Your racing vehicle is already in peak condition and you’re ready to compete. You can use the conditions at the track to your advantage to get you that much more of an edge. For instance, wind in the atmosphere has more effect on a racer performance than meets the eye. When it kicks up, you can’t make it go away, but you can work with it to maximize its advantages and downplay its disadvantages.
Head Winds during a run
Winds blowing towards your vehicle have both benefits and drawbacks. The obvious drawback is that wind resistance blowing against the body can slow it down. A force pushing against the direction a vehicle is traveling means there is more work to accelerate through that force.
For a typical drag racing funnycar or low-profile Pro Mod at 200 MPH:
- a 20 MPH head wind will add about 21% drag to the racer body, as much as a 200-pound increase in frontal aerodynamic drag force
- a 20 MPH tail wind will subtract about 21% drag to the racer body, as much as a 200-pound decrease in frontal aerodynamic drag force
Increased down forces on the spoiler or wing
While a head wind increases aerodynamic drag on the body, it also increases the down force on those vehicles equipped with a rear wing or spoiler. For high powered vehicles, the increased down force can help traction.
In high-powered racing vehicles on a circle track, more tire spin variation occurs. A head wind in the straight-away may change to no wind in the turn, and then to a tail wind in the return straight-away. On another day or location, a head wind in a turn is a tail wind in the other turn with no wind in the straight-away. Down forces may appear and disappear as the vehicle turns. That can affect the setup.
Positive effects on an air scoop
At the same time, head winds blowing into an engine with a forward-facing air scoop can experience a particular advantage. The extra air blowing in can provide a power boost in the form of added oxygen going into the engine. As the racer goes faster beyond a certain speed, air ramming into the scoop can pressurize the inlet.
CASS, or “critical air scoop speed”, determines the forward velocity where the engine air consumption is the same as the air ramming onto the air scoop.
At speeds below the CASS, there is a low pressure in front of the scoop. At the CASS, the air feed to the engine is balanced with the vehicle speed feeding air into the scoop. At speeds above the CASS, the air scoop is pressurized with forward velocity. If the correct fuel curve is set up to provide extra fuel at high speed, the result is more power.
Effect of head wind on the air scoop
For example, one drag race sedan had a typical setup featuring a large forward air scoop feeding an 1850 CFM engine. The air scoop size was 55 square inches which is also typical for an air scoop. The CASS is 115 MPH. At speeds above 115 MPH, the engine air scoop is pressurized and delivers more oxygen to the engine. Higher speeds make higher pressure and more horsepower.
In another example, a high-powered drag race Pro Mod was running with a typical 3,400 CFM engine. The air scoop size was 65 square inches. CASS is 86 MPH. As a result, any speed above 86 MPH pressurizes the blower hat.
At 200 MPH, for example, about 2 psi of added pressure, above atmospheric, was measured in a dragster air scoop. Our ProCalc jetting calculator determined jetting for the proper fuel enrichment. This supercharged 1,800 HP engine gained 12% more horsepower at 200 MPH. That totaled about 2,000 HP.
With a 20 MPH head wind, approximately 2.4 psi of added pressure in the air scoop occurs at 200 MPH. The extra 0.4 psi of pressure feeding the blower will add about 1 psi of boost after the blower, into the blower manifold. That is over and above the manifold boost at 200 mph with no wind.
This boost in the air scoop from the increased ram air from the 20 MPH head wind will add about 40 more HP to the engine, totaling about 2,040 horsepower at 200 MPH with the head wind. Note that this power bump is only at the high speed. At speeds lower than 200 MPH, the bump up is lower. At speeds higher than 200 MPH, the bump up is greater.
This increase is separate from added boost from compression heating. It is also separate from the boost reduction from fuel vaporization cooling in the manifold. As a result, a change in the boost reading that results from these changes may not be noted in a boost gauge or data recorder at the higher speeds.
At 250 MPH, added pressure will add over 25% more power to a supercharged engine with the proper fuel enrichment with speed. Tuning for head or tail wind gets a little crazy with really fast vehicles.
Tail Winds during a run
Tail winds have the obvious benefit of providing a pushing influence on the vehicle, creating less drag, and providing a small acceleration boost. However, there are other factors going on that effect the run.
Decreased down forces on the spoiler or wing
A tail wind may decrease down force on vehicles with a rear wing or spoiler. With higher powered vehicles, more tire spin can occur. In motorsports such as circle racing, large wings and other aerodynamics are common in the sport. Tire spin variation may occur going around a turn where tail wind may change to no wind in the turn, then to a head wind in the return straight-away as previously described.
Effect of tail wind on the air scoop
In the previous head wind example, we discussed how a 20 MPH head wind provides a boost. In the other direction with a 20 MPH tail wind, about 1.6 psi of added pressure occurs into an air scoop. This is 0.4 psi lower than without the tail wind. In a blown car with a forward-facing air scoop, this tail wind will reduce about 1 psi of boost into the blower manifold. However, because of other influences on manifold boost previously described, the change may not be observed on data recordings from the inlet pressure difference.
