Autocross Rules & Classing

This page is the definitive source for the rules and vehicle classing applicable to the MCO Autocross Championship.


These rules should be read in conjunction with the applicable CASC General Competition Rules (GCRs), as identified in the Autocross Championship Supplemental Regulations. All competitors are expected to familiarize themselves with the applicable rules.

Should amendments be made after initial publication of any of the documents below, the documents linked here will be updated, replacing the out-dated versions.

MCO Members are invited to propose revisions to the documents for the consideration of organizers and to facilitate continuity in the organizer community.  Proposals can be submitted to Autocross Directors.

The versions here will be treated as the authoritative version should there be any discrepancy with any other version.

MCO Supplemental Regulations



Everyone needs a helmet and it needs to be appropriately certified. If you don’t have one, rentals are available at local events. Details on appropriate helmets can be found in Section 4.5 of the CASC Autoslalom Regulations. Snell 2000 helmets are no longer accepted.

Helmet rental cost is $20.00 (HST Included) – please bring cash (exact change appreciated) to the event.  Your helmet entitles you to the helmet for the whole day of the event.

Participation by Minors

Any prospective participant under the age of 18 must contact in advance of registering. Ideally, this contact should precede registration cut-offs by two weeks.

Vehicle Classing

With the move to the SCCA classing system, we will now be following the same rules as most autocross clubs in North America. If this is all new to you, it can be daunting looking at the SCCA Solo rule book of over 300 pages, but fear not as we have put together some tips to make it easier for you to class your car.

Step 1 – Find your category

The first step will involve identifying your car’s category. The SCCA classing system is designed to provide you with a list of modifications allowed for each category. Your car’s modifications must all be allowed in the category you pick, otherwise you would have to look into the next category to see if your modifications are allowed there and so on. You will find the list of categories in order of modification levels below.


In autocross, vehicles are separated according to their modifications into the following classes:

  • Street (S)
  • Street Touring (ST)
  • Street Prepared (SP)
  • Street Modified (SM)
  • Open (OP) – grouped category where we mix in all the following categories:
    • Prepared (P)
    • Modified (M)
    • Xtreme Street (X)
    • Classic American Muscle (CAM)
    • Heritage Classic (HC)
    • Electric Vehicle Experimental (EVX)
  • Novice (N) – grouped category where we mix in all the newbie drivers regardless of their car category. A newbie is a driver who has previously participated in no more than 3 autocross events in any club.

* Note that the category grouping is subject to change every season based on the number of participants in each category to ensure that we have a more even distribution of drivers across the categories. Changes to the category grouping may occur up to the third event of the season.

Xtreme Street and Classic American Muscle classes  have a separate rule book here. Note that many MCO members’ car probably fall into the Xtreme Street category.

Heritage Classic have a separate rule book here.

Electric Vehicle Experimental have a separate rule book here.

In summary, the allowed modifications for each class are as follows. Note that the official rulebook takes precedence over the following summary. You should only use this summary to help orient yourself to know which class category your car will likely fall in. Use the latest SCCA Solo rule book to confirm your class category. You can also try the SCCA Solo Classification Assistant when you click on the highlighted class for your car to go through a questionnaire referring to the SCCA Solo rule book.


  • Street tires with a minimum treadwear of 200
  • Wheels of approximately +/- 1″ diameter, but with OEM wheel width and offset of approximately +/- 7mm
  • Brake pads
  • Shocks (dampers)
  • One sway bar only with its brackets, front OR rear
  • Air filter drop-in without altering the OEM airbox
  • Exhaust after the catalytic converter without touching it

