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or maybe you want |
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Thought you'd get away from that cursed math book when ya headed out to the garage to play with yer hot rod, eh? Think again! From calculating your compression ratio to degreeing the cam you're gonna need math. Here you'll find formulas I've run across to simplifiy some of the questions that sometimes arise and supply some general info I've found. This area is constantly going to change as I find new information. If you have any little calculations that you don't see here and would like to share them, email me. Have fun.
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:
| A = |
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where |
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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 |
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.
| 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?
A quick calculation for horsepower based on your 1/4 mile trap speed:
| HP = (TS/234)3 * race weight | or | HP = (TS * 0.00426)3 * race weight | where |
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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.
Here's some more:
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or maybe you want |
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Or try:
| HP = |
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or for a quick idea of ideal ET assuming good street rubber and decent traction.... | ET = |
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Calculation assuming sea level and known Volumetric Efficiency
| Horsepower = |
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where |
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Most use Barometric pressure which is in measured in inches of mercury. To get the equivalent pressure in psi:
| Pressurepsi = |
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| Theoretical CFM = |
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and | Actual CFM = |
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| Required CFM = |
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This seems to figure the requirement a bit larger than you'd think necessary. |
Engines are occasionally defined as simply an air pump. While this is definitely an oversimplification, your engine's 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.
| Volumetric Efficiency = |
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or | Volumetric Efficiency = |
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* 100 |
Or, if you know your horsepower at a given rpm (the point of peak tq is going to be your max VE) you can approximate your Volumetric Efficiency at sea level by using a variation of the previous Horsepower calculation:
| VE = |
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All measurements in inches.
There are some rough standards for RPM limits. These are based on pistion 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. Rmember...these are rough....talk with your engine builder or an expert.
| Piston speed (fpm) = |
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and | RPM limit = |
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An optimum set-up will put you thru the traps at the rpm your engine's peak hp. These calculations are useful in selecting rear tire diameters and rear gear ratios. The freeware spreadsheets below allow easier access to these calculations.
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Just as the wrong sized jets in a carb can cause decreeased 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 fo 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 calcuation 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) = |
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With a known injector fuel flow rate you can get a rough estimate of the systems capacity by using:
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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 |
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