Why Was the VC-Turbo Infiniti QX50’s EPA Fuel Economy Cut for 2020?

As MotorTrend’s chief technology cheerleader, I have pep-talked variable-compression engine concepts time and again since 2009, including three big explainer stories from the official fall 2016 Paris Motor Show launch of Nissan/Infiniti’s 2.0-liter VC-Turbo four-cylinder. The first engine to bring variable-compression technology to production features tricky new Rube Goldberg multi-link piston connecting rods and promises […]

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As MotorTrend’s chief technology cheerleader, I have pep-talked variable-compression engine concepts time and again since 2009, including three big explainer stories from the official fall 2016 Paris Motor Show launch of Nissan/Infiniti’s 2.0-liter VC-Turbo four-cylinder. The first engine to bring variable-compression technology to production features tricky new Rube Goldberg multi-link piston connecting rods and promises Big-V-6 power with Little-four-cylinder fuel economy! Rah, Rah, Sis Boom Bah!

First installed in the 2019 Infiniti QX50, the VC-Turbo was rated at 268 horsepower at 5,600 rpm and 280 lb-ft of torque at 4,400 rpm. Those figures aren’t hugely far off from the previous-generation QX50‘s 3.7-liter VQ37VHR V-6, which was rated 325 hp at 7,000 rpm and 267 lb-ft at 5,200 rpm. The engine’s party trick is that it can, by way of complex piston connecting rods, change its compression ratio on the fly—dropping it to allow more turbo punch, and raising it for fuel-stingy cruising at lighter loads.

At launch, the VC-T-powered QX50’s EPA ratings supported the big-engine power, small-engine fuel economy narrative, with a four-cylinder-esque 24 mpg city, and up to 31 highway and 27 combined. That’s an impressive jump from the V-6–powered 2017 QX50’s 17/24/20 mpg rating with either rear- or all-wheel drive. So on paper, switching from V-6 to VC-T four-pot power boosted economy by 30 percent (combined), while the luxury crossover’s quarter-mile performance only dropped about 5 percent. Huzzah, right?

Eager to see how the engine works in the longer term, we got a fully loaded long-term QX50 AWD to test for a year. We promptly sent the Real MPG team out to test those EPA claims. Their results were reasonably close: 21.0/29.7/24.2 mpg, or down 3.0 mpg in the city, 0.3 highway, and 1.8 combined relative to the EPA’s estimates. But sheesh, as we commuted to work in the LA basin, we seemed to be stopping to fill the 16-gallon tank way more often than expected. Sure enough, at the end of 20,000 miles, our QX50 had swilled unleaded premium at a rate of just 20.3 mpg—a figure lower than what we’d recorded on the supercharged V-6 Jaguar F Pace and the (larger) Mazda CX-9 powered by a technologically simpler 2.5-liter turbo four (20.5 mpg for each long-termer).

For 2020, the QX50’s EPA figures have dropped to 23/29/26 for front-drive and 22/28/25 for all-wheel drive. That’s a haircut of 4 to 9 percent in the city, 7 percent on the highway, and 4 percent combined—in a year where very little changed. What happened?

Officially, Infiniti says the drop is “due to model year changes including enhanced standard equipment on some trim levels which increased the weight class, and the adoption of heightened particulate matter emissions controls.” Apple CarPlay and Android Auto are now standard, as is a new infotainment system with dual HD displays, and some formerly optional safety, security, and driver-assistance gear has been made standard. That doesn’t sound like 125 pounds’ worth of stuff (inertia weight classes are in 125-lb increments).

What has actually happened here is that with $23,535 separating a fully loaded QX50 AWD model from a stripper, there’s a lot of optional gear that might push the Infiniti several inertia weight classes (these are approximated EPA classifications for setting up a dynamometer during emissions testing). That doesn’t mean the EPA estimates were “off” before now, but it does reveal something about how fuel economy figures are determined.

At the outset of a new model launch, it’s anybody’s guess how many folks will settle for the advertising lease-rate special stripper model and how many will load the car up, so the manufacturer gets to make that guess. In offering a prediction of the trim level mix, an automaker can choose whichever version is expected to take a large chunk of sales and use that—and its inertia weight class—as the basis for a given powertrain combination’s fuel economy estimates. If that guesstimated sales leader is lighter and therefore posts better fuel economy, great, the EPA figures might be higher than on other trim levels; if the expected volume leader is a heavier spec using the same engine and therefore thirstier, so be it—but that means the EPA figures might be lower than the best a given model can do.

Once there’s a year of sales history in the books, however, the inertia ratings (and the MPG-estimating equations and data they feed into) need to be updated to reflect the versions people actually bought more than others. No more guessing or fudging. The “average” QX50 folks are buying, it turns out, is simply a whole lot heavier than what Infiniti initially estimated. Good for them, because a rich mix equates to higher profits. And our $59,085 long-termer weighed in at 4,163 lbs—fully 353 pounds up on the factory curb weight of a front-drive “Pure” trim QX50. That’s why, it seems, the EPA figures were revised downward.

As for our year-long experience with the (pre-revised-MPG) QX50 with the VC-Turbo engine, weight may not have been the only factor in its disappointing efficiency. Utilizing the engine’s turbocharger—and in this case the 8.0:1 compression that goes with it—is a lot more fun than tiptoeing the QX50 around at small throttle openings, where a driver fully exploits the fuel economy benefits of the VC-Turbo’s highest compression ratio of 14.0:1. The truth is, we seldom see the big fuel-economy numbers downsized, highly boosted engines claim. Nissan’s variable-compression turbo engine is no different.

This cheerleader is declaring his Variable-Compression pep-rally over.

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