Post on 21-Aug-2015
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