Horsepower Correction, Power Management, And Science

The goal of racing engine tuning is to generate the best power for your setup. Knowing the limits of your vehicle—and tuning within those boundaries—ensures racing success. By maintaining a good air/fuel ratio when air density is fluctuating, among other controls, you keep your engine operating in top form. And the knowledge of how to prepare for these varying conditions can make or break your race program’s success.

Weather conditions affect the amount of oxygen in the atmosphere, which then has an effect upon engine performance. More oxygen with the right amount of fuel makes more horsepower, and values such as air density and density altitude help to determine how much fuel should the engine will require to burn that oxygen. Air density has a simple proportionate effect on fuel needs for racing engines. More fuel is needed in a linear proportion with higher air density. Less fuel is needed in a linear proportion with lower air density. Each of these changes needs to be made by the operator after careful consideration of each of the factors which alter performance.

The goal of all this tinkering is to get the best horsepower out of your engine. While air density affects horsepower, the effect is not a simple linear proportion. Mathematically, horsepower effects differ from the simple air density percentage value. A modified combination of the air temperature ratio and the air-pressure-with-humidity ratio determine those horsepower effects.

Blown hydro racing boats such as this gorgeous SEMA Show entrant often compete in bracket drag racing classes with precise elapsed-time targets. Tuners chase tune-ups up and down from dramatic weather changes that occur around water; numerical horsepower corrections determined from numerical control offer dramatic improvements to help get the first run off the trailer dialed in. Rather than making guesstimated adjustments to the engine and hoping for the best, using more precise mathematical corrections will help to minimize surprises on the race course.

Air Density Alters Horsepower

In each cubic-foot of atmosphere, the amount of air fluctuates, as temperature, humidity, and barometer changes affect how much oxygen is present in that cubic-foot. Tracking these in order to be aware of the current air density is important to maintaining an ideal air/fuel ratio. If you would like to learn more about this, check out our previous article on weather changes and engine tuning.

These atmospheric changes affect how much horsepower the engine will generate, which in turn can alter how the competition vehicle performs. For example, some drag racers will run a racecar at the limit of tire traction. With no changes to the tune-up, high air density will increase horsepower and can break the tires loose.

In another example, a competitor runs a racecar or race boat in a performance bracket at an elapsed-time limit established by specific racing class rules: examples are the 10.90, 9.90, 8.90, and 7.0-second quarter-mile bracket-racing classes. Low air density reduces horsepower and slows the racer away from their target bracket. High air density increases horsepower that may cause a breakout of elapsed-time beyond the bracket, disqualifying the racer.

In the early years of racing, seat-of-the-pants tuning and tuning by experience on early-model nitro engines were common. Tuning was an art that was dependent on experience. Seat-of-the-pants tuning—especially for nitro—has been in use since then, mostly absent the advantages of horsepower correction calculations.

For the racer to truly excel, numerical tuning and extensive record keeping provides more help, especially with nitro and/or superchargers. It is a better alternative to the dilemmas and unknowns from expensive trial-and-error tuning that was faced by racers throughout the years, especially with respect to the cost of replacement parts which have failed.

Air density changes affect the horsepower in a gasoline-fueled racing engine in one manner. In an alcohol-fueled racing engine, air density changes affect horsepower, but not as much as when gasoline is used. And in a nitro-fueled racing engine, air density changes affect horsepower differently from both of those, and also not as much.

The blown nitro-injected Hemi V8 in a Top Fuel Funny Car for quarter-mile drag racing is often tuned to the limit of tire traction, with an excess of 60-plus gallons per minute nitromethane fuel flow to create over 11,000 horsepower. When power is too much and the tires break loose, engine damage usually results from engine over-speed, as the engine can often explode from ingesting so much nitromethane.

How Air/Fuel Ratios Affect Horsepower

For a properly tuned air/fuel ratio that is maintained across different air densities, the amount of fuel fluctuation changes proportionately, but the amount of horsepower fluctuation varies in a non-linear fashion.

Racing engines are typically run at high compression and/or high boost, if they are forced-induction engines. Detonation is usually managed by an air/fuel ratio that is richer than the stoichiometric ratio common in highway vehicles. Racing engines are most often not run on the lean side of the stoichiometric ratio.

