We said it last year and we’ll say it again: ProMod drag race cars are the wildest motorsports door slammers around! Last year we wrote an article about the weather and engine tuning for the Mid-West ProMod Racing Series held at the Texas Motorplex on November 1-3, 2018. This year that same event is happening a couple weeks earlier on October 16-19, 2019. The weather will be a little bit different and engine tuning will need to be adjusted accordingly.
The event is happening at the NHRA Fall Nationals. Top drag racing ProMod racers will be running at power levels well in excess of 2,000 HP and putting on a spectacular show. For maximum performance, engine, launch RPM or shift RPM changes can be affected by air density changes. Air density knowledge is followed religiously by most competitors and RacingJunk is filled with race cars and parts servicing this category of racing.
A Basic Review of Air Density Science
The air density affects the power output of a racing engine:
• less air density >> less oxygen in the air
• less oxygen in the air >> less power.
Humidity also affects the amount of oxygen in the air:
• more humidity >> less oxygen in the air
• less oxygen in the air >> less power.
Fuel system tuning for these kinds of air density & humidity changes are part of racing. Although the trends are shown, the change in horsepower from air density is not exactly proportional to humidity changes. More info about horsepower corrections from air density is in our publication, Blown Nitro Racing on a Budget.
The track is at an altitude of 515 feet. When correcting (or un-correcting as general barometer values are corrected to sea level) barometer, the theoretical nominal barometer is about 29.4 inches HG or about 98% air density for STP (standard temperature pressure) weather conditions.
Weather at last year’s event
Last year’s MWPMS event at this track was on Nov. 1-3, 2018. That was later in the year than this year’s event in mid-October, 2019. A few weeks later in the year meant colder temperatures among other different weather patterns compared to this year. The hourly weather trends for that 2018 event are listed below:
Texas Motorplex weather for Nov 1, 2018: partly cloudy; air density 99% @ 8:00 am down to 97% @ 4:00 pm up to 100% @ 11:00 pm
Weather for November 2, 2018: clear weather; air density 100% @ 8:00 am down to 98% @ 11:00 pm
The air density in the AM was in the 97 to 98% range, for good power making. In the afternoon and remainder of the day, it fell to the 94 to 96% range for a power reduction.
For those who pay attention to water grains, they rose from 47 grains in the morning up to 70 grains in the evening, indicating an increase in moisture from a weather front change. That reduces power although that effect is folded into the air density percentage. For those who tune from air density, adjustments to the fuel system were necessary from both effects. A 3 to 4% reduction in fuel weight would have been necessary to keep a consistent air to fuel ratio, or lambda for those setups so equipped. Ironically, air density and water grains were not necessarily in sync. That is, the percentage of water grains change was different than the percentage of air density change. That complicated tuning decisions for tuners who watch both.
More important, this increase in moisture also reduces traction. These high powered drag cars with slew control over launch power would need careful adjustment if the track is moist. Note that as the evening progressed, the change between the temperature and the dew point fell to only 10 deg. F. That is an indication of increased effect on traction from moisture. Common spray-on traction compounds should be tolerant to that however. In addition if the racetrack surface builds up with a thick layer of rubber from numerous race car launches, the increase in humidity can also affect delamination between the track surface and the rubber coating. In this case, it would be important to pay attention to racetrack personnel to see if scraping of excess rubber is done to minimize this effect. Once the rubber starts to delaminate, further launch power reduction is most likely necessary.
What Happened a Year Ago: Air Density & Weather from October 16-19, 2018
Air density information was examined for this year’s event dates from a year ago.
The air density details for this October 16, 2018 link are shown below.
Weather on October 16, 2018: Rain throughout the day. Most likely the event would have been rained out.
Weather on October 17, 2018: light rain until the evening with an air density of 100%. So no racing.
Weather on October 18, 2018: the day was overcast with an air density range from 99% down to 97% and then back up to 98%. For any racing that day, the tune-up would be consistent with a 98% air density baseline. However with overcast skies, the track would be cooler from the absence of direct sunlight, necessitating possibly a power reduction on launch.
