# Automotive Formulas

• 0
• 19K

Here's a bunch of equations for solving for "stuff" related to cars. Car Math!

## Automotive Formulas​

### Formulas for displacement, bore and stroke:​

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 / indocated 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

odometer 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

Replies
8
Views
610
Replies
15
Views
9K