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Calculate your Compression Ratio


TurbochargedBerserker

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Math:

 

Formula 1:

 

CR =

 

[{(B/2)^2 * S*Pi}+Vcc+Vp+{(B/2)^2*Pi*(DH+TG)}]

[{(B/2)^2*Pi*(DH+TG)}+Vcc+Vp]

 

Where:

 

CR= Compression Ratio

B = Cylinder bore (Stock LT1 = 4.00in)

S = Stroke (Stock LT1 = 3.48in)

Vcc= Combustion Chamber Volume (Stock LT1 = 58cc = 3.539 in3)

Vp= Piston Volume (Stock LT1 = 4.5cc = 0.274606 in3)

DH = Deck Height (Stock LT1 = 0.015 in)

TG = Head Gasket Crush Thickness (Stock LT1 = 0.05 in)

Pi = 3.1415

 

 

Formula 2: (I prefer this one)

 

Compression Ratio = (GV+DV+HV-VV+PV) / (GV+DV+HV-VV)

 

 

GV (Head Gasket Volume) = Bore(in) X Bore(in) X 12.87 X (Head Gasket Thickness in Inches)

 

DV (Below Deck Volume) = Bore(in) X Bore(in) X 12.87 X (Inches Below the Deck)

 

HV (Head Volume) = CC's

 

VV (Dish, Valve Pocket, Dome Volume) = CC's

(Minus for dish or valve pockets, Plus for dome)

 

PV (Volume displaced by Piston) = Bore X Bore X Stroke X 12.87

 

 

Some links:

 

Smokeup's CR Calculator

 

Smokeup's Dynamic CR calculator

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Try some of these LSR formulas

 

Formulas:

 

The horsepower needed to overcome aerodynamic drag at a given speed is:

HP = k*A*Cd*v^3

Where HP is horsepower, k is the density of air, A the frontal (cross-sectional) area, Cd the coefficient of drag,

and v the velocity. The density of air depends on conditions but is nominally .08 lb/ft^3.

 

Aerodynamic drag is calculated as:

 

F = 1/2 CDAV^2

Where: F - Aerodynamic drag force, C - Coefficient of drag, D - Density of air (nominally about 0.08 pounds

per cubic foot..

yes I know that's not a technically accurate mass but it saves converting to and then back from metric), A -

Frontal area, V -

Velocity of object

 

displacement, stroke & bore:

 

pi = 3.1415927

pi/4 = 0.7853982

cylinder volume = pi/4 x bore2 x stroke

stroke = displacement / (pi/4 x bore2 x number of cylinders)

 

 

piston speed:

 

piston speed in fpm = stroke in inches x rpm / 6

rpm = piston speed in fpm x 6 / stroke in inches

 

brake horsepower:

 

Horsepower = rpm x torque / 5252

torque = 5252 x horsepower / rpm

brake specific fuel consumption = fuel pounds per hour / brake horsepower

bhp loss = elevation in feet / 1000 x 0.03 x bhp at sea level

 

indicated horsepower & torque:

 

horsepower = mep x displacement x rpm / 792,00

torque = mep x displacement / 150.8

mep = hp x 792,000 / displacement x rpm

mep = hp x 792,000 / displacement x rpm

mechanical efficiency = brake output / indicated output x 100

friction output = indicated output - brake output

taxable horsepower = bore2 x cylinders / 2.5

 

air capacity & volumetric efficiency:

 

theoretical cfm = rpm x displacement / 3456

volumetric efficiency = actual cfm / theoretical cfm x 100

street carb cfm = rpm x displacement / 3456 x 0.85

racing carb cfm = rpm x displacement / 3456 x 1.1

 

weight distribution:

 

percent of weight on wheels = weight on wheels / overweight x 100

increased weight on wheels = [ distance of cg from wheels / wheelbase x weight ] + weight

 

center of gravity:

 

cg location behind front wheels = rear wheel weights / overall weight x wheelbase

cg location off-center to heavy side = track / 2 - [ weight on light side / overall weight ] x track

cg height = [ level wheelbase x raised wheelbase x added weight on scale / distance raised ] x overall weight

 

g force & weight transfer:

