Understanding Weather Tuning Basics
By WFO, November 2011
Changing weather can have a profound impact on your engine performance. Many top racers actually take advantage of weather readings as part of the engine tuning process. Your first look at weather may seem confusing with unending contradictions and possibilities to ponder. Tuning using the weather is not an exact science. First, the weather is always changing. What is useful is being able to predict a direction and need for change before you find yourself shelling out thousands of dollars for burnt and broken parts. Tuning using the weather may be used as a defensive strategy that protects your investment. The goal is to produce fast efficient runs down the race track with some degree of advanced certainty in how the engine will perform. Tuning with the weather will also help you understand how to take advantage of making the best possible power under a given condition. Once you learn to read the weather and predict changes needed in engine tuning, it will become second nature like anything else.
In order to fully apply tuning by the weather you must also have a working knowledge of your engines behavior. This comes from reading the spark plugs and knowing which cylinders are predominately lean or rich, and how the overall engine is using the air/fuel mixture. Defined below are the parts of the weather typically used in engine tuning for naturally aspirated and nitrous engine applications. Remember that the definitions are relative and directional. You will still have to estimate how much and what change is needed for your engine, which will only come with practice and observation over time.
Absolute Barometric Pressure is measured in inches of Mercury (in. Hg) and has to do with the atmospheric pressure conditions of a given location. Normal pressure gauges have a scale that sets the zero point for atmospheric pressure at 14.7 psi or 29.92 in Hg. (Barometer), which is gauge pressure. This is defined as the Standard Day Absolute Barometric Pressure at sea level and gives a point from which to measure. 14.7 psi is pounds per square inch of pressure exerted on an object by the air at sea level. So what we are really talking about is the pressure of air and how it impacts conditions in the engine as it changes.
Absolute Barometric Pressure is not the same as what is reported for an area on weather forecasts, which is a sea level corrected pressure. For this reason, it is important to measure the Absolute Barometric Pressure and all weather samples with a weather data instrument at your location for an exact reading. The Absolute Barometric Pressure will only encounter very minor fluctuations during a typical racing weekend. Once measured, the reading will remain fairly consistent varying by a few hundredths. So how does this factor play out in tuning the engine?
When the barometric pressure is higher there is more oxygen available for combustion in a given volume. Consequently the opposite is also true. For every thousand feet of elevation climbed, the barometric pressure is reduced by approximately one inch of mercury and there is less oxygen available for combustion in a given volume. Absolute Barometric Pressure also has an affect on cylinder pressure. The engine is subject to increases and decreases in compression ratio as the pressure of air changes. When the barometer reading is low, for example 24 in Hg, the air quality is both comparatively oxygen poor and the cylinder pressure in the engine is low. This creates a noticeable drop in power. Technically one could run less fuel to air ratio for low oxygen and more timing to increase cylinder pressure if Absolute Barometric Pressure was all the measure needed. It’s however only once piece of the equation for consideration.
Racers coming from sea level tracks are often surprised when their engine burns up coming to a higher altitude low Barometer track. This has to do with dry air conditions and high Density Altitude. High Density Altitude and extremely low water grains in the air (almost none) create a lean condition when combined in the equation that reverses the direction of timing and fuel. Because this lean condition is produced, a little fuel may need to be added and timing taken out. Ultimately the engine cannot make power in these conditions and the car will slow down. There is a bell curve exception rule somewhere around 5000 ft in the Density Altitude where fuel needs to be added despite the fact that most other weather conditions will indicate that less fuel is needed. Dry air is rather self explanatory. Even though the oxygen is relatively poor with a low Barometer, you are getting all of it with no water grains to offset any oxygen that is in the air that you may have been used to at a humid track. In this case, one needs to use caution in not taking too much fuel out. I will get to how timing fits in the discussion of water grains.
Temperature affects how tightly molecules of air are packed together. As air cools it releases energy, slows down, and the molecules become more condensed. The opposite is true in warmer weather where the molecules excite and begin to bounce off one another moving rapidly. Temperature rising and falling can be used to predict directional changes in the Density Altitude. A cooling temperature will produce a lower Density Altitude reading resulting in a need for more fuel because you are getting more air. If the engine was on lean edge in the day time, you might consider that you will need to add fuel to race later that evening if there is a significant drop in temperature. The night air will be more condensed. If the engine ran a lean mean pass at night in cool temperatures and performed well, it’s relatively safe to run in hotter temperatures the next day. The engine will be richest at the hottest point of the day with the same tune up because the air is least condensed at this point. In measuring the temperature and all weather conditions, it is important to at least implement a consistent practice and its best to measure a condition most similar to what the engine will actually intake.
