Weird and Wonderful Indy Cars: 1973 and the Clean Rear Wing Revolution

Published On February 8, 2017 » 13460 Views» By Marshall Pruett » Blogs, IMS History, Indy 500

Regular #SpeedRead readers will recall Marshall Pruett’s eight-part series on “Weird and Wonderful Cars of the ‘500’” that appeared in the weeks leading up to the 100th Running of the Indy 500 presented by PennGrade Motor Oil. Now, the feature continues with one car featured regularly.

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Leave it to an eagle to fully grasp the importance of clean airflow above and below its wings.

The 1972 Indy 500 marked the single craziest year for aerodynamic exploration ever seen at the Speedway, and with many of those wild concepts transformed into shapes that embraced sanity the following year, the value of efficient downforce became a priority in 1973.

One area of significant year-to-year aerodynamic development stood out more than others, and in almost every corner of Gasoline Alley in 1973, chassis designers went to great efforts to clean up the airflow around the turbocharged four-cylinder Offenhausers and Ford-based Foyt V8 turbos.

Those giant rear wings, used to settle and secure power figures that were flirting with 1000 hp, were important for the downforce they generated, and provided clear paths to those wings were created, aerodynamic drag would be reduced. In turn, the loss in drag would help top speeds rise which, in the early Seventies, was a new frontier to blaze.

Dialing up the horsepower was the key to setting new lap records before wings took over the Speedway, but once those wings became commonplace and power figures equalized, the next performance trail to explore was found in achieving aerodynamic purity.

As many of the winged Indy cars showed prior to 1973, attention to detail between the roll hoop and the rear wing was often an afterthought, and with a variety of protrusions and blockages limiting the flow of air to those wings, downforce suffered and drag figures hindered top speeds. Lloyd Ruby’s 1972 Atlanta chassis (modeled after McLaren’s successful M16B) and Foyt V8 turbo engine (below) offer a glimpse of the pre-1973 thinking.


As qualifying for the 1973 race demonstrated, the concept of clean rear wing airflow went from limited use to widespread adoption. The separation from the pole winner in 1972 to the 33rd qualifier was a full 17.1 mph; in 1973, the bottom of the field pulled much closer with just 9.8 mph separating first from last. In another interesting comparison, 1973’s pole speed of 198.4 mph was only 2.5 mph faster than 1972’s pole, but the slowest car in 1973 was 9.8 mph faster than 1972’s slowest car.

The numbers, in simple terms, speak to modest improvement at the front of the field in 1973 while the bottom half—well, most of the bottom half…there were still a few holdouts—embraced the clean rear wing airflow concept. The blueprint for the idea hailed from a familiar den of ingenuity in Southern California in 1972.

Among the enlightened constructors, All American Racers further refined its 1972 Eagle chassis coming into 1973 and used an incredibly elegant rear aero treatment to deliver a race-winning solution in the hands of Gordon Johncock. His Offy-powered No. 20 Patrick Racing Eagle sported a familiar, low, intertwined exhaust system that sat below the cam covers, and on the other side of the motor, the turbo’s intake plenum was parallel with the top of the sidepod.

Viewed from the side (below), the smooth lines–highlighted by the flat exhaust-engine-plenum arrangement–worked wonders at the back of the car.


The flat layout is also visible from this angle (below).


And here’s another look from the other side of the car taken with Mark Donohue’s Penske Racing Eagle-Offy.


In an era where aerodynamic freedom meant teams and constructors went down their own development paths, some went about cleaning up rear airflow in different ways. Take Joe Leonard’s Offy-powered Parnelli VPJ2 chassis produced by the Vels-Parnelli team, for example.

It used the same flat plenum and exhaust installation, but the Parnelli (below) also included a large streamlined engine cover/intake that tapered and terminated atop the rear wing. In time—after Indianapolis—the improvements offered by removing the engine cover were made clear. The car duly ran in the latter stages of the 1973 season without the bulbous piece in front of the rear wing.


England’s McLaren Cars, winners of the 1972 Indy 500 with Penske and Donohue, used an approach that mirrored the Parnellis, but rather than use a low plenum solution, it chose to send the Offy’s plenum skyward. Housed in dedicated engine covers that partially exposed the plenum, the McLarens of Gary Bettenhausen and Johnny Rutherford (below) reveal the odd choice to add weight above the engine.



Foyt’s turbo V8s also received unique treatments in 1973.

In Bob Harkey’s Kenyon-Eagle, the twin plenums consume a considerable amount of open space in front of the rear wing, and with the engine’s exhausts sprouting from the middle of the V—surrounded by shrouding to contain the heat—its low rear wing was fed a bare minimum of clean air.


The same Foyt V8 motor in the back of Dick Simon’s Eagle (below), minus the exhaust shrouding and with the rear wing slightly elevated, is a kindred spirit of the flat Offy installation in other Eagles.


With the central exhausts hugging the top of the engine (below), Simon’s car looks relatively clean compared to the rest of the Foyt V8 installations.


One look at the Coyote-Foyt Johnny Parsons briefly drove shows another take on rear wing airflow. Unlike Simon’s Eagle, which rotated its plenums downward to free flow paths to the rear wing, this Foyt engine (below) filled the airstream with two blunt plenums to hit on the way to the back of the car. Again, while most cars took the clean rear wing aero lessons to heart in 1973, there were a few that held onto the old ways.


The Indy 500’s turn towards aero refinement was inevitable after so many crazy concepts were tried without success in 1972. Since AAR’s Eagle set the trend and proved its worth with Johncock’s win, eliminating drag through intelligent design has become the standard practice at the Speedway.

Let’s close with one more look at the beautiful simplicity and effectiveness of 1973’s winning Indy 500 chassis and engine package. And before you go, take a moment to ponder the awesomeness of the surfboard-size rear wing…



About The Author

Marshall Pruett

Marshall Pruett transitioned from more than 20 years as an open-wheel and sports car mechanic, engineer and team manager into his current role as a writer and reporter in 2006. Follow him @MarshallPruett []

I love the Indy 500. Me and my Dad first started going in 1985. The Spin and Win from Sullivan. Last year was our last race we saw some great ones hope for many more.