It's hard to overstate how incredible the sixth-generation Mercedes-Benz S-Class is. The interior's opulence is more in line with a Bentley or Rolls-Royce than a BMW 7 Series, yet we tested a 2014 S63 hitting 60 mph in 3.7 seconds. Toss the car into a corner, and it'll handle far better than a nearly 5,000-pound luxury sedan has any right to. In the more than four years since the S-Class made its debut, however, Mercedes' competitors have been working hard to catch up. In an attempt to stay at the head of the pack, the 2018 S-Class receives a major refresh, and the S63 gets a new engine.

Under the hood, the S63 drops its 577-hp 5.5-liter twin-turbocharged V-8 in favor of a 4.0-liter twin-turbo unit that makes 603 hp and 664 lb-ft of torque. Power is still sent to all four wheels, but the updated all-wheel-drive system can now vary its torque distribution instead of sticking to a 33/67 front/rear split. The aging seven-speed automatic has also been replaced by a newer nine-speed unit.

But does that extra power, upgraded transmission, and improved all-wheel drive actually make the refreshed S63 a better performer? We took a new 2018 Mercedes-AMG S63 to our test track to find out. And then had to pull the car from testing because the brakes overheated.

Unfortunately, we don't mean the brake discs got "flaming hot." Actual flames were licking out through the spokes—the result of our punishing brake-testing regimen. That said, only the brakes burned, and the thermal event didn't cause any markings or damage to the vehicle. In other words, there was not a caustic fire that burned any components inside or outside the wheel well. A quick cool-down lap took care of the situation.

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Rather than blaming the car, testing director Kim Reynolds attributed the source of the overheating to our figure-eight brake-torture test (see below for Reynolds' thoughts on the brake failure). In short, the S63 is a heavy car, with loads of horsepower, being run through an extreme test that stresses the brake components. Igniting the brakes, while rare, is not unheard of in the testing community. If you were going to regularly, seriously flog your new S63, you might tick the options box for ceramic brakes.

Regardless of the source of the burn, the incident shut down our testing for the day, and we returned the S63 to Mercedes' press fleet representatives so they could figure out what went wrong. One week later, we were back at the track with a replacement car, ready to see what the S63 could do. Thankfully, that second testing attempt went off without a hitch. In fact, not only did the new S63 stay cool and shoot zero flames, but it also beat the first car's initial figure-eight results.

From a stop, the 2018 S63 accelerated to 60 mph in a scant 3.4 seconds and run the quarter mile in 11.6 seconds at 121.5 mph. That's impressive acceleration by any standard, but for a 5,105-pound car, it's mind-boggling. For comparison, when we tested the 650-hp track monster that is the Chevrolet Camaro ZL1 1LE, it took 3.6 seconds to hit 60 mph and ran the quarter mile in 11.7 seconds at 123.0 mph. The 2014 S63 AMG, meanwhile, laid down a 0-60 time of 3.7 seconds and did the quarter mile in 12.1 at 115.5 mph.

Mercedes-AMG has also simplified the S63's launch control, making it easy to repeat those results every single time. "Race start could not be any easier: Simply mashed both pedals then release the brake. Good for AMG for finally making it simple," wrote road test editor Chris Walton after testing the S63. "It first revs to 3,500. After releasing the brakes, the engine jumps to 4,000, then holy smokes, does it go. The nose lifts, there's a little bit of front wheelspin, then it digs in and goes. This limo is seriously quick."

The S63's braking performance was also impressive, but surprisingly, it lagged behind the 2014 version we've also tested. That car needed only 100 feet to stop from 60 mph, but the refreshed 2018 S63 we tested required 109 feet. Walton liked the firm feel of the brake pedal but noted it had "longer travel than expected." Still, he was impressed that hard braking resulted in "very little dive" and "zero wander."

Although a powerful engine, all-wheel drive, and large brakes can help compensate for a car's weight in straight-line tests, it's much more difficult to beat physics in our figure-eight test. Despite that challenge, Reynolds managed a 24.3-second time with average lateral acceleration of 0.83 g. Compared to the 2014 S63, that's an improvement of 0.7 second and 0.04 g.

As Reynolds noted, it "seems like a car with a ton of weight and a ton of horsepower." He also found that after a few laps, the brake pedal began to get soft, potentially risking the same fiery incident that stopped our earlier test. "It is a fun car, though," he said. "Doesn't like to turn in but is great fun exiting."