 

drive wheel torque = flywheel torque x first gear x final drive x 0.85

wheel thrust = drive wheel torque / rolling radius

g = wheel thrust / weight

weight transfer = weight x cg height / wheelbase x g

lateral acceleration = 1.227 x radius / time2

lateral weight transfer = weight x cg height / wheel track x g

centrifugal force = weight x g

 

shift points:

 

rpm after shift = ratio shift into / ratio shift from x rpm before shift

drive-shaft torque = flywheel torque x transmission ratio

 

instrument error:

 

actual mph = 3600 / seconds per mile

speedo error percent = difference between actual and indicated speed / actual speed x 100 indicated distance = odometer reading at finish - odometer reading at start odometer error percent = difference between actual and indicated distances / actual distance x 100

 

MPH RPM gears & tires:

 

mph = rpm x tire diameter / gear ratio x 336

rpm = mph x gear ratio x 336 / tire diameter

gear ratio = rpm x tire diameter / mph x 336

tire diameter = mph x gear ratio x 336 / rpm

 

tire size & their effect:

 

tire diameter = 2 x selection width x aspect ratio / 2540 + rim diameter

effective ratio = old tire diameter / new tire diameter x original ratio

actual mph = new tire diameter / old tire diameter x actual mph

 

Simple Conversions:

 

1 inch = 25.4 mm

1 mm = 0.039371 inch

1 cubic inch = 16.387 cc's

1 liter = 1000 cc's = 61.024 cubic inches

Liter to cubic inch conversion: 61.026 ci/L

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  • 11 months later...

ok i did good in all my math classes but cant seem to find the #'s i need. i plan on doing a 408 (supercharged). going to port and polish the stock heads, or maybe a cheaper aftermarket head, patriots most likely, but im open to suggetions. i just need to know what cc pistons to go with to get 10-0-1 cr . thanks in advance!

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ok i did good in all my math classes but cant seem to find the #'s i need. i plan on doing a 408 (supercharged). going to port and polish the stock heads, or maybe a cheaper aftermarket head, patriots most likely, but im open to suggetions. i just need to know what cc pistons to go with to get 10-0-1 cr . thanks in advance!

Using the following figures:

 

Stroke Length = 4.000 in

Bore Diameter = 4.030 in

Head Combustion Chamber Volume = 72 cc

Head Gasket Thickness = 0.040 in

Head Gasket Bore Diameter = 4.010 in

Piston Deck Height = 0.000 in

Piston Dish Volume = 13 cc

Nominal CR = 9.96:1

 

Double-check my head gasket thickness number and piston deck heights, but that should get you very close.

 

Mr. P.

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ok i did good in all my math classes but cant seem to find the #'s i need. i plan on doing a 408 (supercharged). going to port and polish the stock heads, or maybe a cheaper aftermarket head, patriots most likely, but im open to suggetions. i just need to know what cc pistons to go with to get 10-0-1 cr . thanks in advance!

Using the following figures:

 

Stroke Length = 4.000 in

Bore Diameter = 4.030 in

Head Combustion Chamber Volume = 72 cc

Head Gasket Thickness = 0.040 in

Head Gasket Bore Diameter = 4.010 in

Piston Deck Height = 0.000 in

Piston Dish Volume = 13 cc

Nominal CR = 9.96:1

 

Double-check my head gasket thickness number and piston deck heights, but that should get you very close.

 

Mr. P.

 

thanks mr p those were the # 's i was looking for! but will the head cc chanmer volume change once the heads are ported?

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thanks mr p those were the # 's i was looking for! but will the head cc chanmer volume change once the heads are ported?

Port work has no effect; you loose head CCs if the deck surface is remachined. Every machine shop should have a burette and be able to tell you the actual measured chamber volume post-machining.

 

Mr. P.

 

FYI - resurfacing the deck also directly affects the intake manifold seal as well, you are making the 'V' between banks wider by the amount being removed from the deck. Surfacing to excess will require additional corrective machining to prevent vacuum leaks between the intake manifold and head.

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Piston deck height is -0.008

 

If you are using MLS gaskets, the compressed thickness is 0.058, but you need to pick a gasket to get this number locked in.

I plugged in those numbers and wow that extra .010" of gasket makes more difference that I imagined, dropping the CR to 9.6:1; if using that gasket you need to reduce Piston Dish Volume to 9cc to maintain 10:1 CR (assuming 72cc head).

 

Mr. P. :)

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