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The Great Weldini
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Discussion Starter #1 (Edited by Moderator)
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 COMPRESSION RATIO WOULD BE 13.09-1




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
 

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Well?
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10,814 Posts
nice V8 info lol
 

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The Great Weldini
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8,901 Posts
Discussion Starter #7
Id love to see this without the caps lock on, but maybe thats just me.
me to but i cut and paste and i didn't want to rewite it for your sweetass so i said fuck it.
 

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when your talking about piston speed your forgot to multiply the stroke x2.

When the crankshaft does one full revolution the piston goes (if starting from bdc) to tdc and then back down to bdc. So that would be one full stroke upwards and then a full stroke downwards. So stroke x2 ;)
 

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when your talking about piston speed your forgot to multiply the stroke x2.

When the crankshaft does one full revolution the piston goes (if starting from bdc) to tdc and then back down to bdc. So that would be one full stroke upwards and then a full stroke downwards. So stroke x2 ;)
It's already accounted for in his conversion factor. That's why you divide by 6, not 12.
 
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