Auto Math 101

CUBIC INCH DISPLACEMENT

BORE X BORE X STROKE X .7854 X # CYLINDERS

EXAMPLE: SBC 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 COMPRSSION 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 LI9FT 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.​

 

A comment on carburetor CFM requirements

The constant 3456 is not a magical number pulled from thin air.

If one looks at the equation CFM = CID x RPM / 3456 one will note that cubic feet exists on the left side of the equation and cubic inches exist on the right side of the equation. To correct this, one must divide CID by (12 x 12 x 12) which is 1728. Since a four-stroke engine only draws on every other revolution, RPM must be divided by 2. Thus 12 x 12 x 12 x 2 is equal to 3456, the constant in the equation.

Understanding (opinion) is more important than equations.

Jon.