard atmospheric conditions, then you can get a torque estimate for your engine. My A80 Continental is rated at 80 horsepower at 2700 rpm. Torque at that point would be:

T = HP / N x 5252 = 80 / 2700 x 5252 = 155.6 ft-lbs

What is it that creates torque? It’s the average Brake Mean Effective Pressure (BMEP) acting on the pistons. BMEP is an old WW2 measure of power that was used with radial engines and read directly on the engineer's panel. Now let's take a leap of faith and assume that Manifold Pressure is similar to BMEP. That seems reasonable to me, that the pressure of the air coming into the cylinder is closely related to the pressure created on the piston, which in turn creates the torque. So... Manifold Pressure (MP) is related to torque.

MP is an absolute pressure, and at constant temperature air's mass is uniformly compressible. That is to say that at half pressure, you have half the mass of air, or air/fuel mixture in this case. So if you have half the pressure you'll have half the ability to make torque, and in turn, power.

We've all seen the toque curves for an engine at various RPMs. Those curves are generated by having a brake attached to the crankshaft, loading the engine running at full throttle. The RPM is then adjusted by the applied brake load, not by changing the wide open throttle. We can't do that with our fixed pitch props, at least not with such control. What does change the load on our engines is the aircraft speed. As airspeed increases, angle of attack on the propeller decreases, and the drag load is relieved. RPMs go up and MP decreases because that great big air pump is pumping more air, at a lower torque, causing a higher drop in pressure if the throttle plate is it is not moved.

Enough rationalization blabbering. Torque may be roughly, linearly equated to MP, and thus horsepower can be estimated from MP and RPM as follows:

Manufacturer's engine ratings are given for the ideal condition of a sea level pressure of 29.92 inches of mercury, at 59 degrees F standard temperature. A Manifold Pressure gage should read 29.92 if the aircraft is on an ocean beach and conditions are standard. Every naturally aspirated engine has a pressure loss through the carburetor. I'll just guess that it's . 92 inches of mercury, or about 1/2 of a psi. That means that the rated engine power was achieved at 29" MP. So now using the MP to estimate torque, and applying that to the original horsepower equation, you can calculate various conditions under which 75% power can be obtained from your engine. I'll use my engine for an example:

75% = .75 x 80 = 60 horsepower

T = MP/29 x 155.6 (this is the linear relationship assumed between MP and torque)

N = RPM

HP = T x N / 5252

60 = MP/29 x 155.6 x RPM / 5252

60 x 5252 x 29 / 155.6 = MP x RPM

MP x RPM = 58,730

So a set of points that would deliver 75% power can be calculated:

RPM = 58730 / MP (for 75% HP of the A80 engine) MP"hg RPM TO GET 75% POWER 16 3671 17 3455 18 3263 19 3091 20 2937 21 2797 22 2670 ~ rated RPM 23 2553 24 2447 25 2349 26 2259 27 2175 28 2098 29 2025

From the table it can be seen that this engine cannot generate 60 horsepower, without exceeding the rated RPM of 2700, with less than about 22 inches of MP. Notice that required RPM decreases as MP increases. If my goal is to set 75% power, then I would have to take this table up with me, climb to altitude, establish straight and level flight, and then starting from the low power side, increase MP until the RPM runs into the curve. Intuitively, RPM will increase with an increase in MP as you open the throttle. But since the table has decreasing RPM as MP increase, at some point you WILL hit the 75% power curve. I know that if I set 19" MP, my engine will not be going 3091 RPM, it will be much slower. But if I set 22", it very well may be going 2670 RPM. That would be 75% power. And if I set 27", I also know very well that my engine will be going much faster than 2175 RPM.

I know most of you have VWs. Just recalculate the rated horsepower torque for your engine. The numbers below are assuming the following information which I got from the GP website: http://www.greatplainsas.com/specsfd.html

Engine VW 2180 HP = 76 RPM = 3600

T = HP / N x 5252 = 76 / 3600 x 5252 = 110.9 ft-lbs 75 % power = 76 x .75 = 57 57 = MP/29 x 110.9 x RPM / 5252 57 x 5252 x 29 / 110.9 = MP x RPM RPM = 78283 / MP (for 75% HP of the VW 2180 engine)

MP"hg RPM TO GET 75% POWER 16 4893 17 4605 18 4349 19 4120 20 3914 21 3728 22 3558 23 3404 ~ max continuous RPM for the 2180 24 3262 25 3131 26 3011 27 2899 28 2796 29 2699

Keep in mind that available MP decreases with altitude. At about 8000 ft msl only 75% of the sea level pressure is even available. Engine RPMs will increase because the thinner air provides less drag, but the table relationship remains intact. If your engine will not over speed at wide open throttle and 8000 feet, than that is also a 75% power point and should come close to the curve. I'd be interested in anyone's feed back on these numbers at 8000 ft.

Dave AZ