AUTO MATH 101

DISPLACEMENT= 

BORE X BORE X STROKE X .7854 X # CYLINDERS =

CUBIC INCH DISPLACEMENT

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 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 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.