Street Touring

  • Pedal cover
  • Replaced front seats, minimum of 25 lbs each with installation kit
  • Strut and sway bar
  • Tires with a minimum treadwear of 200 (street).
  • Maximum tire width:
    • STR (AWD), STS – 225mm
    • STX (AWD), STH (AWD) – 245mm
    • STR (2WD) – 255mm
    • STX (2WD), STH (2WD) – 265mm
    • STU (AWD, RWD mid-engine, & RWD forced induction) – 295mm
    • STU (RWD N/A & FWD) – 315mm
    • SST – Unlimited
  • Wheels, maximum width:
    • STR (AWD), STS – 7.5″
    • STX (AWD) – 8.0″
    • STR (2WD), STX, STH – 9.0″
    • STU – 11.0″
    • SST – Unlimited
  • No fender work except fender roll (no fender pull, fender cutting etc.)
  • Rotor (brakes), braided hoses
  • Suspension (Coil, shock, coilover)
  • Alignment outside of OEM spec (refer to rule book for which alignment parts are allowed for your type of suspension)
  • Air intake
  • Exhaust (headers, high-flow catalyzer at less than 6″ from the OEM catalyzer’s original position)
  • Engine management system tuning within these limitations:
    • Any OE OBD2 or newer communications port functionally must remain
    • 2005 and older model year vehicles, supplementary (“piggyback”) ECU is permitted provided that it is plug compatible and connects only between the standard PCM/ECU and its wiring harness
    • 1995 and older vehicles may implement a replacement “stand-alone” PCM/ECU
  • Fuel pressure regulator.
  • For cars originally equipped with forced induction cars in SST, STU and STH without removing body panels or structural members:
    • Intercoolers or charge air coolers only cooled by the atmosphere unless its standard parts
    • Charge pipes
    • Compressor Bypass Valves, blow-off valves, and pop-off valves
    • Boost regulation systems electric or mechanical (no changes to turbocharger or wastegate and wastegate spring)
  • LSD differential with exceptions:
    • Only the viscous OEM is allowed in STS
    • Only 1 LSD change is allowed for AWD/4WD
  • Battery relocated but not in the passenger compartment (trunk is allowed).

Street Prepared

  • All modifications permitted in Street and Street Touring class.
  • First class that allows R-Compound except SSR class
  • Replacement of parts with the same category model (e.g., Golf and Jetta).
  • ECU
  • Removal of air conditioning
  • Complete suspension
  • Fender work, tires may exceed
  • Front seats
  • Steering wheel even if original airbag
  • Spoiler and splitter
  • Tires and wheels
  • Complete brakes
  • Battery relocated but not in the cabin space (trunk OK)
  • Complete exhaust, removal of anti-pollution system including the catalyzer
  • Throttle body, intake manifold, and injection
  • LSD differential

Street Modified

  • All modifications from previous classes
  • Engine modifications
  • Engine swap
  • Body parts such as hood and trunk can be replaced
  • Rear seat removed.


  • Similar to SP for parts and inter-model engines
  • First class where chassis reinforcements are allowed
  • Lightened bumpers and interior
  • Lexan windows permitted
  • Aerodynamic restriction such as spoiler and splitter for several classes. (XP is more permissive)
  • Engine restrictions from Street Modified for BP to FP classes based on the OEM engine and restrictions for turbo and superchargers, and XP class for engine swaps

Step 2 – Find your class

After you have identified the right category for your car, its time to find the class for your car to sign up with. Within each categories, there are classes and your car will be in one of them. To make it easier, use this SCCA Solo Classification Assistant which will highlight all the classes your car would go in, and you pick the category and class that applies to your car. When you click on the class, it will also prompt you with a questionnaire referring to the SCCA Solo rule book to help you confirm if you have the right class. Here is the list of classes below:

Street (S)

  • S Street (SS)
  • A Street (AS)
  • B Street (BS)
  • C Street (CS)
  • D Street (DS)
  • E Street (ES)
  • F Street (FS)
  • G Street (GS)
  • H Street (HS)

Street Touring (ST)

  • Street Touring Sport (STS)
  • Street Touring Hatchback (STH)
  • Street Touring Xtreme (STX)
  • Street Touring Ultra (STU)
  • Super Street Touring (SST)

Street Prepared (SP)

  • Super Street Prepared S (SSP)
  • C Street Prepared (CSP)
  • D Street Prepared (DSP)
  • E Street Prepared (ESP)
  • F Street Prepared (FSP)

Street Modified (SM)

  • Super Street Modified (SSM)
  • Street Modified (SM)
  • Street Modified FWD (SMF)

Open (OP) – grouped category where we mix in all the following categories:

Prepared (P)

