Auto Formulas

Please note: these formulas are for estimating only. Contrary to popular belief, using this site will not increase your horsepower.
•ET from HP •HP from ET •HP from MPH •HP from Torque
•Torque from HP •Max HP from CFM •in³ to Liter •Liter to in³
•Displacement (standard) •Displacement (metric) •HP/ET Chart •1/8 to 1/4 mile ET
•1/4 to 1/8 mile ET •Piston Speed •Find RPM from Piston Speed •Find Gear Ratio
•Find Tire Diameter •Find RPMs •Find Trans Ratio •Find MPH
•Tire Diameter •HP Loss @ Elevation •Max CFM Needed •Volumetric Efficiency
•rpm after shift •Street Carb CFM •Racing Carb CFM
•Wheel Thrust •G (force) •Weight Transfer •Lateral Acceleration
•Lateral Weight Transfer •Centrufugal Force •Driveshaft Torque •Drive Wheel Torque
•Complete List of Formulas
Estimate E.T. from Weight (lbs.) and wheel HP
Weight:
Wheel HP:
ET in seconds:

ET from HP (1/4 mile) = (WEIGHT / HP)^.333 * 5.825

Estimate Maximum H.P. from Cylinder Head Flow
CFM:
Number of Cylinders:
Flywheel HP:

HP = CFM x 0.2575 x number of cylinders
Estimate wheel H.P. from Weight (lbs.) and M.P.H.
Weight:
Trap Speed (MPH):
Wheel HP:

HP from MPH (1/4 mile) = WEIGHT * (MPH/234)^3
Estimate wheel H.P. from Weight (lbs.) and E.T.
Weight:
ET in seconds:
Wheel HP:

HP from ET (1/4 mile) = WEIGHT * (5.825/ET)^3

Estimate H.P. from Torque and RPMs
Torque:
RPMs:
HP:

HorsePower = TORQUE * RPM / 5252
Estimate Torque from HP and RPMs
HP:
RPMs:
Torque:

TORQUE = (5252 * HP) / RPM

Rear-Wheel HP Required to push your brick through the quarter mile
This chart was borrowed and modified from http://www.jason.fletcher.net/tech/combos/et_hp.htm

Use the chart below to see how much wheel horsepower is required to run the elapsed time you want.

ET & SPEED

(seconds @ mph)
VEHICLE WEIGHT (pounds)
24002600280030003200 34003600380040004200
16.0 @ 85 116125135145154 164174183193203
15.5 @ 88 127138149159170 180191202212223
15.0 @ 91 141152164176187 199211223234246
14.5 @ 94 156169182194207 220233246259272
14.0 @ 97 173187202216230 245259274288303
13.5 @ 101 193209225241257 273289305321337
13.0 @ 105 216234252270288 306324342360378
12.5 @ 109 243263283304324 344364385405425
12.0 @ 114 275297320343366 389412435458480
11.5 @ 119 312338364390416 442468494520546
11.0 @ 124 356386416445475 505535564594624
10.5 @ 130 410444478512546 580615649683717
10.0 @ 136 474514553593632 672712751791830
9.5 @ 143 553599645692738 784830876922968
9.0 @ 151 651705759813868 9229761,0301,0841,139
8.5 @ 160 7728379019651,030 1,0941,1591,2231,2871,352
8.0 @ 170 9261,0041,0811,1581,235 1,3121,3901,4671,5441,621

Calculate Cubic Inches from Liters
Liters:
# of Cylinders:
Cubic Inches:
Cubic Inches per Cylinder:

1 Liter = 61.02374 Cubic Inches

Calculate Liters from Cubic Inches
Cubic Inches:
# of Cylinders:
Liters:
Liters per Cylinder:

1 Cubic Inch = 0.016388 Liters
Calculate Displacement from Bore and Stroke (inches)
Bore (inches):
Stroke (inches):
# of Cylinders:
Cubic Inches:
Cubic Inches per Cylinder:
Liters:
Liters per Cylinder:

Displacement (in^2) = Number Of Cylinders * 3.14159 * (Bore/2)^2 * Stroke
Calculate Displacement from Bore and Stroke (metric)
Bore (millimeters):
Stroke (millimeters):
# of Cylinders:
Cubic Inches:
Cubic Inches per Cylinder:
Liters:
Liters per Cylinder:

Displacement (in^2) = Number Of Cylinders * 3.14159 * (Bore/2/25.4)^2 * Stroke/25.4
Calculate Tire Diameter
Section Width (P275/40R17):
Profile (P275/40R17):
Wheel Diameter (inches) (P275/40R17):
Diameter (inches):

Diameter (in.)=(((section width * profile)/2540) + (wheel diameter/2)) * 2
Find Rear Gear Ratio
RPMs:
Tire Diameter (inches):
Transmission Gear Ratio:
MPH:
Rear Gear Ratio:

Rear Gear Ratio = (Tire Diameter * RPM) / (Trans Ratio * MPH * 336)
Find Tire Diameter (inches)
RPMs:
Rear Gear Ratio:
Transmission Gear Ratio:
MPH:
Tire Diameter (inches):

Tire Diameter = (Trans Ratio * MPH * 336 * Rear Gear Ratio) / (RPM)
Find RPMs
Tire Diameter (inches):
Rear Gear Ratio:
Transmission Gear Ratio:
MPH:
RPMs:

RPM = (Trans Ratio * MPH * 336 * Rear Gear Ratio) / (Tire Diameter)
Find Transmission Gear Ratio
RPMs:
Rear Gear Ratio:
Tire Diameter (inches):
MPH:
Transmission Gear Ratio:

Trans Ratio = (Tire Diameter * RPM) / (Rear Gear Ratio * MPH * 336)
Find MPH
RPMs:
Rear Gear Ratio:
Transmission Gear Ratio:
Tire Diameter (inches):
MPH:

MPH = (RPM * Tire Diameter) / (336 * Rear Gear Ratio * Trans Ratio)
Find 1/4 mile ET from 1/8 mile ET
1/8 ET:
1/4 ET:

1/4 mile ET = 1/8 mile ET x 1.5832
Find 1/8 mile ET from 1/4 mile ET
1/4 ET:
1/8 ET:

1/8 mile ET = 1/4 mile ET / 1.5832
Calculate Piston Speed
Stroke (inches):
RPMs:
Speed (Feet/Minute):
Speed (Feet/Second):

piston speed in fpm = stroke in inches x rpm / 6

Calculate RPM from Piston Speed
Stroke (inches):
Piston Speed (FPM):
RPM:

RPM = piston speed in fpm x 6 / stroke in inches

HP loss based upon elevation
Elevation (feet):
HP @ Sea Level:
HP Loss:
HP @ Elevation:

bhp loss = elevation in feet / 1000 x 0.03 x bhp at sea level
RPM After Shift
Shift from Ratio:
Shift to Ratio:
RPM Before Shift:
RPM After Shift:

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

Calculate Maximum CFM Needed
Max RPM:
Displacement (inches^3):
Max CFM Needed:

Theoretical cfm = rpm x displacement / 3456
Volumetric Efficiency
Theoretical CFM:
Actual CFM:
Volumetric Efficiency:

Volumetric Efficiency = acutal cfm / theoretical cfm x 100

Street Carb CFM Needed
Max RPM:
Displacement (inches^3):
CFM Needed:

Street carb cfm = rpm x displacement / 3456 x 0.85

Racing Carb CFM Needed
Max RPM:
Displacement (inches^3):
CFM Needed:

Racing carb cfm = rpm x displacement / 3456 x 1.1

Drive Wheel Torque
Flywheel Torque:
Transmission Gear Ratio:
Rear Gear Ratio:
Drive Wheel Torque:

drive wheel torque = flywheel torque x trans gear x final drive x 0.85

Wheel Thrust
Drive Wheel Torque:
Rolling Radius (inches):
Wheel Thrust:

wheel thrust = drive wheel torque / rolling radius

G Force
Wheel Thrust:
Weight:
G Force:

g (force) = wheel thrust / weight

Weight Transfer
Weight:
CG Height (inches):
Wheelbase (inches):
G (force):
Weight Transfer:

weight transfer = weight x cg height / wheelbase x g

Lateral Acceleration
Radius (feet):
Time (seconds):
Lateral Acceleration:

Lateral Acceleration = 1.227 x radius / time^2

Lateral Weight Transfer
Weight:
CG Height (inches):
Wheel Track (inches):
G Force:
Lateral Weight Transfer:

lateral weight transfer = weight x cg height / wheel track x g

Centrufugal Force
Weight:
G Force:
Centrufugal Force:

centrufugal force = weight x g

Driveshaft Torque
Flywheel Torque:
Transmission Ratio:
Driveshaft Torque:

driveshaft torque = flywheel torque x transmission ratio 

Here is a list of formulas:			[back to top]
•Rear Gear Ratio = (Tire Diameter * RPM) / (Trans Ratio * MPH * 336)
•Tire Diameter = (Trans Ratio * MPH * 336 * Rear Gear Ratio) / (RPM)
•RPM = (Trans Ratio * MPH * 336 * Rear Gear Ratio) / (Tire Diameter)
•Trans Ratio = (Tire Diameter * RPM) / (Rear Gear Ratio * MPH * 336)
•MPH = (RPM * Tire Diameter) / (336 * Rear Gear Ratio * Trans Ratio)
•Tire Diameter (inches) = (((section width * profile) / 2540) + (wheel diameter / 2)) * 2 
•HorsePower = TORQUE * RPM / 5252
•TORQUE = (5252 * HP) / RPM  
•1 Liter = 61.02374 Cubic Inches 
•Displacement (in^2) = Number Of Cylinders * 3.141592653589 * (Bore/2)^2 * Stroke 
•HP from MPH (1/4 mile) = WEIGHT * (MPH/234)^3 
•HP from ET (1/4 mile) = WEIGHT * (5.825/ET)^3 
•ET from HP (1/4 mile) = (WEIGHT / HP)^.333 * 5.825 
•HP from cylinder head intake flow  = CFM x 0.2575 x number of cylinders
•1/4 mile ET = 1/8 mile ET x 1.5832 (prestage.com)
•1/8 mile ET = 1/4 mile ET / 1.5832 (prestage.com)
•piston speed in fpm = stroke in inches x rpm / 6 
•rpm = piston speed in fpm x 6 / stroke in inches 
•bhp loss = elevation in feet / 1000 x 0.03 x bhp at sea level 
•theoretical cfm = rpm x displacement / 3456 
•volumetric efficiency = acutal 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 g force & weight transfer 
•drive wheel torque = flywheel torque x trans gear x final drive x 0.85 
•wheel thrust = drive wheel torque / rolling radius 
•g (force) = 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 
•centrufugal 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 
->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 
•cg 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 

Check out Prestage.com for more useful formulas.

email me at aaronmurray@msn.com if you have any formulas you would like to see
added (as a calculator) or if you have any suggestions.