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Table of Constants for Variable Speeds and Strokes


Stroke in Ins.

500 550 600 650 700 750 800 850 900 950 1,000 1,050 1,100 1,150 1,200



.257) .268

2.00.....| .205 214 2.25.. . .190 .200

.231 .241 3.50. . .223 .233 .245 2.75.. ... .245 256 .270 .282 .295 3.00... . .214 .227.241.254 .267 .279 .294 .308) -322 3.25..

.145.158.173-188.203.2191. .303 .319 .333 .349 3.50...... -326 -343 •359| .375 3.75...... .284.301.317 .334 •349 .368 .385.401 4.00.... .178.1961. -356.373 392 411 .429 4.25...... -341 -360 .378.396.416.436.456 4.50..... . 381 :400 .419 1441 .461 .485 4.75...... .423 .443.466 .486.509 5.00......223 .245.266.2901-312-335-357) -379-401-423 -445 .466.491 .512.536 5.25......2341 .257.279-304-327) -3511-375) -3981-421-444 .467/ .490 •5151-538.563 -5.50.. .245.270.293-318-343 - 368-393-417.441.465 .489 513 -540 -564-590 5.75.. -258.282-307-332-369-385-410.436 461-486-512-5361 .564.589 .616 6:00.. .3681.2941.320-348-375-402.428 455-481-508 535 559 .589 .615 .643

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1.00 3.35 1.50 1.75 3.00 3:25 3.50 3.75 4.00 4.25

Rule:-Multiply number to right of bore by number in upper table at intersection of proper R.P.M. and stroke columns.

Example:-Find, approximate horsepower developed by motor having 4.25" bore, and 5" stroke at 800 revolutions per minute.

Under 4.25 bore we find 18.05.

At intersection of s" stroke line and 800 R.P.M, column in upper table we find

.357. The product of these numbers gives the horsepower. thus:

Approximate norsepower - 18.05 X-357 -6 45 H.P.

7.55 9.00 10.56 12.25 14.05 16.00 18.05 20.20 11.55 35.00 27.50 30.30 33.os 36.00

These figures have been computed for the average M.E.P. as found in the ordinary motor car engine, but of course will vary with increase or decrease in compression and with different mechanical efficiencies.

•Por multi-cylinder engines, multiply by Qumber of cylinders, Abova formula gives H.P. for only ono cylinder.


4.75 5.00 5.35 5.50 5.75 6.00

D Diameter of cylinder in inches. L Stroke of piston in inches. R - Revolutions per minute of crank shaft. n = Number of cylinders.

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*The above are for automobile racing boat engines; for others the rating is taken as two-thirds of the above formulas. For ergines having a displacer cylinder or cylinders the above rating is increased in the ratio that the displacer piston's displace meni bears to that of the working cylinders.


On account of the great difficulty of securing good indicator cards at the high-piston and rotative speeds of automobile motors this is very little employed. The manograph which is employed for obtaining cards or diagrams from high-speed motors, as a general thing does not have an equal pressure scale and therefore does not readily lend itself to the accurate determination of I. H. P.

The indicated H. P. may of course be determined approximately by assuming a certain mechanical efficiency for the motor under consideration. This varies from as high as 90 per cent. in some cases to lower than 70 per cent. in others; the average would probably be not far from 80 per cent.

The formula for I. H. P., the bore and stroke being known as well as the R. P. M. and the mean effective pressure, is,

D' XL XR. P. M. Xn X M. E. P. - I. H. P. (for 4-cycle).

33,000. X 12 X 2


Di XL Xn X M. E. P. XR This constant equals 550,000 and the formula becomes

550,000. The constant for 2-cycle is 275,000.

A formula which is given by Grover for the mean effective pressure, the compression being known, is as follows:

M. E. P. - 2C

-0.01 C2 C - Compression pressure above atmosphere in pounds per square inch.

This formula does not hold good for compression pressures over 100 pounds per square inch above atmosphere. Comparatively recent data seems to prove the reliabihty of this formula, also the fact, which Grover points out, that under favorable conditions, the values given by this formula may be slightly exceeded. Tests with higher pressures than 100 pounds show that the M.' E. P. tends to remain at 100 pounds per square inch.

The compression pressure may be obtained by some form of gauge, the ignitica for that cylinder being cut off, or in case the volume of the explosion chamber is knoro it may be obtained from the formula.

PV - Constant. P being the absolute pressure.

V being taken in the one case as the volume of the combustion chamber and in the other as the above plus the piston displacement. P being in one case atmospheric pressure and in the other the absolute compression pressure. Of course for C in Grover's formula the assumed atmospheric pressure should be subtracted from the result ob tained by this formula.

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If it be assumed that the base of the triangle represents a line 1,000 feet long and that the first sloping line represents a road having a rise that brings it so feet above the starting point, this is figured as 50 feet in a thousand, or 5 per cent. In other words, one foot of rise for every 20 feet, but the latter instance does not mean distance actually traveled by a car in ascending such a slope, but distance measured horizontally with reference to that slope. The grade is measured by the tangent of the angle of inclination and not by its sine, so that a grade which represents 100 per cent. Cortesponds to an angle of inclination of but 45 degrees, and not go degrees, or perpendicular, as is commonly supposed. At the upper end of the next sloping line the elevation would amount to 669 feet, which is equivalent to a rise of one foot for every 64 feet traveled horizontally. So one in three cortesponds to a 33 1-3 per cent. grade, one in two to a 50 per cent. grade, and so on until a 100 per cent. grade is reached, which, as noted, is the equivalent of a 45 degree angle.

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