Automotive
Formulas
Heres a bunch of Formulas pertaining
to all things Vehicular. I make no promises as to their accuracy
Carburetor
Cfm Calculator:
Formula:
(Cubic Inches/3456) X Volumetric Efficiency
X Maximum RPM = Ideal Carb size
For instance, if you had a 440 you wanted
to spin a maximum of 6000 rpm, and assuming 90% efficancy, you could
use 687.5 CFM, I usually use the idea that you move one step up from
that, so in this instance, a 750 cfm carb might be in order.
(440/3456)x0.90x6000=687.5
Formula to figure approximate
volumetric efficiancy
Theroetical CFM = RPM X Displacement/3456
Volumetric Efficiancy = Actual CFM / Theroetical
CFM X 100
Potential Horsepower with
heads
All flow values are for 10in-H20 because
you can estimate potential HoresPower using these numbers.
HorsePower (per Cylinder @ air flow) = 0.43 * (Head flow CFM @ 10in-h20)
or 3.44 * CFM for an 8-cylinder engine.
For Instance, A stock "906" head flows 126 cfm at 0.500"
lift. The potential HP is 126 * 3.44 = 433 HP
This formula shows at what
RPM peak HP should occur based on engine displacement
This lets you select the proper camshaft duration to match to the
heads airflow and engine size
Formuls: Peak HP RPM = [2000 / Engine CID (per Cylinder)] * Headflow
CFM @ 10in-h20
Or HP RPM = [16000 / CID (for 8 cylinders)] * CFM
For instance, the head head as above on 440 engine. HP RPM = [16000
/ 440] * 126 = 4,582 RPM
CALCULATING PERFORMANCE
HP = Torque * RPM/5252
HP = (((MPH / 234)^3) * Weight)
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 SPEED AND GEARING
mph = (rpm x tire diameter) / (gear ratio
x 336)
rpm = (mph x gear ratio x 336) / tire daimeter
gear ratio = (rpm x tire diameter) / (mph
x 336)
tire diameter = (mph x gear ratio x 336)
/ rpm
FORMULAS FOR ENGINE STROKE
AND VOLUME
cylinder volume = pi/4 x bore^2 x stroke
stroke = displacement / (pi/4 x bore^2 x
number of cylinders)
FORMULAS FOR COMPRESSION
RATIO
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
swept vol (cc) = bore^2 * stroke * .0031416
/ #cyl
Deck height (cc) = bore^2 * deck height
(inch) * .01996
Compression Ratio = (CylVolume + ChamberVolume)
/ ChamberVolume
Compression Ratio = [swept vol + deck height
+ head vol] / [deck height + head vol] (all in cc's)
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 AIR INTAKE FLOW
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
