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F2 = |
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* F1 |
where |
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Mustang Math
Disclaimer: Information provided below is meant to give you a better understanding of a vehicles capabilities, component needs, and fuel requirements. Use only calculated requirements for a good basic starting point, and work from there. Pro-M / Best Products will not be responsible for damage to your vehicle, for any reason, if your calculations are not accurate. However, these are the same formulas that Pro-M / Best Products uses to determine their vehicle requirements. Please note that you will get the most accurate results by testing on the track or at a dyno.
Horsepower and Torque:
Horsepower comes from torque. Torque is a result of the combustion process forcing the piston downward and rotating the crank. This output is measured as Torque. The idea is to generate high enough pressure on each stroke often enough (rpm) to generate the necessary Horsepower.
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Horsepower = |
|
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Horsepower and
Torque, incidentally, are always equal at 5252 rpm.
Wanna figure out what that factory horsepower rating is at your height above sea
level?
Corrected BHP = BHP * (1 - ((elevation/1000) * .03))
Note:
BHP = Brake Horse Power
.03 = 1/30 mercury
Horsepower, ET, and Weight:
A quick calculation for horsepower based on 1/4 mile trap speed:
|
HP = (TS/234)3 * race weight |
or |
HP = (TS * 0.00426)3 * race weight |
where |
|
This horsepower output is the minumum required for the specified trap speed. It assumes ideal track conditions, weather conditions, traction, and vehicle aerodynamics. It will understate horsepower required at speeds exceeding 100 mph.
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Or try:
|
HP = |
|
for a quick idea of ideal ET assuming good street rubber and decent traction.... |
ET = |
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Horsepower:
Calculation assuming sea level and known Volumetric Efficiency
|
Horsepower = |
|
where |
|
Most use Barometric pressure which is in measured in inches of mercury. To get the equivalent pressure in psi:
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Pressurepsi = |
|
Air Filter Selection:
An average foam filter will flow 4.38 cfm/sq-in.
A good paper filter will flow 4.95 cfm/sq-in. An oiled cotton gauze (K&N)
will flow 6.03 cfm/sq-in.
To get your required filtered surface area for a oiled cotton gauze filter
use the following formula:
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A = |
|
where |
|
Then using the following modifying factors if using an alternative filter media:
|
A * 1.3767 = required surface area for foam element |
|
A * 1.2181 = required surface area for paper element |
Cubic Feet per Minute:
|
Theoretical CFM = |
|
and |
Actual CFM = |
|
|
VE |
= |
volumetric efficiency |
|
CID |
= |
cubic inch displacement |
|
RPM |
= |
revolutions per minute |
Carburetor Cubic Feet per Minute:
|
Required CFM = |
|
This seems to
figure the requirement |
Volumetric Efficiency:
Engine output is based
on how much air and fuel it can burn. It's proficiency at burning the
air/fuel mixture is defined as it's Volumetric Efficiency. If you know the
amount of air your engine can move at a specific rpm you can use this
calculation to estimate volumetric efficiency.
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Volumetric Efficiency = |
|
or |
Volumetric Efficiency = |
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* 100 |
Or, if you know your horsepower at a given rpm (peak HP is what you want to use here) you can approximate your Volumetric Efficiency at sea level by using a variation of the previous Horsepower calculation:
|
VE = |
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Cubic Inch Displacement:
CID = Number of cylinders * 0.7854 * bore * bore * stroke
All measurements in inches.
Rev Limits:
There are some rough standards for RPM limits. These are based on piston speed measured in feet per minute. Cast crank and rods should aim for under 3500 fpm. Forged crank, rods, and beefed main caps can handle closer to 3800-4000 fpm. This is only a rough estimate.
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Piston speed (fpm) = |
|
and |
RPM limit = |
|
fpm = feet per
minute
RPM vs. MPH:
These calculations are useful in selecting rear tire diameters and rear gear ratios.
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Fuel Injectors:
Just as the wrong sized jets in a carb can cause decreased performance and driveability problems, so can incorrectly sized injectors. The following calculation can be used for approximating fuel flow per injector based on horsepower (HP) and Brake Specific Fuel Consumption (BSFC).
Note:
1) Engine HP must be a realistic estimate.
2) BSFC is determined from engine dyno measurements. It typically ranges from 0.4-0.6 for gasoline engines. A BSFC of 0.5 is a safe, initial estimate.
BSFC =
Pounds of fuel per hour
-------------------------- Brake Horse Power
3) The 0.8 multiplier for the "Number of Injectors" helps derive a practical "Max Injector Flow Rate" for each injector based on an effective real world injector operating pulse time and fuel flow. It is unrealistic to establish the fuel flow to an engine based on an injector operating pulse time of 100% (wide open all the time). This calculation uses an injector operating cycle of 80%. Some full race engine management systems may operate at 85-95% duty cycle, but extended operation may eventually overheat the injectors and cause irregular flow rates and poor low rpm operation.
|
Injector Flow Rate (lbs/hr) = |
|
With a known injector fuel flow rate you can get a rough estimate of the systems capacity by using:
|
where |
IFR = Injector Flow Rate (lbs/hr) |
Increasing the fuel pressure can often provide increased fuel flow and better atomization. If you know an injector's static (non-pulsed) fuel flow at one system pressure you can find its static flow at another pressure with this:
|
F2 = |
|
* F1 |
where |
|