According to author Bob Szabo, “Higher compression ratios need exorbitant amounts of extra fuel for cooling and detonation control. Unfortunately that increase in extra fuel provides power-robbing cooling throughout the engine cycles. That extra fuel is always there occupying space during intake, providing cooling during compression, providing cooling during power, and occupying space during exhaust.”

To a certain point, running leaner will increase horsepower. However, once that point is passed, the engine may start to burn up from the high compression or high boost. In fact, running on the lean side of the stoichiometric ratio makes less horsepower if the engine does not overheat or burn up, as there is simply not as much fuel to burn to make power.

Because of this complexity, knowing the optimum air/fuel ratio and how it performs under different conditions dramatically simplifies tuning to achieve optimum horsepower.

Horsepower Correction

Dyno tests often reveal how sensitive a racing engine is to the amount of extra fuel. Changes in the measured horsepower output can occur from changes in fuel enrichment. However, a dyno test is performed with that environment’s air density. Some dyno service providers determine a horsepower value for the air density in the test and a “corrected horsepower” value for standard air density conditions.

That horsepower correction determination is complex. Unless it takes into account the specific type of fuel or the type of engine, it may not be a valid conversion. That could explain why horsepower is often different when engines are moved from the dyno to the race course.

If done properly, horsepower correction depends on special math for changes in air density with exponential modifications. A horsepower correction factor relies on extensive math and is dependent on the type of fuel and the type of engine. According to former Competition Eliminator racer Patrick Hale in his book, Motorsports Standard Atmosphere and Weather Correction, “The HP correction factor depends on the fuel system of the engine, that is, the type of fuel being burned and if the engine is naturally aspirated or not. The mechanical efficiency (i.e. friction) of the engine also comes into play for this calculation.”

However, if you know the exponential modifications for your engine, the math becomes relatively simple. “The HPC was designed by the SAE committee to be used to correct the observed torque from the dyno to standard day conditions,” he says.

Horsepower correction as a result of air density changes illustrated for a supercharged racing engine is as follows:

HP correction factor = [1 – total loss factor] x [(temperature ratio)0.4 / (pressure w humidity ratio)0.7] – [total loss factor]

The temperature ratio exponent is 0.4 for this engine. The pressure with humidity ratio exponent is 0.7 for this engine. These exponents are higher for naturally-aspirated engines and lower for supercharged engines. They are more for gasoline, less for nitro, and less for methanol.

The engine’s output is also affected by friction and accessory losses.

The total loss factor in this first equation is illustrated as follows:

total loss factor = long block frictional factor + blower drive factor + accessory loss factor

Nitro Engine Example

In a nitro-fed engine, specific changes in the nitro percentage can correct the horsepower. Changing the nitro percentage for air density changes is the method that Gene Adams and Dean Adams use to tune Kin Bates’ well-known national-championship-winning Nostalgia A-Fuel dragster. In an engine with forced-induction, correct the horsepower with a change to the supercharger or overdrive speed. For a blown nitro engine, either or both can be the horsepower correction method.

Below is an example of the horsepower effect for a racing engine running in an 80 deg. F temperature, 30.0 in-Hg uncorrected barometer, and 40-percent humidity. This combination of weather values compute to an air density of 95-percent with a fuel adjustment to 95-percent.

However, the horsepower correction is not as simple. A value of 97-percent was computed for a supercharged racing engine. In other words, even though a 5-percent reduction in fuel was needed for the temperature/humidity/pressure combination in the example, the horsepower reduction was only 3-percent.

This example demonstrates the magic of supercharging. It is a great band-aid for horsepower losses from drops in air density. This is a reason for its popularity in racing around the world!

Horsepower corrections done for a string of 7.0 second elapsed-times for this fast bracket drag racecar tuned by AA Fred Myrick.

