CUBIC INCH DISPLACEMENT
BORE X BORE X STROKE X .7854 X # CYLINDERS
4.030 x 4.030 x 3.750 x .7854 x 8 = 382.6 ( 383 Chevy )
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
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
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
NOTE: ALL CALCULATIONS MUST BE IN cc’s
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
NOTE: 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 BY 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 1309-1
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
PISTON SPEED
STROKE IN INCHES X RPM ÷ 6
RESULTS MEASURED IN FEET PER MINUTE
EXAMPLE: 3.48 X 6000 ÷ 6 = 3480 FPM
CARBURETOR CFM
BIGGER IS NOT ALWAYS BETTER
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
STREET CARB. CFM = RPM X DISPLACEMENT ÷ 3456 X .85
RACING CARB. CFM = RPM X DISPLACEMENT ÷ 3456 X 1.10
CAM LIFT vs. GROSS VALVE LIFT
CAM LIFT OR LOBE LIFT X ROCKER RATIO = GROSS VALVE LIFT
THIS LEADS ME TO THE NEXT FORMULA TO DETERMINE MY GROSS VALVE LIFT IF I CHANGE MY ROCKER RATIO SUCH AS , INSTALLING 1.6 SBC ROCKERS WITH A COMP. CAMS XTREME ENERGY # XE 268H CAMSHAFT. CURRENT LISTING STATES .477/.480 LIFT w/1.5 ROCKERS
VALVE LIFT ÷ ROCKER RATIO = LOBE LIFT [ CAM LIFT ]
LOBE LIFT X ROCKER RATIO = GROSS VALVE LIFT
EXAMPLE: .477 ÷ 1.5 = .318 [ LOBE LIFT ]
.318 X 1.6 = .5088 [ GROSS VALVE LIFT ]
NOTE: IF THE CAM HAS A DIFFERENT LIFT FOR THE EXHAUST, USE THE SAME FORMULA FOR THE EXHAUST. ALL OTHER SPEC’S FOR THE CAMSHAFT REMAIN THE SAME.
THEREFORE THE COMP. CAMS #XE 268H USING 1.6 ROCKER RATIO PRODUCES .508/ .512 LIFT.
Quick Reference – “Bore Spacing”
Bore spacing is the distance from the center line of one cylinder bore to that of the adjacent cylinder. Assuming the bores are perfectly round, this distance can be determined by measuring the distance from one cylinder wall edge to the far cylinder wall of the adjacent cylinder. To save all of our Greaseralley readers the trouble, here’s the bore spacing (in inches) on common American V8’s:
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AMC: 4.75
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BUICK 350: 4.24
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BUICK 400-430,455: 4.75
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CADILLAC 472, 500: 5.00
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CHEVY SMALL BLOCK: 4.40
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CHEVY BIG BLOCK: 4.84
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MOPAR SMALL BLOCK: 4.46
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MOPAR BIG BLOCK: 4.80
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FORD SMALL BLOCK: 4.38
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FORD FE: 4.63
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OLDSMOBILE: 4.625
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PONTIAC: 4.62
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