Air Density Online

Weather Accuracy in Engine Tuning with Handheld Weather Stations

Kestrel at the track

Using a handheld weather station at the race track

Throughout the years, we've seen many racing teams documenting their engine specs after a run.  Fewer teams keep meticulous records of the weather they experienced during that run.  It seems obvious to note that it’s hot and humid or windy.  Perceived weather and actual measured weather could provide conflicting information.

In open drag racing competition such as Top Alcohol, the tuning goal for maximum performance is to get the pistons so hot that oil is burnt on the underside.  This is a smidgen away from piston dome melting damage.

One of the key tuning goals is even fuel distribution around the engine for the entire run.  That included both the launch and the high end.  In addition, the high end is affected by ram air as the speed increases.  That increases the oxygen to the engine.  Fuel must be increased to accommodate that.

Fuel distribution is done with testing and changes to port nozzles.  Unfortunately air density is usually changing throughout a test outing.  Without proper adjustment to the overall fuel gallons per revolution for the air density changes, tuning results are often misleading.

Having accurate weather data is vital to having accurate engine tuning data.  Going by the feel of the weather or by non-localized data can severely impact tuning decisions down the line.

Wind and humidity affect perceived temperature

A hot and dry day is a lot more tolerable than a hot and humid day.  Humidity inhibits the body’s ability to cool itself through sweat.  Wind can not only feel cool against the skin but it can facilitate sweat evaporation, making it feel even cooler.

Check out the National Weather Service’s heat index calculator.  A 90 degree F day with 10% humidity feels like an 86 degree day.  A 90 degree day with 50% humidity feels like a 95 degree day.   That is almost a 10 degree difference in how the temperature feels to the body.

How does this affect your engine?  It means that what you think is the amount of oxygen in the air due to perceived weather conditions could be off.  Even slight miscalculations can throw off your engine’s performance. Having accurate weather data means having an accurate engine tuneup.

Find the actual weather before a tuning decision

It is important to keep track of the current weather, whether you use our site or a handheld device, such as Kestrel’s 5100 weather station.

The Kestrel weather station identifies actual raw numerical weather data: temperature, humidity, and barometer values.  It also computes summary data frequented by racing tuners: air density, density altitude, and water grains.  It additionally provides added weather data such as heat stress, and wet bulb temperature.  Through the years, we have met many racing tuners with different preferences in tuning data study.

For example, air density often starts out high in a cool morning, goes down in the hot afternoon, and goes back up in the cool evening.  Weather fronts with strong winds can make impulsive changes, often overlooked by the crew chief in a tune-up preparation for the next run.

Numerical weather values by the hour can identify the over-all fuel system changes to accommodate for that.  Then fuel distribution changes can be done appropriately, run after run, to achieve an even burn in the racing engine.

Once fuel distribution is optimized, the over-all engine enrichment can be adjusted for the maximum power.  That is the point where the pistons are so hot after a run that the oil is burnt on the underside of the pistons even though the domes are not melted nor are the ring lands collapsed.

Once you achieve this, welcome to racing heaven.

That level of engine characteristic can only be achieved with precise engine tuning for weather changes.  The Kestrel 5100 portable weather station along with the Air Density Online forecast and historic data base is an optimum combination to support that achievement.

Bob Szabo's funny car dragster"

Bob Szabo's funny car dragster during a 200mph+ run.


At a recent drag racing test outing, we prepared for the Saturday test by examining the forecasted weather on Air Density Online the day before race day.  We determined an air density change from 98% in the morning to 94% in the afternoon caused primarily by forecasted temperature changes.  This required a 5% change in the number of gallons of fuel per revolution to the engine over the full day of racing.  We examined our mechanical fuel injection jetting setup.  First we determined that we had the necessary inventory of fuel injection nozzles and bypass jets to control the difference in fuel gallons.  Second, we set up a jetting change plan by the time of day, according the Air Density Online forecast printout.

On Saturday, we reexamined the weather with a Kestrel 5100.  The air density was very close for the time.  There was a small change in the barometer, although it was not enough to change our jetting plan.  The wind direction changed.  And the wind speed changed.

According to the track map, the track went from the west to the east.  Wind direction was from the west at 20 MPH.  This was a tail wind that would have a positive effect on the aerodynamic drag of the race car.  Wind was gusty. This also had a negative effect on ram air. We noted the difference and reduced our ram air enrichment by about 1% fuel volume.

We were in staging with continued wind gusts.  We reexamined the weather and wind direction with the Kestrel just prior to our first run.  The wind had changed from the west to from the northwest causing a cross wind on the racetrack.  That would affect handling.

The wind was increasing throughout the day.  When it reached 30 MPH, the racetrack reduced their test track length from one quarter mile to on eighth mile for safety.  Examining the wind direction and the weather on-site with the Kestrel forewarned us of the weather effects.  We were not surprised by the reduction in test track length.

As the wind increased, the temperature felt cooler due to increased evaporation on our skin.  It felt like cool weather with high air density.  In fact the air density was still dropping from temperature increase.  That still necessitated a change to the fuel injection for the hotter weather that may have been ignored without the weather numbers.

The combination of the forecast from Air Density Online and the on-site weather measurements of air density and wind direction provided key information for tuning that may have been ignored by feelings from the weather.

Keep detailed records

Because of the differences between perceived weather and actual weather, it is very important to keep detailed records of the weather at the time of your run and how your engine’s combination performed under those weather conditions.  When you made that near-perfect run last summer, was it a warmer afternoon than usual or was it just the humidity rising due to a nearby summer storm?  With good records, you would know for sure.

Newer handheld weather stations like Kestrel’s enable data logging so you can export this information after the fact.

Engine tuning for better performance

With a regular record of runs, making engine modifications down the line becomes easier.

Tuning for the proper enrichment or leaning is too sensitive to be 'guestimated'.  After examination of many engineering dyno and road test data sets from various fuel systems, we came to the conclusion that numerical control tuning has the same advantage over guestimated tuning as numerical control machining has over 'guestimated' machining setup.

In a one-off machining task, the journeyman can manually do a good job.  But in production where repeatable accuracy is key, numerical control machining can make even an inexperienced apprentice accomplish a great job.

Similarly, in competition, fast turn-around between rounds is demanding.  Weather changes are frequent and excessive in some locations.  Those changes affect the engine and, in racing on land, traction and handling.

Recent technology brings numerical control to the race tuning task, just like it did for machining.

Related articles on

Using this site with a Kestrel Weather Station Tuning with Methanol vs. Nitro

Other resources

Kestrel 5100 weather station Kestrel Instruments
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By Jennifer on August 20th, 2020 in Info and Education, Main News


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