By now, you might have seen our dyno tests of both the 2016 Ford Mustang Shelby GT350 and its more extreme GT350R sibling.

To the seat of our pants, the R felt stronger than the base GT350. Our Vbox agreed—though there was a negligible 67-pound (30 kg) weight difference between the two, the R beat the GT350 on both 0-60 (by 0.2 second, 3.9 seconds versus 4.1) and through the quarter mile (12.1 seconds at 119.6 mph (192 km/h) versus 12.4 at 117.8 mph (190 km/h)). We chalked the slight differences up to tire grip (the R wears much more aggressive rubber), and indeed the two cars posted identical 1.7-second 45-65-mph (72-105 km/h) passing numbers.

And then we drove them back-to-back, and the R felt far quicker. Livelier, too—but also much lighter. We could explain most of that away because of the wheels. The GT350R wears wheels made of carbon-fiber, saving a claimed 60 pounds (27 kg) over conventional wheels. That much rotating mass is a lot—and that could easily explain the difference in what we felt. But since the engines have such different exhaust notes (the R is missing the GT350’s intermediate resonator), we wanted to make sure Ford hadn’t given the R a special tune. So we strapped both to the dyno to find out.

The results? Indeed the GT350R made more power throughout the rev range. Here are the peak numbers:

GT350: 467 hp @ 7200 rpm (wheels)

GT350R: 471 hp @ 7200 rpm (wheels)

Oh, but before you go telling your friends that the GT350R’s engine is more powerful: Don’t. A chassis dyno measures how quickly a car is able to accelerate a fixed mass and then calculates the engine’s output based on that rate of acceleration.

The GT350R’s lower rotating mass, thanks to its carbon-fiber wheels, will give its engine an apparent advantage on the dyno. Without swapping the wheels between the two cars, which we weren’t able to do, we can’t account for that difference.

But more important, the 6-hp difference between the two cars is insignificant. At just over 1 percent difference, it’s far less than the variance we saw between multiple runs on the same car. The GT350R did two runs, both at 471 hp, but the GT350’s runs varied by a massive 27 hp at the wheels.

The ambient air temperature was to blame for that significant difference. Before we installed a sixth (!) fan to blow cold air directly at the intake, the Mustang’s computer reported intake air temperatures over 115 degrees. With the fans, both cars provided consistent results with intake air temperatures around 100 degrees.

Additionally, the shape of the curves is so similar that it’s hard to believe there’s any tuning difference between the two engines—and indeed watching the ignition-advance curves, the two engines were doing the same thing at the same time.

So does the GT350R make more power? Yep, a couple horsepower more at the wheels thanks to its lighter wheels. But Ford’s identical 526-hp rating at the engine? Legit.

See the graphs below.