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