Automotive Calculations SBC / BBC
#1
Automotive Calculations SBC / BBC
Some math calculations for commonly asked SBC / BBC engine building questions.
CUBIC INCH DISPLACEMENT
BORE X BORE X STROKE X .7854 X # CYLINDERS =
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PISTON TO DECK CLEARANCE VOLUME
BORE X BORE X 12.87 X PISTON TO DECK CLEARANCE [PLUS OR MINUS ]
EXAMPLE:4.030 X 4.030 X 12.87 X .009 = 1.88cc
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COMPRESSED GASKET VOLUME
BORE X BORE X 12.87 X COMPRESSED GASKET THICKNESS
EXAMPLE: 4.030 X 4.030 X 12.87 X .030 = 6.27cc
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NET COMBUSTION CHAMBER VOLUME
1. DOME PISTON EQUATION
NET COMBUSTION CHAMBER VOLUME = COMBUSTION CHAMBER VOLUME--PISTON DOME VOLUME + [PISTON DECK CLEARANCE + COMPRESSED GASKET THICKNESS]
EXAMPLE: 64cc HEADS, 12cc DOME PISTON, 1.88cc OF PISTON TO DECK CLEARANCE AND 6.27cc OF COMPRESSED GASKET VOLUME
64-12=52
1.88+6.27=8.15
52+8.15=60.15cc
ALL CALCULATIONS MUST BE IN cc's
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2. DISHED OR FLAT TOP PISTON EQUATION NET COMBUSTION CHAMBER VOLUME = COMBUSTION CHAMBER VOLUME + PISTON DISH OR VALVE POCKET VOLUME + PISTON DECK CLEARANCE + COMPRESSED GASKET THICKNESS
EXAMPLE: 64cc HEADS, A-12cc PISTON DISH OR VALVE POCKET, 1.88cc OF PISTON TO DECK CLEARANCE AND 6.27cc OF COMPRESSED GASKET VOLUME
64cc + 1.88 + 6.27 =84.15cc
ALL CALCULATIONS MUST BE IN cc's
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COMPRESSION RATIO
CUBIC INCH DISPLACEMENT ÷ # CYLINDERS x 16.39 ÷ NET COMBUSTION CHAMBER VOLUME + 1
EXAMPLE: DIVIDE 355 BY 8 = 44.375
MULTIPLY 44.375 NY 16.39 = 727.306
DIVIDE 727.306 BY YOUR NET COMBUSTION CHAMBER VOLUME, LETS SAY IT'S 60.15 = 12.09 add 1
YOUR COMPRESSION RATIO WOULD BE 13.09-1
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BORE / STROKE RATIO
BORE ÷ STROKE
EXAMPLE: 4.030 ÷ 3.48 = 1.158
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ROD RATIO
CONNECTING ROD LENGTH ÷ CRANKSHAFT STROKE
EXAMPLE: 5.700 ÷ 3.489 = 1.637
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GROSS VALVE LIFT
LOBE LIFT X ROCKER RATIO = GROSS VALVE LIFT
EXAMPLE: .400 x 1.7 = .680
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CARBURETOR CFM
WE MUST KNOW THE THEORETICAL AIR CAPACITY TO DETERMINE THE VOLUMETRIC EFFICIENCY
THEORETICAL CFM = RPM X DISPLACEMENT ÷ 3456
VOLUMETRIC EFFICIENCY = ACTUAL CFM ÷ THEORETICAL CFM X 100
*GENERAL GUIDELINES FOR CARBURETOR, REQUIREMENTS WILL VARY*
STREET CARB. CFM = RPM X DISPLACEMENT ÷ 3456 X .85
RACING CARB. CFM = RPM X DISPLACEMENT ÷ 3456 X 1.10
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PISTON SPEED
STROKE IN INCHES X RPM ÷ 6
