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Racers Math - 1

Racers Math 1:  This section of the site is for calculating aspects of your own cars performance and is specifically for drag racing.  If you have an idea of the types of things you'd like to be able to calculate, let us know.  Please bear in mind that these calculators are approximations only.  Although the information is based on a 5.0 liter Mustang, it is general enough to apply to any car used for drag racing. Note, you must have a JavaScript enabled browser and it must be turned on to use these calculators.  Test your browser by clicking the button below.  If nothing happens, you're not enabled.  For more detailed calculations, check out the engine analyses available at the SlowGT store.

Links to all of the calculators on all pages in the site.

Fuel Pump Capacity Versus Horsepower

This calculator is useful for calculating the fuel pump flow capacity you will need to support a given horsepower.  Please note that these calculations are estimates only.

• Estimated Maximum Flywheel Horsepower can be calculated using the calculator below.

• Brake specific fuel consumption (BSFC) is a measure of how many pounds of fuel are required to generate one horsepower for one hour (lb/hp-hr).  More efficient engines will have a lower BSFC, less efficient combinations will have a higher BSFC.  Most will fall between 0.4 and 0.6.

• The fuel pump you select will have to be capable of supplying the calculated flow capacity at the pressure that you intend to run your fuel injectors (see calculator below).  The calculated minimum required flow capacity is (as it's called) the minimum flow rate you should consider.  If available, you should select a fuel pump with a higher rated capacity at your desired fuel pressure.

• After you have calculated a result, if you would like to see a curve showing how the required flow capacity changes with horsepower, select "Plot It".

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Fuel Injector Sizing Versus Horsepower

This calculator is useful for calculating the maximum horsepower that can be supported by a given fuel injector size.  Please note that these calculations are estimates only.

• The Injector flow rating is the manufacturers flow capacity rating and is usually given as lbs/hour.

• The Injector Flow Rating Pressure is the pressure at which the Injector Flow Rating above is measured.  Usually between 42 and 44 psi.

• The Number of Engine Cylinders, I believe, is self explanatory.

• The Operating Fuel Pressure is the maximum fuel pressure that will be applied to the injector at wide-open throttle and (we believe) should be set between 40 and 65 psi for normal use.

• The Duty Cycle is a number that represents how long the injector is open during an entire four-stroke cycle and we prefer that it be no larger than 90%.  The duty cycle number you enter should be between 30% and 100%.

• The BSFC is defined above.

• When selecting fuel injectors, they should be selected to work properly in concert with the mass air flow (MAF) sensor (if so equipped).  We hope to cover this issue in a future Tutorial.

• After you have calculated a result, if you would like to see a curve showing how the maximum engine horsepower support varies with fuel pressure, select "Plot It".

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Evenly Spaced Transmission Gears for Off-the-line Gearing and Rear End Ratio 

This calculator is for manual transmission cars and is useful for calculating evenly spaced transmission ratios based on desired off-the-line gearing and rear end gear ratio.  The term evenly spaced means that the engine RPM will drop by a fixed amount on each gear shift thereby keeping the engine in a specific RPM range.  Please note that these calculations are estimates only.

• Vehicle Weight (in pounds), Maximum Engine Flywheel Horsepower, Number of Forward Transmission Gears, High Gear Ratio (usually 1:1) and Desired Rear End Ratio should be self explanatory.

• Launch Ratio is the product of multiplying your first gear ratio by your rear end ratio.  This number for drag-only applications is usually 13.5:1 or numerically greater.  For example, SlowGT uses a 3.08 first gear with a 5.13 rear end ratio for an off-the-line ratio of 13.5*5.13=15.8.

• Shift Point is the RPM at which you intend to shift; normally 500 to 1,500 RPM above the RPM at which the engine makes peak horsepower.

• Engine Red Line is the RPM limit for the engine.  The calculator will warn you if your chosen combination will require you to exceed this RPM value in high gear.

• Rear Tire Diameter should be given in inches.

• The calculator will display each of the calculated gear ratios as well as the maximum RPM and speed in each gear.

• After you have calculated a result, if you would like to see a curve showing how engine RPM varies with gear changes, select "Plot It".

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  Horsepower, Weight, Shift Points, Gears and Tires

This calculator is for manual transmission cars and is useful for calculating the relationship between horsepower, weight, shift points, transmission ratios, rear end ratios and tire sizes.  Use this calculator if you know your horsepower, desired shift points, minimum RPM points and tire diameter pretty well and you want to calculate the gear ratios required to make the engine stay in its' power band.  Please note that these calculations are estimates only.

• For each of the Transmission Gears, enter the maximum RPM and minimum RPM that you would like the engine to rev.  For instance, if you would like to shift out of first gear at 7,000 RPM and drop to 5,000 RPM in second, enter 7000 as the Max RPM for first gear and 5000 RPM for the Min RPM in second gear.  First gear has no Min RPM because that's where you launch.

• 4th Gear Ratio, Maximum Engine Flywheel Horsepower, Vehicle Weight (in pounds) and Rear Tire Diameter (in inches) should be self-explanatory.

• The calculator calculates your estimated maximum mph through the traps based on horsepower and weight.  The rear gear ratio is then calculated based on the tire diameter and 4th gear ratio you.  Each of the other gear ratios is calculated from the shift points you have entered; the Max RPM and Min RPM values (Max RPM is the RPM at which you shift out of a gear, Min RPM is how low the engine RPM will drop when you shift into a gear).

• After you have calculated a result, if you would like to see how the engine RPM changes as a function of vehicle speed, select "Plot It".

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RPM for Speed In Gear

Use this calculator to calculate the relationship between rear end gear ratio, transmission gear ratio, tire diameter, vehicle speed and engine rpm.  Please note that these calculations are estimates only.

• Rear End Ratio, Transmission Gear Ratio, Tire Diameter and Vehicle Speed should be self-explanatory.

• There is currently no plot function for this calculator.

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Speed for RPM In Gear

This calculator is similar to the calculator above.  Please note that these calculations are estimates only.

• All inputs are the same except for Engine RPM.  

• There is currently no plot function for this calculator.

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Calculate Horsepower from Known Performance and Weight

Use the form below to calculate your cars estimated peak engine horsepower from current performance and weight.  You can also use it to calculate how much horsepower you will need to get your car to a given performance level.  Please note that these calculations are estimates only.

• Vehicle Weight is given in pounds in full race trim with driver.

• Best MPH is the highest MPH the car has run in the quarter mile.

• After you have calculated a result, if you would like to see a curve showing how the calculated value changes, select "Plot It".

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Calculate Performance from Known Horsepower and Weight:

Use the form below to calculate your cars estimated performance from known horsepower and weight.  Please note that these calculations are estimates only.

• Vehicle Weight is given in pounds and is the full race trim weight including driver.

• Estimated Maximum Engine Horsepower is just what you think it is.

• After you have calculated a result, if you would like to see a curve that shows vehicle MPH and ET as a function of horsepower, select "Plot It".

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Calculate Minimum ET From Known MPH

Use the form below to calculate your cars best estimated ET based on maximum MPH.  After you have calculated a result, if you would like to see a curve that shows minimum ET for a given maximum MPH select "Plot It".  Please note that these calculations are estimates only.

[ Parent Page ]

[ Racers Math 1 ] [ Racers Math 2 ]

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For more detailed engine-building information, check out the analyses and books available at the SlowGT store.

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This web site is dedicated to 5.0 Liter Mustang Drag Racing.
The technical information on this site is provided based on the owners personal experience.
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Last Updated: June 10, 2001

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