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.