you can check out the spec numbers over at their website,
www.sfifoundation.com/
basicly the difference is for weight and/or speed of vehicle, just different configurations....use what ever spec for the particular class and speed you want to run....
the 2.7 spec is for rear engine dragster, which is what you have, the 10.x is for altereds/funny cars...
here is a list ripped off the site
Spec # Dragsters Price (US$)
2.1 Rear Engine Dragster (Top Alcohol/Methanol) – 6.29 Seconds & Quicker
2.2B Front Engine Dragster – 6.29 Seconds & Quicker
2.3N Rear Engine Dragster (Top Fuel) – 6.29 Seconds & Quicker
2.4B Front Engine Dragster – 6.30 to 7.49 Seconds
2.5B Rear Engine Dragster – 6.30 to 7.49 Seconds
2.6 Front Engine Dragster – 7.50 Seconds & Slower
2.7B Rear Engine Dragster – 7.50 Seconds & Slower
Spec # Funny Cars, Altereds and Roadsters Price
10.1E Funny Car Roll Cage (Alcohol, Advanced ET, etc.)
10.2 Altered Car Roll Cage – 6.00 to 7.49 Seconds
10.3 Altered Car Roll Cage – 7.50 Seconds & Slower
10.4 Side Steer Roadster Roll Cage – 7.50 Seconds & Slower
10.5 Fuel Funny Car Chassis
Spec # Full Bodied Cars Price (US$)
25.1E Full Bodied Car Tube Chassis – 7.49 Seconds & Quicker; 2,800 lbs. Maximum
25.2 Full Bodied Car Tube Chassis – 7.49 Seconds & Quicker; 3,200 lbs. Maximum
25.4 Full Bodied Car Tube Chassis – 7.50 to 8.49 Seconds; 3,600 lbs. Maximum
25.5 Full Bodied Car Stock or Modified/OEM Floorpan and Firewall with OEM Frame or Uni-Body Construction – 7.50 to 8.49 Seconds; 3,600 lbs. Maximum