# Auto Math 101

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

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:

• AMC: 4.75
• BUICK 350: 4.24
• BUICK 400-430,455: 4.75