RAGENZ28

iTrader: (5)

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