That said, the S63 was never going to give a star performance on the figure-eight course. It's far too large and heavy for that. Put it in Sport+ mode on an open road with plenty of fast sweepers, however, and it's a completely different story.

On a road like that, the S63 is hilariously fun to drive. The engine is so powerful and the snarling exhaust is so loud that it's impossible not to have fun. Sure, you're still driving a 5,000-pound car that's more than 17 feet long, but because body roll is so well-controlled, you really don't feel it. You also don't necessarily have to use the paddle shifters, thanks to the new nine-speed and its smarter programming.

Unlike some other high-powered cars, though, when it's time to act like a mature adult again, the S63 can also play that role perfectly. Switch to Comfort mode, and the AMG-badged monster that cracked and popped with every downshift transforms into a quiet, refined luxury sedan.

To the right (well-heeled) buyer, that's exactly the S63's appeal. It's a comfortable, understated daily driver with an incredibly well-crafted, luxurious cabin. The car handles stop-and-go traffic without an issue and eats up highway miles like they're nothing. But when you want to let loose and have a little fun, the S63 is also quicker and more powerful than most people would expect.

It's too large to be called a sport sedan, but how many other cars can hit 60 mph in 3.4 seconds while chilling a bottle of champagne in the back seat?

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A Word From our Testing Director on the S63's Brake Failure

During the figure-eight test, I noticed the brake pedal starting to get soft. Given the car's speed and considerable mass, that wasn't surprising, so I slowed down, let the brakes cool for a few laps, and tried the test again. Almost immediately, they went soft again, so I let the brakes cool down and pulled in to call it quits.

A few minutes after I parked the car, I turned around and saw small flames licking out between the spokes of the right front wheel. I jumped back in the car and drove it around in an attempt to put out the flames and cool the brakes even more. But after pulling in again, road test editor Chris Walton and I could still feel a considerable amount of heat still radiating from the brakes.

After letting the car sit for a bit to cool down more, I drove it around slowly to check the brakes. I heard a faint chafing sound, so to avoid doing any more damage, I decided to stop and have it flat-bedded back to Mercedes-Benz.

On a recent press trip, a guy who's done brake testing for a supplier told me the pads had probably been burning. Using a fire extinguisher would have made a colossal mess and maybe made things worse. The best thing to do was jump in and cool the brake down with lots of air.

Besides being fun to do, our figure-eight test gives us interesting lap times, lateral g numbers, and subjective impressions. But coincidentally, it's also a severe brake fade test.

We actually used to do a brake fade test that amounted to six stops from 60 mph, decelerating at 0.5 g as measured by a crude g meter. We'd repeat it as quickly as possible using a hydraulic pressure sensor screwed onto the brake pedal to read pedal force. After several years, we stopped seeing any major fade issues and quit doing the test. We also quit checking that the parking brake was strong enough to hold the car on a steep hill.

Our figure-eight test is much more severe than that old test, as it's repeated limit braking from higher speeds, and tires these days are way grippier. You almost can't configure a test to stress brakes any more than this.

Because of that, I'm reluctant to get too excited when I see smoke afterward. Nobody punishes brakes more than we do in that test, so it has almost zero meaning in the real world. I say "almost" because every now and then, a car (usually a Ford) gets a soft pedal after maybe two laps, which is too soon.

If you check the options list, the S63 also has optional ($8,950) carbon-fiber-reinforced ceramic disc brakes, which likely would have precluded this event from happening. —Kim Reynolds

2018 Mercedes-AMG S63 4Matic+ BASE PRICE $148,495 PRICE AS TESTED $192,395 VEHICLE LAYOUT Front-engine, AWD, 4-pass, 4-door sedan ENGINE 4.0L/603-hp/664-lb-ft twin-turbo DOHC 32-valve V-8 TRANSMISSION 9-speed automatic CURB WEIGHT (F/R DIST) 5,105 lb (54/46%) WHEELBASE 124.6 in LENGTH x WIDTH x HEIGHT 208.5 x 75.0 x 59.0 in 0-60 MPH 3.4 sec QUARTER MILE 11.6 sec @ 121.5 mph BRAKING, 60-0 MPH 109 ft LATERAL ACCELERATION 0.95 g (avg) MT FIGURE EIGHT 24.3 sec @ 0.83 g (avg) EPA CITY/HWY/COMB FUEL ECON 17/26/20 mpg ENERGY CONS, CITY/HWY 198/130 kW-hrs/100 miles CO2 EMISSIONS, COMB 0.96 lb/mile