Estimating Horsepower
This quick calculation for when you have vehicle weight and 1/4 mile trap speed:
(Trap Speed/234) x Race Weight = Horsepower
Or
Horsepower = (Trap Speed x 0.00426) x Race Weight
This horsepower output is the minimum 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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Rear Axle Ratio
(RPM x Tire Diameter) / (MPH x 336)
Example in this instance: 28.7 Tire Diameter
RPM = 2500
MPH = 65
(2600 x 28.7) / (65 x 336)
74620 / 21840 = 3.42
Rear Axle Ratio - Stationary Vehicle
Raise both rear wheels of the vehicle off the ground, and for safety reasons set in on jack stands. Put your vehicle in neutral. Make a reference point on the driveshaft and on the rear housing. Also mark a reference point on the wheel and on the fenderwell (masking tape). Have a "friend" watch the driveshaft rotate both tires on revolution. The number of turns the driveshaft makes indicates the ratio.
Example 3 1/2 turns = 3.50 ratio
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Carb Size
Cubic inches x max RPM/3456 = CFM of carb needed
Example in this instance: 350 Chevy
Max RPM = 6200
350 x 6200 / 3456 = 627 cfm
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Piston Compression Height (Pin Height)
Total Deck Height- Rod Length- 1/2 of stroke= Compression Height
Example in this instance: 350 Chevy
Deck Height= 9.00" Ι Rod Length = 5.7" I Stroke = 3.48"
9.00 - 5.7 - 1.74 = 1.56
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Fuel Injector Requirement
Just as you size carb CFM's and jetting to an engines power requirements, fuel injectors have to be sized to engine power requirements.
(Max Horsepower x .50 x .80)/ Number of injectors = Injector lb rate required
Notes:
- Engine HP must be a realistic estimate of power at wide open throttle
- The .50 is the BSFC again. A BSFC of 0.5 is a safe, initial estimate. Turbocharged engines will need the formula changed to .60.
- The 0.8 multiplier assumes that no injector can operate at 100% duty cycle and uses 80% cycle capability
Example in this instance: 8 cylinder engine
Horsepower = 400
(400 x .50 x .80)/ 8 = Injector lb rate required
160/8 = Injetor lb rate required
20= Injector lb rate required
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Fuel Pump Requirement
Will you have enough fuel to make all the power your engine can produce?
2(Flywheel Horse Power x BSFC/6)=GPH-Gallone Per Hour of Fuel
Exampel in this instance: 500 horsepower engine
BSFC is often not known. This number ranges from .4 for efficient engines to .6 for blown or turbo charged engines. Most builders use .5 for basic calculation
2(500 x 5/6) = 83.3 GPH-Gallons Per Hour of Fuel
When selecting a fuel pump note the following. The advertised GPH rating is free flow and not indicative of in-use ratings. Consult the manufacturers catalog for GPH ratings for your maximum RPM and PSI needed.
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Rear Tire Diameter
(Gear Ratio x 336 x Trans Gear Ratio x MPH)/RPM = Tire Diameter
Example in this instance: 3.42 Gear Ratio
Trans Gear Ratio = 1.0
MPH = 65
RPM = 2500
(3.42 x 336 x 1.0 x 65)/2500 = Tire Diameter
74692/2500 = 29.8768 Tire Diameter
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Air Cleaner Size Formula
This formula can be used to determine the size your free flowing K&N filter needs to be.
(Cubic inches x Max RPM) /20.839
Example in this instance: 350 Chevy
RPM = 5500
350 x 5500 / 20.839 = 92.374 sq in
To convert the square inches to a round filter use the following formula to determine the height. We have chosen our diameter to be 14" because most aftermarket cleaners available in primarily 14".
Filter Area from above formula/(Filter Outside Diameter x 3.14) + Filter Rubber End Cap Heights = Filter Height
92.374/(14x3.14) + .75 = 2.85" rounded 3.00"
We need a minimum element for our 350 at 5500 RPM to be 14"x3"
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