RESULTS MEASURED IN FEET PER MINUTE
EXAMPLE: 3.48 X 6000 ÷ 6 = 3480 FPM
CUBIC INCH DISPLACEMENT
BORE X BORE X STROKE X .7854 X # CYLINDERS =
__________________________________________________ ______
PISTON TO DECK CLEARANCE VOLUME
BORE X BORE X 12.87 X PISTON TO DECK CLEARANCE [PLUS OR MINUS ]
EXAMPLE:4.030 X 4.030 X 12.87 X .009 = 1.88cc
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COMPRESSED GASKET VOLUME
BORE X BORE X 12.87 X COMPRESSED GASKET THICKNESS
EXAMPLE: 4.030 X 4.030 X 12.87 X .030 = 6.27cc
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NET COMBUSTION CHAMBER VOLUME
1. DOME PISTON EQUATION
NET COMBUSTION CHAMBER VOLUME = COMBUSTION CHAMBER VOLUME--PISTON DOME VOLUME + [PISTON DECK CLEARANCE + COMPRESSED GASKET THICKNESS]
EXAMPLE: 64cc HEADS, 12cc DOME PISTON, 1.88cc OF PISTON TO DECK CLEARANCE AND 6.27cc OF COMPRESSED GASKET VOLUME
64-12=52
1.88+6.27=8.15
52+8.15=60.15cc
ALL CALCULATIONS MUST BE IN cc's
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2. DISHED OR FLAT TOP PISTON EQUATION NET COMBUSTION CHAMBER VOLUME = COMBUSTION CHAMBER VOLUME + PISTON DISH OR VALVE POCKET VOLUME + PISTON DECK CLEARANCE + COMPRESSED GASKET THICKNESS
EXAMPLE: 64cc HEADS, A-12cc PISTON DISH OR VALVE POCKET, 1.88cc OF PISTON TO DECK CLEARANCE AND 6.27cc OF COMPRESSED GASKET VOLUME
64cc + 1.88 + 6.27 =84.15cc
ALL CALCULATIONS MUST BE IN cc's
__________________________________________________ _____
COMPRESSION RATIO
CUBIC INCH DISPLACEMENT ÷ # CYLINDERS x 16.39 ÷ NET COMBUSTION CHAMBER VOLUME + 1
EXAMPLE: DIVIDE 355 BY 8 = 44.375
MULTIPLY 44.375 NY 16.39 = 727.306
DIVIDE 727.306 BY YOUR NET COMBUSTION CHAMBER VOLUME, LETS SAY IT'S 60.15 = 12.09 add 1
YOUR COMPRESSION RATIO WOULD BE 13.09-1
__________________________________________________ ____
BORE / STROKE RATIO
BORE ÷ STROKE
EXAMPLE: 4.030 ÷ 3.48 = 1.158
__________________________________________________ ____
ROD RATIO
CONNECTING ROD LENGTH ÷ CRANKSHAFT STROKE
EXAMPLE: 5.700 ÷ 3.489 = 1.637
__________________________________________________ ____
GROSS VALVE LIFT
LOBE LIFT X ROCKER RATIO = GROSS VALVE LIFT
EXAMPLE: .400 x 1.7 = .680
__________________________________________________ ____
CARBURETOR CFM
WE MUST KNOW THE THEORETICAL AIR CAPACITY TO DETERMINE THE VOLUMETRIC EFFICIENCY
THEORETICAL CFM = RPM X DISPLACEMENT ÷ 3456
VOLUMETRIC EFFICIENCY = ACTUAL CFM ÷ THEORETICAL CFM X 100
*GENERAL GUIDELINES FOR CARBURETOR, REQUIREMENTS WILL VARY*
STREET CARB. CFM = RPM X DISPLACEMENT ÷ 3456 X .85
RACING CARB. CFM = RPM X DISPLACEMENT ÷ 3456 X 1.10
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PISTON SPEED
STROKE IN INCHES X RPM ÷ 6
RESULTS MEASURED IN FEET PER MINUTE
EXAMPLE: 3.48 X 6000 ÷ 6 = 3480 FPM