“Twilight Zone” is Lee Sicilio’s 1969 Dodge Charger Daytona, completed just before the 2012 Bonneville Speed Week. The car’s fabricator/engineer Ryan Fain (center) makes last minute preparations at the starting line for a record qualifying run.

There is no such thing as “good enough” for a racer. Every detail of our personal performance, and our machine’s performance, has to be refined to the absolute best it can be to win races. That’s because our competitors are engaged in the same quest, and the strong ones have created highly refined racing programs. Finding performance gains in high-payoff areas that your competitors aren’t even thinking about is a great way to produce a big performance jump over the field. If the tracks you run are fast and open, with long straights and big corners, then improving the aerodynamic performance of your car can produce a big performance gain.

To get a direct feel for how much airflow affects your car’s performance, try sticking your hand out of a car window on the highway, palm down. Then consider that the frontal area of your car is about 500 times larger than the edge of your hand. Then consider that your top speed on the track may be twice your highway speed, which generates four times as much drag as highway speeds. That’s 2,000 times the drag on your hand, which is a lot!

Because we cannot see airflow, not many racers have a solid understanding of where the air goes and what it does as a car travels through it. Part of that is because the shape of a car is highly complex, with constantly varying curves, edges, corners, junctions, free edges, open windows and wheel wells, every imaginable shape under the car, and airflow through the coolers, engine and cabin.

What if we could see airflow? That would answer a lot of questions about what is actually happening to the air while the car punches through it. It would tell us where the airflow is clean, efficient and logical. More importantly, it would tell us where the zones of trouble are, and how big those zones are. After making a change to the shape of the car, we would know whether that change improved the airflow in a zone of trouble, and if that change affected anything downstream of that new feature. In addition, it would define what the local direction of “downstream” actually is since a car pushes air in all sorts of directions that aren’t straight along the car’s path.

Well, there is a way to see airflow. Even better, all it will cost you is some time. It doesn’t get any cheaper than that. One way to see airflow on the surface of your car is called oil streak flow visualization, and preparing your car for it is as simple as anything ever gets. Save some used engine oil from your next oil change to use for this investigation. Used oil is a good choice for the dots because it is a lot darker than new oil, so the streaks are easier to see. It doesn’t cost anything either, which is great. New oil will work too, but it doesn’t show up as well in photos. Other fluids like spray lubricants and pink antacid liquids also will do the job, but antacids can stain plastic windows.

Just before a high speed trip on the track, put a series of oil dots on the body with a finger dipped in your used oil. Repeat as many times as it takes to cover the area of interest. Oil dot spacing can vary from 1” to 6” between dots depending on the level of detail that you want to see in the streaks. Dot the horizontal surfaces first, because the oil will run down angled and vertical surfaces slowly.

These oil dots were applied moments before a record-breaking run at the Bonneville Salt Flats. New two-stroke oil was used for the dots. Used engine oil would have made the dots and streaks easier to see.
These oil dots were applied moments before a record-breaking run at the Bonneville Salt Flats. New two-stroke oil was used for the dots. Used engine oil would have made the dots and streaks easier to see.

During the track run, airflow on the surface of the body will push the oil in the direction of local airflow. The length of each streak is a rough indicator of how fast the air is moving there. If the track run is fast enough and long enough, the car will carry a record of local airflow that will last long enough to get good photos before gravity takes over again and pulls the streaks toward mother Earth. After taking photos, cleanup will take several paper towels, spray glass cleaner and auto detailer spray.

Here is a best-case example of oil-dot streaks on the roof. All of the streaks show that the airflow direction was almost exactly straight back along the direction of travel of the car. The streaks are all long and nearly the same length, indicating uniform, high speed airflow. This is great for low drag, but the high speed airflow here created some lift.
Here is a best-case example of oil-dot streaks on the roof. All of the streaks show that the airflow direction was almost exactly straight back along the direction of travel of the car. The streaks are all long and nearly the same length, indicating uniform, high speed airflow. This is great for low drag, but the high speed airflow here created some lift.
Here is another example of what you want to see. The oil dots on the bottom of the rear wing streaked straight back, all the way to the trailing edge. This means that the airflow was fully attached, and the wing was working efficiently. If the oil streaks on your wing suddenly turn 90 degrees, that indicates flow separation at the turn point.
Here is another example of what you want to see. The oil dots on the bottom of the rear wing streaked straight back, all the way to the trailing edge. This means that the airflow was fully attached, and the wing was working efficiently. If the oil streaks on your wing suddenly turn 90 degrees, that indicates flow separation at the turn point.