Weather on October 19, 2018: rain throughout the day. Most likely the event would have been rained out.
Because of the stormy weather in October, 2018, we look further and analyze data from two years ago in 2017.
What Happened Two Years Ago: Air Density & Weather from October 16-19, 2017
The air density details for this October 16, 2017 link are shown below.
Weather for October 16, 2017: Air density started at 100%, went down to 96%, then back up to 99%. The tune-up would have been similar to the November 2018 event. Higher 100% air density in the morning would possibly require slew control adjustment to reduce launch power from that of a 98% air density baseline.
Weather for October 17, 2017: clear weather; air density range from 101% down to 95% back up to 98%.
Weather for October 18, 2017: clear weather; air density range from 100% down to 94% back up to 97%.
Weather for October 19, 2017: clear weather; air density range from 98% down to 93% back up to 96%. That is a dramatic one day change.
More Air Density Variation Means More Oxygen Variation Means More Power Variation
Air density varies from a high of 98% to a low of 93%. If this event was held during the weather that occurred on these dates in 2017:
1. Mechanical fuel injection engines would need a 5% change in fuel weight to maintain a constant air to fuel ratio.
2 EFI engines with open loop controls would need a 5% change in fuel control to maintain lambda with a constant air to fuel ratio
3 EFI engines with closed loop control probably would not need adjustment. If the oxygen sensor is providing a valid indication of enrichment, the tune-up should be maintained by the engine management system. However, rich mixtures, typical of high boost, high compression engines often create misleading oxygen sensor readings due to after burning in the exhaust. Setup and knowledge of those limitations are needed to maintain a tune-up.
4 For carbureted engines, a 5% change in fuel weight from jetting or float level or booster adjustments would be needed to maintain good fuel mixture.
5 For engines running nitrous oxide, some tuners adjust the nitrous oxide for weather changes to compensate for the “on-the-motor” fuel system. That can get tricky. Adjustment of the “on-the-motor” fuel system may be an alternative that helps keep the “non-nitrous” gasoline part of the tune-up correct to the amount of air density.
Moisture Effects on the Race Track
As stated in our previous article, the best racetrack surface is usually right after a strong rain storm where the pavement surface is washed clean from the rain. Since rain occurred on the 2018 chart, it’s worth considering. If rain clears on race day and is then followed by a good coating of traction compound and rubber dragging deposit before the race track, traction is at it’s best.
When it does not rain ahead of time, national event preparation often includes manually washing the racetrack surface as clean as possible prior to traction compound application and dragging. That is done in order to minimize dust, dirt, and oil residue. However, track preparers say when it does not rain ahead of an event, washing is way more work than applying the subsequent traction compound & dragging. Prior pavement cleaning is more critical to maximum traction for a subsequent event.
What if the Event was Run Earlier in the Year?
Earlier in the year, it looks like the weather may be warmer with a comparable power reduction in most engine combinations. The EFI turbo engines may automatically compensate however other engine combinations would not. If the event was run a month early in September, air density values from airdensityonline.com indicate the low 90% range from warmer weather with even more power reduction management. The pesky rain is often the ruling influence on a race at this track, regardless of the month.
• Air density forecasts are available through Airdensityonline.com for racetracks all over the US, International racetracks, and many now south of the equator.
• Current air density in metric units is now available for metric tuners.
• Historic air density from a track can be reconstructed from AirDensityonline.com to get an indication of tuning needs for planning a subsequent event.
• Air density data was analyzed on the same dates from this year, but from a year before; unfortunately indicating rain.
• Air density data was further analyzed on the same dates but from two years before for further tuning predictions.
• Most engine tuners for high powered race cars such as MWPMS ProMods rely on air density data for fuel system and traction tuning.
• Air density tuning is a vital part of motorsports setups bought or sold through racingjunk.com.