Car Math Formulas

teamzr1

Supporting vendor
• 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 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

Formulas for 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

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 radius / time^2
lateral weight transfer = weight x cg height / wheel track x g
centrifugal force = weight x g
Formulas for shift points

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

Compression Ratio Formula​

DV (Below Deck Volume) = Bore(in) X Bore(in) X 12.87 X (Inches Below the Deck)
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
Compression Ratio = (GV+DV+HV-VV+PV) / (GV+DV+HV-VV)
Cubic Inches
= Bore x Bore X Stroke X Number of Cylinders X .7854

Conversions​

Convert Cubic Inches to CCs = Cubic Inches X 16.386
Convert Cubic Inches to Liters = Cubic Inches X .016386
Engine in Liters = (Bore(mm) X Bore(mm) X Stroke(mm) X Number of Cylinders X 12.87) / (16386 X 1000)
Engine in CCs = (Bore(mm) X Bore(mm) X Stroke(mm) X Number of Cylinders X 12.87) / 16386
Convert CCs to Cubic Inches = CCs / 16.386
Convert Liters to Cubic Inches = Liters / .016386

• TimP, Invetterate and HJG

HJG

CCCUK Member
Makes me appreciate the metric system even more.

teamzr1

Supporting vendor
In today's Corvettes electronics do not forget
Amps x Ohms = Volts 