Now the tricky part begins: interpreting what the oil streaks mean. All you get is surface flow direction and a rough indicator of flow intensity. Sometimes it takes an educated and experienced eye to tell what the off-body airflow is doing to cause the streaks that you can see. We will describe and illustrate a few common flow patterns here, but covering this topic fully is beyond the scope of this article. If you see something that you aren’t sure about, contact your favorite aerodynamic expert for a good bench racing session and an in-depth education.

Zooming in to the tip of the wing shows a small trouble zone. The oil streaks flowed away from the corner junction, indicating that there was a small airflow separation at the corner. Airflow separation at interior corners like this is common, particularly where the airflow decelerates along both panels. The step-in shape between the wing and pylon/end plate contributed to this trouble zone.
Zooming in to the tip of the wing shows a small trouble zone. The oil streaks flowed away from the corner junction, indicating that there was a small airflow separation at the corner. Airflow separation at interior corners like this is common, particularly where the airflow decelerates along both panels. The step-in shape between the wing and pylon/end plate contributed to this trouble zone.

The cheap and easy nature of oil streak flow visualization makes it a practical research technique for every track racer. Knowing what is really happening to the airflow on the surface of your car is the first step toward modifications that will improve your car’s performance. Retesting with your modifications in place will provide an indication of their effectiveness. If your oil streaks show the need for a modification that no one has ever done before, and it’s legal for your class, go for it. Stepping into the twilight zone of aerodynamic modifications is the essence of innovation.

Not even 283 mph at Bonneville will move an oil dot if the airflow is separated from the surface. The oil dots in the hood cove and behind the fender outlet scoop remained stationary. The stationary oil dots behind the fender outlet indicated that not much air flowed in or out, even though the outlets are open to the under-hood area.
Not even 283 mph at Bonneville will move an oil dot if the airflow is separated from the surface. The oil dots in the hood cove and behind the fender outlet scoop remained stationary. The stationary oil dots behind the fender outlet indicated that not much air flowed in or out, even though the outlets are open to the under-hood area.
These oil streaks are evidence of a major airflow separation. The top row of oil dots streaked down and aft. The next row of dots streaked down and forward. All of the other dots streaked up and aft. These streaks indicated that there was a definite pattern to the airflow, but the airflow was not attached to the surface. Almost every car has this problem. If your racing class rules allow it, reshaping this area can result in less drag and more downforce.
These oil streaks are evidence of a major airflow separation. The top row of oil dots streaked down and aft. The next row of dots streaked down and forward. All of the other dots streaked up and aft. These streaks indicated that there was a definite pattern to the airflow, but the airflow was not attached to the surface. Almost every car has this problem. If your racing class rules allow it, reshaping this area can result in less drag and more downforce.

Although these photos were taken at the Bonneville Salt Flats, airflow is airflow and cars are cars. Oil streak flow visualization works the same there as at your home track, but without the occasional effects on the oil streaks from cornering and braking.

Oil streaks on the cowl panel at the base of the windshield show that the local airflow direction here was nearly parallel to the lower edge of the windshield. This shows the profound effect that major protuberances have on the flow field. The oil streaks are difficult to distinguish from paint blisters that were caused by exhaust heat soak after the run.
Oil streaks on the cowl panel at the base of the windshield show that the local airflow direction here was nearly parallel to the lower edge of the windshield. This shows the profound effect that major protuberances have on the flow field. The oil streaks are difficult to distinguish from paint blisters that were caused by exhaust heat soak after the run.
The oil dots on to of the rear quarter panel streaked away from the base of the rear wing pylon, indicating a small zone of airflow separation. This is another example of junction flow separation.
The oil dots on to of the rear quarter panel streaked away from the base of the rear wing pylon, indicating a small zone of airflow separation. This is another example of junction flow separation.
The long, straight oil streaks on the rear deck and trunk lid indicated high speed attached airflow on the rear window and deck. This excellent flow quality was one of many innovations that made the 1969 Dodge Charger Daytona the dominant car of its day in NASCAR.
The long, straight oil streaks on the rear deck and trunk lid indicated high speed attached airflow on the rear window and deck. This excellent flow quality was one of many innovations that made the 1969 Dodge Charger Daytona the dominant car of its day in NASCAR.
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Image courtesy of Neil Roberts