Relative Humidity, Vapor Pressure, and Water Vapor- Relative Humidity is a value used to indicate how much water vapor is present in an air sample. It is a relative figure as relating the amount of water vapor present to the maximum amount that air could hold at a specific temperature. Air has the ability to hold more water vapor as temperature increases in theory. However, it is common to see this figure increase at night during a racing event. 50% Relative Humidity is 50% of the possible saturated water vapor content for air. This differs from Vapor Pressure and Water Vapor. Vapor Pressure is calculated from both temperature and Relative Humidity. The Vapor Pressure number indicates the portion of the available air pressure that is absolutely water vapor. Water vapor is expressed in grains and is basically the same as Vapor Pressure however expressed in a different measurement. Some racers use the Vapor Pressure readings, while it has been more common for most to use Water Vapor or grains of water as the reading. Water Vapor is grains of moisture per pound of dry air at Standard Day Density Altitude (14.7 psi or 29.92 in Hg, at 60 deg with 0% Humidity.) The below chart indicates a general trend relationshipindic
| Water Vapor in Air (grains water vapor/lb dry air) |
Relative Humidity
(%) |
Temperature (oC/oF) |
| -1 |
4 |
10 |
18 |
22 |
27 |
32 |
38 |
| 30 |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
| 10 |
3 |
4 |
6 |
8 |
11 |
16 |
21 |
29 |
| 20 |
5 |
7 |
10 |
16 |
21 |
30 |
42 |
58 |
| 30 |
7 |
10 |
14 |
22 |
34 |
46 |
65 |
87 |
| 40 |
9 |
14 |
20 |
30 |
44 |
62 |
85 |
116 |
| 50 |
12 |
16 |
26 |
39 |
55 |
78 |
108 |
147 |
| 60 |
14 |
18 |
32 |
48 |
66 |
92 |
128 |
176 |
| 70 |
17 |
20 |
38 |
54 |
78 |
108 |
158 |
208 |
| 80 |
19 |
22 |
42 |
62 |
88 |
125 |
173 |
|
| 90 |
21 |
24 |
48 |
70 |
100 |
140 |
195 |
|
(http://www.engineeringtoolbox.com/wa...air-d_854.html)
The power output of the engine depends on oxygen intake, so the engine output is increased or decreased as the equivalent "dry air" density increases or decreases, and produces more or less power as moisture displaces oxygen in more humid conditions.
Water Grains are an indicator of the margin of safety for our purposes as related in figuring the amount of fuel needed. The more water grains there are in the air, the greater the window for margin of a lean error. When water grains are low there is more oxygen in the air and fuel needs to be added with the opposite being true. With increased grains of water in the air, the air/ fuel ratio becomes richer as a result.
Low water grains also indicate a need to put less timing in the engine. In dry conditions, timing is taken out and with higher water grains you can put timing in the engine. Advanced timing in the engine increases cylinder pressure, can make more power, but also increases the chance for detonation. Advancing the timing will lean out the air/ fuel ratio of an engine and is something to consider in dry air. In advanced weather tuning, timing can be used to compensate for water grains at the track or as an adjustment for Density Altitude. Adjusting timing can be a first line of defense consideration before changing carburetor jets or the nitrous spread. This is where you can take the greatest advantage of individual cylinder timing. Timing knocked out of one or two cylinders that look a little lean or glazed according to the spark plug can allow you to fix the problem without having to take an action the affects the power of the whole engine. Too much timing will burn up an engine more quickly than an imbalance in the fuel.
Density Altitude is a value expressed in feet above or below sea level. It is a combined calculation that considers Absolute Barometric Pressure, temperature, and Relative Humidity. In simple terms, density is the mass of anything, including air, divided by the volume it occupies. Density altitudeis defined as the altitude at which a given air density is found in the Standard Atmosphere or Standard Day. It’s actually a height, which is not the same as the physical elevation of a location. For example if an elevation of approximately 6100 ft is equal to the pressure of the Standard Day at that elevation, and the temperature is roughly 100 degrees, the density would be the same as that found at about 10,000 feet. You cannot therefore simply depend on track elevation as a valid measure although track elevation will be an indicator of what you may expect to find. Density Altitude can swing dramatically during a racing weekend and some times even in-between rounds. You will hear people asking and discussing good or bad air at the race track. Typically this stems from looking at the Density Altitude reading. The lower the Density Altitude reading, the better the air is for making power. It is even possible for Density Altitude to fall below zero. Lower readings generally mean a need for more fuel with the opposite being true also less the exception rule stated above. For a given altitude, the Density Altitude changes with changes in Barometer, air temperature, and Humidity with each capable of impacting the reading.The Barometer and temperature both have a greater impact on Density Altitude than Humidity. Being that we already know that the barometer stays fairly stable during an average racing weekend, temperature then becomes the greatest predictor in the rising or falling direction of Density Altitude.
An increase in barometer or pressure increases air density, therefore decreasing density altitude
Anincrease in temperature decreases air density, therefore increasing density altitude
An increase in humidity decreases air density, and therefore increases density altitude
(Increased air density = decreased Density Altitude) = Increased need for fuel with the opposite being also true less the exception rule
Air Density Ratio (ADR) is another way of looking at the engines need for changes in fuel. The ratio is produced by dividing the calculated density of the air by the Standard Day Altitude Density. At the zero point, the ADR is equal to 1.00, which is 100% Standard Day. As the ADR goes up or down, the amount of fuel needed by the engine also goes up and down by that same percentage in order to maintain the same air/fuel ratio in the engine
In order to make power, the idea is to cram as much condensed air and fuel into the cylinder as possible. To keep the engine from burning up, the ratio of air and fuel must fall into a given middle ground window. If the engine is too rich or too lean problems will occur. Changing weather changes this ratio without a single thing ever being mechanically changed in the engines tuning.
