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This weekend, one door closes and another one opens.

The first 17-race segment of the new Countdown to the Championship sweepstakes comes to a dramatic conclusion at the Toyo Tires Nationals at Maple Grove Raceway outside Reading, Pa. In all four NHRA POWERade classes, there are open slots in the eight-racer tournament brackets that will be filled by the end of the day Sunday. That will give this national event unprecedented significance -- a race that up until this year was scheduled following the star-studded Mac Tools U.S. Nationals in Indianapolis on Labor Day weekend but now precedes it.

Countdown in a nutshell

The NHRA's new playoff system is an elimination process. That's the biggest difference between the Countdown to the Championship and NASCAR's Chase for the Nextel Cup. It starts with the NHRA's biggest event -- the U.S. Nationals on Labor Day weekend at Indianapolis. In the 18th of 23 annual events, the top eight racers in each pro category will begin competing for the championship. It means the 17th event, the Toyo Tires NHRA Nationals at Reading, Pa., on Aug. 16-19, is the last chance to make it to the playoffs. Starting at Indy, the standings are reset, with points awarded in 10-point intervals from the first- to last-place finishers for the eight contenders in each class: Top Fuel, Funny Car, Pro Stock and Pro Stock Motorcycle. Indy begins the Countdown to Four over the next four events. The drivers who rank in the top four in each class after the Richmond race on Oct. 7 will compete for the title in the last two events: Las Vegas and Pomona, Calif. Points are reset again at Las Vegas with a 10-point margin between the four competitors in each category. "The one thing most of the drivers wanted was to keep the top eight in it until the end,'' said six-time NHRA champion Kenny Bernstein. "No one was complaining about going to the top-eight playoff. But a lot of teams and crew chiefs don't like cutting it down to four in the last two races. "It's a brutal way to end it, but this always has been a brutal sport. You line up at 23 events and can lose in the first round each time." -- Terry Blount

The emphasis at Maple Grove this time around will be directed at which drivers in Top Fuel, Funny Car, Pro Stock and Pro Stock Motorcycle will head to Indy as part of the Countdown to Four (the next step in how this year's champions will be determined) rather than who will occupy the winner's circle on Sunday. This is still one of the NHRA's most prized and prestigious races; earning event honors at Maple Grove remains something every drag racer takes tremendous pride in. But the 2007 edition of the Toyo Tires Nationals has more riding on it than ever before.

Here's a preview:

Top Fuel
There's very little elbow room at the Countdown cutoff in the sport's quickest and fastest category.

The math here is relatively easy. Only two spots are available in the top eight, but seven drivers are within striking distance of squeezing their way in, and anyone with even the slightest understanding of mathematics knows that seven into two won't go.

The No. 7 and No. 8 racers heading into this weekend's action are Whit Bazemore and David Grubnic, respectively. Last week in Brainerd, Minn., these two veterans met in Round 1 of the Lucas Oil Nationals, with Bazemore advancing, only to lose in Round 2 to Morgan Lucas. Both know that an early-round loss in Reading -- with Melanie Troxel, Cory McClenathan, Doug Herbert, Hillary Will and Doug Kalitta parked on the doorstep within 63 points of them -- could end their title hopes.

With points leader Rod Fuller comfortably out in front of Brainerd winner Brandon Bernstein, who's in second, don't look for a change at the very top of the standings this weekend. Instead, look for some changes somewhere around the No. 8 neighborhood.

Funny Car
Jim Head is a marked man.

The only spot in the Funny Car top eight that hasn't yet been clinched is the one in which Head finds himself -- No. 8. Within range of Head are no fewer than five competitors as far away as 122 points (Cruz Pedregon at No. 13) and as close as 83 points (Tommy Johnson Jr.).

Head and his five pursuers have had sketchy 2007 campaigns. Last week in Brainerd, Bernstein nearly picked up his first Funny Car event title since 1989 when he barely lost to 14-time POWERade champion John Force in the final round, while none of the other racers chasing Head -- Johnson Jr., Del Worsham and Jeff Arend -- advanced past the second round. (Pedregon failed to qualify.)

Twenty points are awarded for each round's win in the NHRA's professional classes, and for Head to hold his ground, he'll have to avoid a disastrous DNQ and keep the pressure on the drivers hoping to hunt him down by staying alive in the early rounds of eliminations. Meanwhile, the five Funny Car hopefuls needing some last-minute lightning must take advantage of any opening Head may provide them.

Pro Stock
Six drivers. Three openings. One more shot. And a jumbled weather picture.

As any experienced drag racing fan can attest to, the non-supercharged, 500-cubic-inch Pro Stock engine is a cantankerous creation when weather conditions fluctuate. This weekend, weather forecasters are predicting warm and humid conditions on Friday (as professional qualifying begins) and cooler, drier weather for Saturday, with a chance of showers on Sunday -- all at an event that has been plagued by rain many times in the recent past.

With three Countdown berths hanging in the balance, the tuning calls made this weekend by the crew chiefs directing the battle plans for Warren Johnson and Kurt Johnson, Larry Morgan, Richie Stevens Jr., V. Gaines and Jim Yates had better be on the money.

Reading has traditionally been a track where Pro Stock race cars love to romp with plenty of oxygenated air and a grippy racing surface. It appears as if those ideal conditions may present themselves on Saturday (and perhaps Sunday if the rain holds off), but late-summer weather in the Northeast can be an ever-changing animal.

The teams needing to bully their way into the Countdown to Four that can unravel the atmospheric mysteries they'll encounter in Maple Grove over the weekend stand the best chance of making the cut.

Pro Stock Motorcycle
There's a bit more democracy in the bikes.

Only points leader Matt Smith and two three-time POWERade champions, Andrew Hines and Angelle Sampey, have locked up a Countdown to Four invitation. That leaves five vacancies and eight riders fighting over them. Wild is a word that doesn't do justice to the way this skirmish will be settled. And the point spreads between those competing racers is tight.

Only 75 points separate Chip Ellis and Karen Stoffer, who are tied for fourth, from No. 8 Ed Krawiec. Outside the top eight, a scant 43 points lie between No. 9 Steve Johnson and No. 11 Antron Brown -- both first-round losers last week in Brainerd. Since it's a real possibility that the final Countdown to Four lineup could see several positions decided by less than a single round's worth of points, every one of these eight Pro Stock Motorcycle contenders had better be dialed in this weekend.

Bill Stephens covers NHRA for ESPN.com