Fifty-five IMSA WeatherTech SportsCar Championship entries take to the grid in, coincidentally, the 55th edition of the Rolex 24 At Daytona. There are four classes, two of them Prototype and Prototype Challenge, both populated by purpose-built race cars that resemble street cars only in passing.

The other two classes are both GT – that stands for “Grand Touring” or “Gran Turismo,” in Italian. There are 11 GT Le Mans entries, which are primarily factory-supported. And there are 27 GT Daytona entries, the largest class in the field. Both GT Daytona and GT Le Mans cars are based on production cars, so GT competitors look very much like the street-going models they are based on.

We’ll take a look at the GT field today, and tomorrow, we’ll preview the Prototypes.

For casual fans, GT may take a little getting used to: Some cars run in both the GT and GT Le Mans classes – an example would be the Porsche 911 and the Ferrari 488. We once asked a competitor what the difference between a GT Daytona and GT Le Mans car, and he replied, “About a half a million dollars.”

The GT Le Mans cars are more sophisticated than the GT Daytona cars, but what once was a substantial performance gap between the two classes is narrowing. Yes, there may be Porsche 911s and Ferrari 488s running in both GT Le Mans and GT Daytona, but count on the GT Le Mans models to be significantly faster.

Speaking of the Porsche 911, that’s the big news in GT Le Mans for 2017. Daytona marks the premiere of the Porsche 911 RSR, which was first shown to the public at the Los Angeles Auto Show. While it resembles the last-generation 911, this RSR has the six-cylinder engine moved closer to the front, from its traditional rear-engine configuration. It’s been called a mid-engine car now, which may be a bit of a stretch, but moving the engine forward should improve the overall balance of the RSR, as ideally, a race car should have 50 percent of the weight on the front, and 50 percent on the rear. The engine shift will help the RSR in that regard.

While the other GT Le Mans entries haven’t undergone such dramatic surgery, the Ferrari, Corvette, BMW and Ford teams have not been standing still. Especially the Ford Chip Ganassi Ford GTs, which made their world competition debut at the 2016 Rolex 24 At Daytona, then went on to a class win at the 24 Hours of Le Mans, 50 years after the Ford GT won Le Mans outright.

Ganassi fields four Ford GTs, two in North America, and two in Europe. This year, all four Ford GTs will be in attendance, as the company very much wants a Rolex 24 win on the car’s resume, right beside the Le Mans victory.

This year, of the 11 entries in the class, there are two Chevrolet Corvette C7.Rs, two BMW M6 GTLMs, two Porsche 911 RSRs and a lone Ferrari 488 GTE. You can make a case that, in general, some of the GT Le Mans teams may be the most experienced, the most professional of any class on the grid. Corvette Racing team has been together a long time – there are people still with the team that were there in 1999 for the Corvette C5.R’s debut at Daytona.

The biggest difference since then? There was a time when teams worked hard on race strategy, perhaps sending out one car to run flat-out for as long as possible, while the other entry takes it much easier, saving the car, hoping to be in contention for a late-race dash to the checkered flag, theoretically the tortoise passing the crashed and broken hares. No longer.

“It’s an all-out, full-speed battle, every minute of the 24 hours,” said Doug Fehan, Corvette Racing program manager, and the man in charge of the Corvette team – since that 1999 Daytona debut. Count on some close, brutal racing in the GT Le Mans class.

If you’re looking for more widespread changes in the GT ranks, then you’re looking at the GT Daytona class for 2017. Last year’s rules change that made the class a home for cars built to international GT3 specifications has resulted in interest from multiple manufacturers.

The GT3 class is raced in about 50 series around the world, and that has resulted in substantial interest by manufacturers, who realized they could build and sell GT3 cars profitably with a moderate investment, since One Size Fits All. The rules are essentially the same in multiple series, which gives owners of GT3 cars some security regarding resale value of their cars, knowing that they won’t become obsolete in a season or two.

That manufacturer interest has made a big impact on the 2017 Rolex 24 At Daytona field, with three new manufacturers joining the WeatherTech Championship field for the entire season, debuting at Daytona.

Acura will be racing its new NSX with a two-car team, as will Lexus, heading to Daytona with a pair of new RC F GT3s. There are three Mercedes AMG GT3s, which are not new to GT3 racing, but are new to the WeatherTech Championship. All three cars could be contenders: Michael Shank Racing, which fielded a winning Honda-powered Prototype last season, is now in charge of the Acura initiative. The Lexus team is headed by former Trans-Am champion Paul Gentilozzi, and a solid driver lineup led by Scott Pruett, who at 56 may be the most experienced and definitely is the winningest driver in the field, with five overall and 10 class wins at Daytona.

The Mercedes teams have enlisted Riley Technologies and its legendary president, Bill Riley, to manage the team, which are really three separate entries: Former Dodge Viper racer Ben Keating needed a new car to race since Dodge discontinued the Viper, and he turned to Mercedes, mostly because his co-driver, Jeroen Bleekemolen, had raced the Mercedes in Europe and loved it.

The second Mercedes team is the WeatherTech entry, which moved over from Porsche during the off-season. The team is backed by company founder David MacNeil and will be co-driven for the full season by his son, Cooper MacNeil, and Gunnar Jeannette, with Australian Supercars reigning champion Shane van Gisbergen and AMG factory driver Thomas Jaeger completing the Daytona lineup. And the third Mercedes entry is the SunEnergy1 Racing car led by Kenny Habul, an Australian businessman who is the president of SunEnergy1, a leading producer of solar energy products.

With or without the seven new cars, competition in GT Daytona is impossible to handicap. Last year, the brand-new Lamborghini Huracán GT3 was the fastest car in the class, but teething pains kept the Lambo teams out of victory lane. This year, there are eight Lamborghinis in the field, and with a year of development they could be the favorites, especially the No. 48 Paul Miller Racing Lamborghini, which did find victory lane late in the 2016 season.

And the Porsche, Aston Martin, Audi (the R8 LMS GT3 is very similar beneath the skin to the Lamborghini Huracán), BMW and Ferrari teams will do all they can to defeat the newcomers. There are some very experienced drivers and teams in this class, as well as some relatively inexperienced competitors, which is one of multiple reasons why the Rolex 24 is so fascinating: At 4 a.m., you may have a driver who had never seen Daytona until this year, exhausted after that initial adrenalin rush has worn off, fighting to stay alert as a herd of Prototypes driven by career racers flash by, lapping the track 10 seconds faster.

There are two kinds of winners in the Rolex 24 At Daytona: The ones who actually cross the finish line first in their class, and all the other drivers and teams that manage to hold it all together for all 24 hours, even if they finish 50 laps behind the winners.

There’s a very good chance that the closest racing will be within the two GT classes. And this may be the strongest lineup ever in these classes.