It's deliberately not difficult -- per FAST - Autocross :
We have eight classes, loosely categorized by displacement, modifications, and tires. Clarifications are listed below the classes.

• A - Modified race tire (all)
• B - Production race tire 3 liters and greater
• C - Production race tire less than 3 liters
• D - Modified street tire 3 liters and greater
• E - Modified street tire less than 3 liters
• F - Production street tire 3 liters and greater
• G - Production street tire less than 3 liters
• R - Open or closed wheel race cars (all)

Modified cars are production-based cars with any of the following modifications:

• Any internal engine modification (crank, cams, rods, pistons)
• Any external engine modification that alters fuel delivery (alternate carb, altered fuel injection, fuel-air ratio adjusters)
• Any non-factory engine replacement
• Welded chassis reinforcement, including welded roll bars/cages
• Alternate or altered springs
• Altered suspension geometry (relocated pickup points, alternate suspension parts to increase factory alignment adjustment range such as camber kits, camber plates, or slotted strut mounts)
• Alternate main body panels (doors, hood, trunk lid, fenders, roof panel, windows)
• Removal of factory interior parts (seats, interior panels)

Other classing notes:

• Any and all modifications not listed are allowed in the production classes.
• Street tires are DOT approved tires with treadwear rating of 140 or above.
• Forced induction (turbo or supercharger) bumps engine displacement by a factor of 1.5. (e.g. a 2.2 liter turbocharged engine would be classed as 3.1 liter)
• Any non-production based vehicle (such as kit cars or tube-framed "look-alikes") will be classed as a "Race Car" unless deemed an appropriate fit for a lesser class.

If you need to change your class, just go back to the entry list on MAE and edit your entry.

 

These might be a little clearer for new people if listed like this:

• A - Modified / race tire (all)
• B - Stock / race tire / 3 liters and greater
• C - Stock / race tire / less than 3 liters
• D - Modified / street tire / 3 liters and greater
• E - Modified / street tire / less than 3 liters
• F - Stock / street tire / 3 liters and greater
• G - Stock / street tire / less than 3 liters
• R - Open or closed wheel race cars (all)

 

there is a small error in the classing, a 1998cc evo or wrx could be considered under 3litre... many rally rules use a 1.7 for petrol and 1.5 for diesel forced induction, in addition a .8 multiplier for Pushrod engines. (so the big v6 GM/ford cars can be under 3 litre) also a 1.5 is used for a rotary so a turbo rotary motor would be. 1.3x1.5x1.7 = 3.3 litre. much more reasonable set of groupings.

 

yeah, my car is the same way....
1998 cc x 1.5 = 2997cc

However, I believe the the rules say 3.0L and above. Using significant digits your displacement would actually have to be 2949cc/1.5= 1966cc or less to be considered under 3.0L.

If the rules said 3.000L and above, you would be correct.

 

This isn't NASCAR, and you're not Smokey Yunick...if it makes you feel better, pretend it's 1.549 rounded to 1.5 for print.

If someone with blown Rabbit Diesel starts showing up regularly and complains about being spanked by turbo Miatas, we'll revisit.

Someone take away Jeff's calculator....

 

So wait, my car has a turbo 1998cc engine. Am I in E Mod or D mod?

 

D
1998cc x 1.5 = 2997cc = 3.0L

And it's not D mod, it's just D.
D Mod is an SCCA class.

 

Finally the classes are revealed

 

i always thought it was pretty easy to figure out what class your car is in.

 

I'm sorry but it's too late to add anyone to the entry list. The airport requires an attendance list well in advance. Your buddy is more than welcome to come out to the next event.