# Automotive Formulae by Dino

1) Estimating wheel horsepower from top speed

HORSEPOWER (Wheels) = (6.114.W.V + Cd.A.V3)/149877

(x1.11 manual gearbox FWD, x1.17 manual gearbox RWD, x1.17 automatic gearbox FWD, x1.25 automatic gearbox RWD for flywheel horsepower)

W= Test weight (lb), V= Top speed (mph), Cd=Drag coefficient, A= Frontal area (ft2).

2) Estimating flywheel horsepower from 0-60 time

HORSEPOWER (Flywheel) = (0.413.W)/T0-60

T0-60 = 0-60 mph time (sec).

3) Estimating wheel horsepower from 1/4 mile trap speed

HORSEPOWER (Wheels) = W. [Mph/247]3

4) Estimating wheel horsepower from 1/8 mile trap speed

HORSEPOWER (Wheels) = W. [Mph/196]3

5) Relationship between barometric pressure and elevation

BP = BP0.e -0.0104.H/K

BP= Barometric pressure, BP0= Pressure (sea level), H= Elevation (ft), K= Air temperature (K) = 273+oC.

6) Dyno horsepower correction factor for air temperature, pressure and humidity

STP air correction factor = [29.92”Hg/(BP-WVP)]. [(459.7+ AIT)/519.7]0.5

AIT= Air intake temperature (oF), WVP= water vapour pressure.

7) Relationship between flywheel horsepower and compression ratio

Horsepower change (%) = [1- 1/Rnew0.4]/[1-1/Rorig0.4] -1 x100%

R= Compression Ratio.

8) Calculation for rear wheel torque

Torque (Wheels) = (W.G.A)/1545

W= Vehicle & driver weight (lb), G= Gearing (mph/1000rpm), A= Acceleration (mph/sec)

9) Formula for mph, rpm, overall gear ratio and tire diameter

Mph = [rpm x tire diameter(in.)]/[overall gear ratio x 336]

10) Calculation for optimum overall gearing

Overall gear ratio = (tire diameter x maximum rpm)/(mph1/4 x 340)

mph1/4= 1/4 mile trap speed calculated using formula 3) where weight and horsepower are known

11) Intake manifold runner length

L (in.) = ((EVCD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

Where: EVCD = Effective Intake Valve Closed Duration (720 - IVO)
RV = Reflective Value (number of harmonics)
V = Pressure Wave Speed (1125 ft/sec)
D = Runner Diameter (in.)

Reflective wave RPM = ((EVCD x 0.25 x V x 2) ÷ (L + 1/2D)) ÷ RV

12) Exhaust header primary pipe diameter

Peak Torque RPM = [Primary Pipe Area (sq. in) X 88200] / Cylinder Volume (cu. in.)

13) Throttle body cross-sectional area at wide open throttle

CSA (in2) = BD2/821.44 – (SD x BD)/645.2

BD = Bore diameter (mm), SD = Shaft diameter (mm)

14) Throttle body airflow velocity at wide open throttle

AV (ft/sec) = (AF x 2.4)/CSA

AF = Airflow @ 28” H2O depression (cfm or cubic feet/min.), CSA = Cross-sectional area (in2)

15) Optimum throttle body bore diameter

Bore (mm) = [Peak HP RPM x Engine CID]0.5 / 16.85