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Old 08-18-2004, 02:38 PM   #1
Daytona
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Here are the formulas for calculating motorcycle and automotive performance.

* 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
1/8 mile ET = 1/4 mile ET / 1.5832

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)

Calculate 0-60 Times
T(0-60) = (2 x vehicle weight/Max. known engine torque) --> all raised to the .6 power



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 displcement 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 raduis / 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



Formula for instrument error

actual mph = 3600 / seconds per mile

speedometer 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 daimeter

rear gear ratio = (rpm x tire diameter) / (mph x 336) OR = (Tire radius X RPM / 168) / MPH

ideal rear gear ratio = (RPM X Tire Diameter) / (336 X Trap Speed MPH)

tire diameter = (mph x gear ratio x 336) / rpm
tire radius = (mph x gear ratio x 168)/ 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
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Old 08-18-2004, 03:40 PM   #2
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Old 08-19-2004, 12:55 AM   #3
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yea... alot of formulas are car/engine specific.

the 1/4 ET formula was off by about a second for me.
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Old 08-19-2004, 08:58 AM   #4
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Quote:
Originally posted by FormulaNERD@Aug 18 2004, 11:55 PM
the 1/4 ET formula was off by about a second for me.
Which formula? Converting from/to 1/8-mile?

Don't forget that these are mathmatical calculations taking into account average textbook results. there are always variable mitigating factors which could sway the actual results one way or another to a certain extent - more so for performance/time calculations than for, say, displacement or compression ratio.
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Old 08-19-2004, 05:46 PM   #5
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yea, i realize that, i was looking somewhere that had some formulas based on different cars. olds was closer to 1.514 or something like that...
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Old 10-23-2006, 09:53 PM   #6
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my brain hurts now, thanks
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