Horsepower correction example

The total loss factor determined for this racing engine example:

  • long block frictional factor = 0.16 (determined for a 478 ci V-8 engine size; greater for larger engines; less for smaller engines)
  • blower drive loss = 0.04 (determined for a 14-71 Roots blower; less for smaller blowers)
  • accessory loss = 0 (no accessories such as water pump, alternator …)
  • total loss factor = 0.16 + 0.04 + 0
  • total loss factor = 0.20 (this value is a lot smaller for very high output nitro engines)

A horsepower correction factor is determined for this racing engine in 95-percent air density derived from 80 deg F, 40-percent humidity, and 30 in-Hg barometer:

  • temperature ratio = (80 deg F + 459.67) / (60 + 459.67) = 1.039
  • pressure value for 40-percent humidity at 72 feet track elevation and 80 deg F = 0.41 in-Hg
  • pressure with humidity ratio = (30 – 0.41) / 29.92 = 0.989
  • horsepower correction factor = [1 – 0.20] x [(1.039)^0.4] / [(0.989)^0.7] – [0.20]
  • horsepower correction factor = 1.03
  • HP adjustment = 1 / horsepower correction factor
  • HP adjustment = 1 / 1.03 = 0.97

This air temperature, humidity, and pressure combination produced a 5-percent reduction in air density. That same combination produced only a 3-percent reduction in horsepower from a supercharged racing engine. The supercharger reduces the air density effects on horsepower changes.

More Examples:

Example 1: This relationship is illustrated with a 2,200-horsepower supercharged drag racing engine that runs a 6.2 second elapsed time at 100-percent air density.

  • horsepower correction for 95-percent air density = 0.97
  • corrected horsepower = 2,200 x 0.97 = 2,134

To recover that 66 horsepower in 95-percent air density, increase the blower overdrive 6-percent to maintain the 6.2 second elapsed time drag race dial-in.

Example 2: Horsepower correction illustrated with a 4,000-horsepower engine running with racetrack traction limitations.

  • horsepower max to hook up rear tire = 3,800 (observed & calculated from racer database records)
  • horsepower = 4,000 at 100-percent air density (observed & calculated from previous run records or dyno test)
  • the horsepower correction for 95-percent air density = 0.97
  • corrected horsepower = 4,000 x 0.97 = 3,880

That is above the upper limit of 3,800 HP. Since the rear tires can only hook up at 3,800 horsepower, the rear tires are going to spin or shake at the above conditions. A 3-percent blower overdrive reduction reduces the horsepower to 3,800. For nitro fuel, a 2-percent reduction of nitro mixture ratio is an alternative to get back to 3,800 horsepower with the original baseline blower speed. When done ahead of time, these determinations are better than a trial-and-error attempt.

These calculations provide good power management information. Consider that one racetrack condition can only hook up 3,600 horsepower and another can hook up 3,900 horsepower as determined from prior outings. The former would do well at a 6-percent nitro reduction or a 10-percent blower overdrive reduction for this air density example. The latter would do well maintaining the original baseline overdrive and nitro percentage for this 95-percent air density example, bringing the horsepower down to 3,880, just under the 3,900 capacity. Horsepower correction can help to chase the racetrack variable in preparation for the first qualifying run of the next outing.

This altered runs a fuel-injected big-block Chrysler wedge V8 with low percentage of nitro for bracket drag racing, and has the nitro percentage changed for different air densities. The tune-up was based on approximately 26-percent nitro, plus or minus a few percentage points for different air densities. This combination accomplished a highly desirable result, a simple setup to go fast with a simple tuning practice: small changes to nitro percentage.


Knowing how changing environments will affect your engine’s performance can mean the difference between a winning outcome and an unexpected failure. Tracking the air density and knowing your engine make the prediction of horsepower fluctuations easier and less surprising.

While mathematically tracking horsepower fluctuations can provide very precise numbers, knowing how changing environments affect your engine setup is useful when preparing for a run. With proper horsepower corrections, the racecar will be ready right out of the trailer.

About the author

Jennifer Szabo

Jennifer Szabo is the creator of, an online resource that assists racers in tuning their engines. The site aims to help racers get the most power and consistency from their engine setups. Jennifer is a professional web developer and the daughter of drag racer, Bob Szabo. Bob is a retired welding engineer and author of 6 books about mechanical fuel injection engine tuning at Bob and Jennifer maintain a blog at their websites in addition to co-writing articles for various auto industry resources.
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