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| 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