Car Math Formulas

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teamzr1

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  • 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​

    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
    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
 
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