Engine Calculations

Displacement, Compression Ratio, Horsepower, Torque

Displacement

• The volume of the cylinder between the top dead center and bottom dead center positions of the piston

• Does not include the combustion chamber or other non swept space in the cylinder

Displacement Formula

• Cylinder displacement • CID = ( )2 x 3.14 x Stroke

• Engine displacement• Cylinder Displacement x Number of Cylinders

• Displacement conversions

• In3 x 16.4 = cm3 (cc’s)

• Cm3 divided by 1000 = Liters

Bore2

Example

• Bore = 4.030”

• Stroke = 3.48”

• 8 cylinder engine

• CID = ( )2 x 3.14 x 3.48

• CID = (2.015)2 x 3.14 x 3.48

• CID = 4.060 x 3.14 x 3.48

• CID = 44.367 in3

• 44.367 in3 x 8 cylinders =354.9 in3

4.0302

Example Cont’

• 354.9 in3 x 16.4 = 5820 cm3

• 5820 cm3 divided by 1000 = 5.8 Liters

Compression Ratio

• The ratio between the volume of the cylinder at BDC and the volume of the cylinder at TDC

Compression Ratio Formula

• Compression ratio = V1 + V2 +V3 +V4

V2 +V3 +V4

• V1= Cylinder displacement volume

• V2 = Deck clearance volume

• V3 = Head gasket volume

• V4 = Combustion chamber volume

Example

• .010 Deck clearance

• .041 Compressed head gasket thickness

• 64 cc combustion chamber

(64cc / 16.4 = 3.90 in3)

• 44.367 in3 + .127 in3 + .523 in3 + 3.90 in3

.127 in3 + .523 in3 + 3.90 in3

• 10.75:1 Compression ratio

Engine Factors Affecting Compression Ratio

• Piston design• Head design (flat top, dish, dome, valve reliefs)• Compression height – distance from the centerline of the

piston pin to the top of the piston • Ring height

• Deck height – distance from the centerline of the main journals to the block deck surface

• Bore• Stroke• Combustion chamber size• Valve head design (flat, dished, domed)• Head gasket thickness (and design)

Torque

• Twisting force

• Measured in lb/ft

Horsepower

• Calculated from torque and RPM measurements

• One horsepower is the power needed to lift 550 pounds one foot in one second

• Horsepower = Torque x RPM5252

• Horsepower and torque values are always the same at 5252 RPM’s