Racing Formulas



Velocity
fps = 1.467 x MPH
MPH = Velocity in miles per hour
fps = Velocity in feet per second

Horsepower and torque
BHP = T x RPM ./. RPM
or
T = 5252 x BHP ./. RPM
BHP = Brake horsepower
T = Torque (ft/lbs)
RPM = Engine speed in revolutions per minute

Displacement
V = K x B* x B x S x N
V = Engine displacement
B = Cylinder bore
S = Stroke
N = Number of cylinders
K = 3.1416 ./. 4 = .7854

Valve Spring Rate
R = (OL - CL) ./. (IH - OH)
R = valve spring rate
OL = open load
CL = closed load
IH = installed height (closed height)
OH = Open height

Rod Ratio
RR = L ./. R
RR = Rod ratio
L = center to center length of rod
R = stroke

Starting Ratio
Manual transmission
SR = AR x FG
SR = starting ratio
AR = axle ratio
FG = first gear ratio of transmission
Automatic transmission
SR = AR x FG x TR
TR = torque converter torque ratio at stall

Mile per hour
MPH = (RPM x R) ./. (168 x AR)
RPM = revolutions per minute
R = rolling radius of driving tire (inches)
AR = Overall gear reduction


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 (thanks to Bobby Mosher for this formula)
1/8 mile ET = 1/4 mile ET / 1.5832 (thanks to Bobby Mosher for this formula)
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

 

where did you find all of this? i had been looking for a while but never had any luck

 

found those here
http://www.netconx.net/~angell/formulas.htm

Most can be found on prestage.com

 

This should be a sticky!

 

all hail "J" all hail "J"

(quote from MIB 2)

 

A more accurate value for pi is 3.1415926535; yours is rounded off! They say man went to the moon using pi = 3.14, so thats all I use, even though I've had the above approximation memorized since high school (yes, I'm an engineer )

Seriously though, this is a great collection and thanks for posting it <starting up printer>

 

Or fps = MPH * 88 / 60

Easy to remember - 60 MPH = 88 fps

 

TTT for new forum

 

subscribing

 

rpm after shift = ratio shift into / ratio shift from x rpm before shift
Someone please show example, or show me were I missing it
1 st gear 3.35 10,000 rpm
2nd gear 2.18
Car actual falls after shift to 7800.

 

Silly question, but would you happen to have an automatic with a fairly loose
non-lockup converter?

 

Doesn't
pi=3.14xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx and go on and on. In fact if I remember correctly, before computers it had been hand written a distance that would have put it to the moon. Even with computers, as far as I know they have never found the end of pi?
Very good helpful stuff, J rod. Now instead of digging through my junk, I know where to get a needed formula easy.
Robert

 

Good list but the formula for displacement is wrong.
Displacement
V = K x B* x B x S x N
V = Engine displacement
B = Cylinder bore
S = Stroke
N = Number of cylinders
K = 3.1416 ./. 4 = .7854
it should be V=pi*(.5*B)^2*S*N since the general formula is the area of the bore (pi*radius^2) times the stroke, times the number of cylinders. That formula uses pi/4 times the diameter squared which doesn't work, it gets used a couple times and I thought it should be pointed out before someones calculations get thrown way way off.

 

Well damn! And all these years I had it memorized as 3.141592653 6 !

 

Well damn! And all these years I had it memorized as 3.141592653 6 !
Yes, another engineer; why do you ask?

 

V = K x B* x B x S x N is a good formula if you leave out the *. (I'm not sure what J-R meant because he uses "X" for multiplication.

I'm also an engineer from the KISS school. I don't always have a scientific calculator handy, or my slide rule needs cleaning, and my memory for stuff isn't all it was so here's a simple method to use J-R's formula on a 4-function (cell phone?) calculator. The stuff in the [ ] are the keys to push:

bore [X = X] stroke [X] number of cylinders [X . 7 8 5 4 =]

Notice that "7854" are the numbers in the top left hand corner of the keypad. Just start in the upper left and go clockwise (or anticlockwise from the 7 on a cellphone ). You need not memorize the numbers.

PI over 4 is about .785398 or .7854 accurate to a few 1000ths of a %.


Example: 4.000 bore 3.480 stroke.

Using .7854, I got 349.8486 cubic inches.
Using PI/4, I got 349.8478 cubic inches or about .001 cubic inch less.

To put that in perspective, increasing the bore to 4.0001 (one ten-thousandth of an inch) increased the displacement (using PI/4) to 349.8652 cubes, which is .0175 more cubic inches (about .3 cc), or about 19.4 times as much as the "error" using .7854.

Hey, that's close enough for most everyone, engineers and tech inspectors included. I haven't seen too many tech inspectors accurately measure a bore to .0001. FWIW that's how much a 4 inch bore changes for every 5 degrees F temp change.

MadBill: Love your signature quote from ZAD.