Short-Track-Package Testing at Phoenix

Six NASCAR drivers participated in short-track-package testing at Phoenix Raceway on December 5th and 6th. They tested variations in tires, and an alternate splitter and diffuser. NASCAR’s goal for this test was learning how to improve racing at tracks a mile and less.


  • Goodyear tires, both the Martinsville tires and a new, softer set, got high marks from the drivers made available for comments.
  • The same drivers rated a new splitter and diffuser neutral to positive. NASCAR engineers came out of this test considerably more heartened than they were after Richmond.
  • Mufflers (to be used only at the L.A. Coliseum and the Chicago Street Race) with different ducting seemed to reduce heating.
  • Due to time constraints, experiments to eliminate shifting couldn’t be run.

The What

Two teams represented each manufacturer for the two-day test.

  • Ford: Ryan Blaney and Chris Buescher
  • Toyota: Christopher Bell and Erik Jones
  • Chevrolet: Corey LaJoie and Kyle Larson

This test presented the first opportunity to see Legacy Motor Club running the new Toyota Camry XSE now that they are no longer a Chevy team. We also got a preview of the on-track behavior of the new Ford Mustang Dark Horse. Erik Jones and Ryan Blaney were made available after the first test day and Kyle Larson at noon on the second day.

What’s Testing Like?

For drivers (and observers), it’s honestly not all that exciting. For the horde of engineers and technicians crowded around computer banks in the Phoenix Raceway garages, it’s Christmas come early.

Engineers drive testing. NASCAR compiled a ‘laundry list’ of possible configurations spanning everything from shifting to mufflers to aerodynamics. Time constraints forced them to pick only a few of the possible combinations.

Testing is a collaborative enterprise. NASCAR meets with the drivers, and with the team engineers, each evening to debrief. NASCAR changed their plans for Wednesday based on input from Tuesday’s debrief.

In addition to driver input, teams get scads of data: force sensor outputs, air pressure measurements, lap times and passes made during short ‘mock races’. They get to outfit their cars during testing in a way not allowed during practices and races.

For those of us observing, it’s a little complicated because we don’t get access to any of that data. I can’t really blame NASCAR: I wouldn’t want to have to report intermediate results because things may change by the time engines are fired in L.A.

What’s Being Tested?

Prior to the test, NASCAR identified four major areas of concern.

  • Aerodynamics. A modified underbody test at Richmond last fall yielded slightly disappointing results. The problem remains: The leading car has such an advantage that it’s difficult for the trailing car to pass.
  • Mufflers. New venues like the Chicago Street race, the L.A. Coliseu, and potentially the Nashville Fairgrounds require toning down the noise a little. The problem is that the sheer volume and temperature of hot exhaust gases passing through mufflers demands a bespoke design. The muffler must get gases through as quickly as possible while not generating additional heat.
  • Shifting. One complaint about last year’s short-track racing is that drivers could compensate for having a weaker car by shifting. NASCAR has real-time data coming in . They could simply tell the drivers not to shift and penalize them if they do. That’s a less-than-ideal solution. This test originally had modifications to the XTrac transaxle that would eliminate the advantage of downshifting on the docket.
  • Tires. With the Gen-6 car, Goodyear had to design the tires so that they didn’t heat too much. The Next Gen car just doesn’t put heat into the tires the way previous generations of cars did.

Did They Really Test All That?

No. There isn’t time to do everything. Every change requires time to make the change, about an hour to dial in the cars, and then 10 or 15 minutes for a mock race. Eric Jacuzzi, NASCAR’s VP for vehicle performance, noted that he would love to test longer and with more cars. But testing is expensive.

And, frankly, sort of exhausting.

That limits what configurations NASCAR can test. At the first day’s debrief, engineers and drivers weighed what they would learn against the 2+ hours it takes to change out a transaxle. They decided that the speeds in the corners at Phoenix were too slow to justify losing that much time.