The reduction in boost from less ram air will take about 40 HP from the engine. It is now down to about 1,960 HP at 200 MPH with the 20-mph tail wind.
In summary, from a 20 MPH head or tail wind, an 80 HP difference can occur for a vehicle with a forward-facing air scoop going 200 MPH. The racetrack orientation and the prevailing wind relative to the racetrack becomes important to recognize for this magnitude of influence.
Cross wind & diagonal wind and their effects on vehicle control
In a cross wind, it becomes necessary to steer the vehicle in the direction of the cross wind to keep a straight pathway. This causes tires to scrub sideways on the track surface. In this scenario, it causes an increase in the resistance to forward motion and a greater load on the rolling resistance of the racer. A significant amount of power can be consumed with this steering effort against a strong cross wind. That can affect the net performance.
For a diagonal wind (in between a cross wind and a head/tail wind), the wind has two components:
- the component into or against the direction of the racer
- the component perpendicular to the racer
The wind portion into or against the direction of the racer causes the head or tail wind consequence (or advantage) as previously described. The wind portion perpendicular to the direction of the racer causes the counter-steering need previously described.
Head/tail wind portion: The cross-sectional area and aerodynamic streamlining of the vehicle determines how much effect from a head or tail wind portion of the diagonal wind.
Cross wind portion: The cross-sectional area and aerodynamic streamlining of the body sides of the racer determine how much effect from the crosswind portion. The distribution of the body side cross-sectional area and aerodynamic streamlining can become significant with more wind. Front-to-rear differences determine handling effects from a cross wind. That includes different front-to-rear body side cross-sectional areas and streamlining.
For example, vehicles with large vertical rear spill plates can be really difficult to handle with a strong crosswind. The rear end can be blown sideways in a cross wind. Steering becomes a matter of keeping the front end in front of a wandering rear end while trying to stay on the track.
The effect is similar to driving a vehicle with rear wheel drive in the slippery snow. The rear end may spin the tires drifting from one side to the other from the loss of stability. The driving skill often becomes keeping the front end in front of the rear end as it drifts side-to-side. Highway surface changes on snowy roads also affect the rear end slide, again making the driving task a real skill.
In a similar manner, a cross wind on a racer with different body side cross-sectional areas front-to-rear can be a challenge. Again, that is keeping the front end in front of the rear end and staying on the track at the same time.
Boat racing and water chop
Race boats experience similar issues with wind as land racing. However, they have an added burden of water chop and effects from hull design.
Some hulls such as hydroplane or tunnel hull boats benefit from a moderate wind to break up the water surface to keep the hull running loose. These hulls run slower on mirror smooth water. The hull sponsons, running against the water surface, tend to “glue to the smooth water surface.” A light chop aerates the surface, and the boat is faster.
That is most likely the case for other plaining hull designs as well including vee bottom, runner bottom, and flat bottom. A little chop aerates the surface, and the boat is faster.
Boat hull lift
Higher wind speeds also can affect hull lift when it is running fast over the water. More head wind makes more lift against the hull on the water. Too much lift can blow over a boat hull at high speeds.
Long boat racing courses also tend to have areas of greater or lesser head wind from surrounding shoreline features such as hills, docks, and buildings. If the race boat is “hung-out to max”, changes in wind speed and direction can cause a boat to blow over if the driver does not compensate with trim or speed.
Personal example with race boats
Years ago, I was running at top speed in a tunnel hull race boat. About halfway through that back stretch, a strong change in head wind occurred during every lap. When I would approach that location, I backed out of the throttle to reduce power to reduce speed to compensate. I was in the last long straight-away nearing the end of the race. In this final lap, that let-up was enough for one of the competitors to pass me.
My competitor was “hung-out for max speed”, and he did not back off. When he hit the transition point, his boat stood straight up. It was a miracle he did not flip. That killed his speed, and he fell behind far enough to not regain his passing position. I finished the race in front of him after the next turn.
Summary of the effect of wind on an overall run
Wind may be an overall benefit or detriment to a racer. The effect from a 20 MPH head wind on a 200 MPH racing vehicle would be:
- an increase in power about 40 hp increases speed
- a 200-pound increase in aerodynamic drag on the racer body decreases speed
- an increase in down force from extra wind on a spoiler or wing increases tire traction that increases acceleration.
The effect from a 20 MPH tail wind would be:
- a decrease in power about 40 HP decreases speed
- a 200-pound decrease in aerodynamic drag on the racer body increases speed
- a decrease in down force from less wind on a spoiler or wing decreases tire traction that decreases acceleration.
Performance may go up or down depending on the combined effect. It can get complex depending on the baseline amount of horsepower and aerodynamic drag and can be different for different vehicles. Knowing the wind direction and how it could affect your racer can give you an advantage.