  • X Prepared (XP)
  • C Prepared (CP)
  • D Prepared (DP)
  • E Prepared (EP)
  • F Prepared (FP)

Modified (M)

  • A Modified (AM)
  • B Modified (BM)
  • C Modified (CM)
  • D Modified (DM)
  • E Modified (EM)
  • F Modified (FM)

Xtreme Street (X)

  • Xtreme A (XA)
  • Xtreme B (XB)

Classic American Muscle (CAM)

  • Classic American Muscle Traditional (CAM-T)
  • Classic American Muscle Contemporary (CAM-C)
  • Classic American Muscle Sports (CAM-S)

Heritage Classic (HC)

  • Heritage Classic Street (HCS)
  • Heritage Classic Race (HCR)

Electric Vehicle

  • Electric Vehicle Experimental (EVX)

Finally, you have your class and you are ready to register and race! If you happen to be eligible for multiple categories, it is generally advisable to select the “slowest” class based on the lowest PAX Index number here so that you get the most competitive edge in your category. For example, if your 1990 Mazda MX-5 Miata can run in STS (PAX of 0.814) or in XB (PAX of 0.848), you would generally be better off running in STS so that your best time 60.000 would be 60.000 * 0.814 = 48.840 instead of 60.000 * 0.848 = 50.880 if you ran in XB.

Street Tire Index

If you are in a category that is based on R-comp tires under 200 treadwear rating (e.g. Street Prepared, Street Modified, Prepared, and Modified), but you have 200 treadwear tires on your car or above, you can benefit from a Street Tire Index which 0.98 multiplied by your class’s index. For example, if your car is in the SM class with an index of 0.868, if you run tires that are equal or over 200 treadwear rating, you can use the Street Tire Index and run in SM-S with an index of 0.868 * 0.98 = 0.851 to make your life easier.

Why this matters?

Picking a class with a lower value PAX will make your times more competitive in your respective category and also in the Overall PAX results.

Novice Category

Novice category is where we recommend you start in if you have competed in no more than 3 autocross events in any club. The class labelling for Novice category will have an “N” prefix so if your class is normally XB, you will be in NXB.

Championship Format

Category Results and Overall PAX Results are the two championships available for all valid MCO members. The event podiums will be awarded to the top 3 drivers of each category as well as the top 3 drivers in the Overall PAX. These two championships counts towards the season’s championship points separately, so you could win in one, but not in the other. At the end of the season, the 3 highest scoring drivers in each category will receive their category championship prize and the 3 highest scoring drivers in the Overall PAX of events will receive their Overall PAX championship prize.

Category Results

The category results are based each driver’s best time of the day multiplied by their class’s PAX index. Many classes are competing against each other in a single category, and each driver’s class PAX index is the equalizer factor to help equalize the car differences and make it a driver’s race. Faster cars tend to be in a class where they have a higher value PAX index which gives them less bonus time, whereas slower cars tend to be in classes with a lower PAX index giving them more time bonus.

For example, if we use Street category, Gary in his Ford Fiesta ST in HS class takes first place in Street with his PAX time of 47.160, which was obtained by multiplying his best raw time of 60.000 by his HS PAX index of 0.786. Eric in his Corvette Z06 is in second behind Gary despite having set a faster raw time of 58.253 because his class uses a higher value PAX index relative to Gary’s to help equalize the two very different car’s performance. In fact, Eric would have had to get a raw time of 57.301 to beat Gary’s PAX time of 47.160

Position Car # Class Driver Car PAX Index Raw Time PAX Time
1 99 HS Gary 2016 Ford Fiesta ST 0.786 60.000 47.160
2 162 AS Eric 2004 Chevrolet Corvette 0.823 58.253 47.942
3 31 DS Ronald 2019 Civic Type R 0.808 60.000 48.480

Overall PAX Results

The Overall PAX Results works the same way as the Category Results, with the exception that instead of only competing against drivers from your category, this one puts all the drivers from the event together to determine the overall top drivers of the day through the PAX time.

Fastest Raw Time Results

This is a third set of results more for bragging rights really which will highlight the top 3 drivers with the fastest raw time of the event in all classes and who got the infamous FTD (Fastest Time of the Day).