I have been wondering this for awhile. Does anyone have an accurate formula to figure out compression ratios? In addition, is there any other formulas that anyone knows that would be useful at all????

I have ported and polished nonPI heads on a 03 Cobra bottom end. From what I understand, my nonPI comprssion was 9.0:1. A Termi is about 8.5:1 w/ those pistons. Porting the heads may change that some and I will be upgrading the valvetrain too when I get her back and running. That may change a bit too. I will be getting some flow numbers on the heads from MMR who ported them too. ANy ideas anyone??? THanks.
[align=left] [/align]

 

You need to know bore and stroke, and the volume of the combustion chamber at TDC (aka "squish" volume). The first two are easy, thecombustion chamber (squish)volume (Vcc) is difficult to calculate (because of the uneven shape of the space, domed/recessed pistons etc.)--it is usually directly measured using fluid.[/align][/align]Anyway, once you have that info then it's:[/align][/align]Cratio = (Vcyl +Vcc) / Vcc[/align][/align]Vcyl = cylinder volume = B^2/4 * pi * S[/align] where B = bore, S = stroke pi = 3.14[/align][/align]Vcc = combustion chamber (squish space) volume[/align][/align]On an assembled engine you can determine the squish volume by turning the engine to TDC on some cylinder and then filling the combustion chamber (through the plug hole, to the bottom of the plug hole) with a measured amount of engine oil. The amount of oil it takes equals the squish volume. Suck the oil out with a vacuum tool or whatever, however be aware that the engine will smoke like heck when you start it and may foul the plug.[/align][/align]Forour 4.6L engines the standard bore is 90.2 mm, stroke is 90 mm;[/align][/align]making Vcyl = 90.2^2/4 * pi * 90 = 0.575 L. [/align][/align]If it took 57.5 mL (0.057 L) of oil to fill the squish then the compression ratio would be,[/align][/align]Cr = (0.575 + 0.0575)/ 0.0575= 11:1.[/align][/align]Hope this is of some assistance...[/align]

 

I got something alittle different.

Formulas for compression ratio

(CylVolume + ChamberVolume) / ChamberVolume

cylinder volume = pi/4 x bore^2 x stroke

chamber volume = cylinder volume / compression ratio - 1.0

displacement ratio = cylinder volume / chamber volume

amount to mill = (new disp. ratio - old disp. ratio / new disp. ratio x old disp. ratio) x stroke

 

Hell lets make this a fun thread here is every formula you will probably ever need.

Many of the formulas use the value of pi which is 3.1415927
Some formulas contain notation such as ^2 which means "squared" or ^3 which means "cubed"
Formulas for Calculating Performance

Convert between 1/4 mile and 1/8 mile ET's

1/4 mile ET = 1/8 mile ET x 1.5832

1/8 mile ET = 1/4 mile ET / 1.5832

Calculate 1/4 mile ET and MPH from HP and Weight

ET = ((Weight / HP)^.333) * 5.825

MPH = ((HP / Weight)^.333) * 234

Calculate HP From ET and Weight

HP = (Weight / ((ET/5.825)^3))

Calculate HP From MPH and Weight

HP = (((MPH / 234)^3) * Weight)

Formulas for displacement, bore and stroke

pi/4 = 0.7853982

cylinder volume = pi/4 x bore^2 x stroke

stroke = displacement / (pi/4 x bore^2 x number of cylinders)


Formulas for compression ratio

(CylVolume + ChamberVolume) / ChamberVolume

cylinder volume = pi/4 x bore^2 x stroke

chamber volume = cylinder volume / compression ratio - 1.0

displacement ratio = cylinder volume / chamber volume

amount to mill = (new disp. ratio - old disp. ratio / new disp. ratio x old disp. ratio) x stroke

Formulas for piston speed

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

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

Formulas for 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

Formulas for indicated horsepower & torque

horsepower = mep x displcement 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 / indocated output x 100

friction output = indicated output - brake output

taxable horsepower = bore2 x cylinders / 2.5

Formulas for air capacity & volumetric efficiency

theoretical cfm = rpm x displacement / 3456

volumetric efficiency = acutal cfm / theoretical cfm x 100

street carb cfm = rpm x displacement / 3456 x 0.85

racin carb cfm = rpm x displacement / 3456 x 1.1

Formulas for tire size & their effect

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

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

Formulas for 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 raduis / time^2

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

centrufugal force = weight x g

Formulas for shift points

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

drivehsaft torque = flywheel torque x transmission ratio

Formula for 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

odo error percent = difference between actual and indicated distances / actual distance x 100

Formulas for MPH RPM gears & tires

mph = (rpm x tire diameter) / (gear ratio x 336)

rpm = (mph x gear ratio x 336) / tire daimeter

gear ratio = (rpm x tire diameter) / (mph x 336)

tire diameter = (mph x gear ratio x 336) / rpm

Formulas for 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

Formulas for center of gravity

cj 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


Tire Height:
( (Width(mm) / 25.4 ) * ( Aspect(%) / 100 ) *2) + Wheel Height
Translation:
Width(mm)/Aspect(%)"R"Wheel Height
Ex. 245/45R17
Tire Width: 245 mm
Aspect: 45% (See Note on "Aspect below)
Wheel Height: 17"

245 mm / 25.4 --> Convert mm to Inches.
45 % x 2 --> Each tire has top and bottom sidewalls used in the calculation. In the formula above it takes the numeric aspect and divides by 100 to give you percentage.
Wheel Height --> obvious.

((245/25.4)*((45/100)*2)) + 17
((9.68)*(0.90) + 17
8.71 + 17 = 25.71 inches Tire Height

Tire Aspect: This is the second series of numbers (45 in the example above) that equals the percentage of the tire width that the sidewalls of the tire measures. In the example, the sidewall is 45% of the width of 245 millimeters.



[hr]


[/align]

Revolutions Per Mile

Formula Used:
(( "Minutes Per Hour" * "RPMs" ) / ( "Feet in Mile" / ( "Tire Size Inches" * "Pi" / "Inches in One Foot" )) / "Final Drive Gear" / "Transmission Gear" )

Variables - Translated
Minutes Per Hour = 60
RPMs = Uses Upper and Lower Limit
Feet in Mile = 5280
Tire Size = ( (Width(mm) / 25.4 ) * ( Aspect(%) / 100 ) *2) + Wheel Size
Pi = 3.1415
Inches in One Foot = 12
Final Drive Gear = Rear Gear Ratio
Transmission Gear = Trans Gear Ratio

 

Now they need to make this a "sticky" and it'll be Miller time!

 

It wont become a sticky. I'll probably have to make a new thread with a different title to people know whats in here.