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Calculate your Compression Ratio


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#1 TurbochargedBerserker

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Posted 04 May 2005 - 09:27 AM

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

Edited by TurbochargedBerserker, 04 May 2005 - 09:27 AM.

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#2 Xero

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Posted 04 May 2005 - 12:14 PM

CR =

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



OW MY HEAD!!!!!!! :crazy: :eek: :cry:
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#3 Past Owner

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Posted 04 May 2005 - 01:20 PM

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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#4 Mr. P.

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Posted 04 May 2005 - 01:39 PM

Maybe just a rough-n-dirty GenIII bore/stroke vs head CC table?
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#5 SuperSport

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Posted 04 May 2005 - 03:18 PM

Thanks for this thread guys, I was looking for some of these formula's :thumbs:
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#6 2bseen

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Posted 05 May 2005 - 09:03 AM

My head hurts just reading this.
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#7 04CHASE

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Posted 24 April 2006 - 01:29 PM

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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#8 Mr. P.

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Posted 24 April 2006 - 02:10 PM

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!

<{POST_SNAPBACK}>

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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#9 04CHASE

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Posted 24 April 2006 - 02:25 PM

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!

<{POST_SNAPBACK}>

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.

<{POST_SNAPBACK}>


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

Edited by 04CHASE, 24 April 2006 - 02:26 PM.

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#10 Mr. P.

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Posted 24 April 2006 - 02:49 PM

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.

Edited by misterp, 24 April 2006 - 02:54 PM.

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#11 TurbochargedBerserker

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Posted 27 April 2006 - 09:28 AM

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.
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#12 Mr. P.

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Posted 27 April 2006 - 10:52 AM

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.

<{POST_SNAPBACK}>

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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