In addition, the transaxle is a pretty well-characterized component. Lots of racing series use them. I’m betting NASCAR could change the gearing and have a pretty good idea of what would happen without testing. That’s much harder to do for the other three areas.

What Happens During a Test?

The main goal for the short-track package is that the fastest cars should be able to go to the front when they start in the back. On Tuesday, drivers did two 15-lap races for each configuration. The car order was changed between the two sequences. Wednesday, drivers ran 50-lap races. The longer races provide a chance to characterize lap time fall-off and wear.

Note that these races — unlike at Richmond — started the drivers in single file rather than a 2 x 3 lineup. Racing in two rows made it harder to pass and NASCAR wanted all the passing data they could get.

The Tuesday tests started with running the cars with the setup teams had used at Phoenix, followed by a series of 30-lap runs to test different tires. In the afternoon, they tested the new splitter and then the new splitter with a modified diffuser.

On Wednesday, drivers tested various combinations of new splitter, new diffuser and different tires to determine which elements worked best together.

How Were the Mufflers?

Many race cars skip mufflers because mufflers slow cars down. Without a muffler, exhaust gases can get out of the cylinder quickly, making way for more gas to combust. The engine can run faster and generate more horsepower.

But without a muffler, you’re hearing 60 or 70 explosions per second, which creates a sound level of 110 or 115 decibels. That’s way above the range of safe listening.

Mufflers minimize sound by bouncing it off surfaces and forcing through absorbent materials on its way out. The muffler steals energy from the sound through all those collisions, which makes the noise coming out quieter.

The sheer volume and temperature of hot exhaust gases from a Next Gen car demands a custom mufflers. The muffler must withstand the hot exhaust gasses, get them out of the system as quickly as possible, and not heat up so much that they’re heating up the cockpit.

This test evaluated ducting changes to reroute the hot exhaust as it makes its way out of the car. The cars didn’t seem much quieter with the new configuration, but you could hear that they changed the car’s tone. I perceived it as a little more growly (which is a good thing in my book.)

The sound meter app on my phone showed the same peak sound intensity (115 db) as without the muffler, but the peak was a shorter in duration with the muffler. I’ll analyze the sounds in a future post.

Kyle Larson said that he could definitely hear the sound difference inside the car with the muffler. It doesn’t affect his driving, but he “thinks the cars can be quieter just to help the fan experience.”

What About the Aerodynamics?

The splitter tested is larger than the current splitter. In conjunction with the modified underbody, the goal is to create a small amount of lift when a car is running alone, and increased downforce when the car is running near other cars. That should make it easier to pass.

The diffuser, which is located under the car’s rear, helps shape the car’s wake. The diffuser normally has five strakes — vertical pieces of metal. At the Richmond test, NASCAR removed the middle three strakes. Here, they used pieces of aluminum offset slightly from the centerline.

For pictures, take a look at Cole Cusumano’s article at Kickin’ the Tires. Cole got some great photographs that are way better than anything I shot.

NASCAR’s in a bit of a box with the aerodyanmics. In order to allow manufacturers to make the car bodies closer to the production vehicles, the majority of the downforce is generated under the car. Just removing the underbody isn’t an option.

Reactions to the aerodynamics changes were varied. Jones said he didn’t feel much change, while Blaney felt both of the modifications were an improvement.

To my eye, they were making progress. In one mock race, Kyle Larson went from third to first in 10 laps. Christopher Bell went from sixth to second in fifteen laps.

So What’s the Short-Track Package for 2024?

The goal of testing is confirming that you’re going in the right direction. On-track testing validates CFD, wind tunnel and vehicle dynamics simulations. Goodyear may make additional modifications to the tire. The underbody could change even more.

As Jacuzzi said, “I’ve never gone to a test and walked out like, ‘Oh my God, this is what we’re doing.” But he left this test feeling much more confident than he felt after Richmond.

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