Sports Car Club of America - Reno Region
Time Trials Production Car Classification Rules

Classify a Car Classification Rules Example Classifications Classification Grid Revision History

Time Trials Production Car Classification Rules


These rules are intended to be used for classification of production-based automobiles prepared for road-course time trials competition. The rules recognize that in addition to driver skill, car modification and preparation play an integral part in an ultimate competitive lap time. These rules are not intended to factor out or limit the car or mechanic in order to compare only driver skill, but instead attempt to group cars with similar performance potential into classes for competition purposes.

Performance potential is grouped based on discrete features of the vehicle being classed, including weight and engine configuration. The performance potential index generated by these rules is not intended to quantify the performance of a particular car--two cars with equal indexes will not likely have identical performance--but is rather intended simply to group cars of similar power-to-weight potential in order to generate a competitive environment for a wide variety of vehicles.


The purpose of the Time Trials Production rules are to provide a formulaic method for classifying cars for road-course time trials competition. The formula is intended to use discrete features of the car, along with the actual weight of the car, to group cars together that have similar performance potential. The use of a generalized formula for classification over the usual list of cars and allowed modifications is intended to drastically reduce the effort needed to maintain the rules, as well as simplify the process to classify a car for competition.

It is understood that these classifications are based on limited data and experience, and will therefore be inappropriate in some cases. In such cases, the event stewards are given the authority to make changes with regards to an individual's car classification in the interest of balanced competition. Any changes to an individual's car classification should be documented by the event officials with the reasons for the change and sent to the rule authors for review and possible inclusion in future revisions.

Eligible Vehicles:

Commonly available production automobiles, intended for street use, modified and prepared for time trials competition (must maintain original frame or unibody), and on DOT approved tires (street or R-compound) are eligible for classification under these rules.

For the purpose of eligibility determination, and to ensure the goal that competition remain between production automobiles, the following modifications violate the "original frame or unibody" requirement and disqualify a vehicle for Time Trials Production competition unless explicitly excused by the event stewards:

At the stewards' discretion, non-production, limited-production, cars with modified frames/unibodies, and purpose built race cars may be classified under the Time Trials Production rules, however it is suggested that these cars should be classified in TT-X to account for their likely advantages over true production based cars.


Cars are classified via Performance Potential Index (PPI). The PPI is a weight/performance potential ratio that is used as an index into a table of classes. Weight is adjusted to compensate for the performance advantage of light cars over heavy cars, as well as for common modifications. Displacement is adjusted to compensate for features of the engine that improve the power potential of the engine.

The PPI ratio is calculated with the following formula:

        [race weight] + [weight adjustment] + [weight penalty]
[PPI] = —————————————————————————————————————————————————————————
       [actual displacement] * [displacement factor]

Race Weight:

The race weight is the actual weight of the vehicle in race trim, including the driver, rounded up to the nearest pound. Weight should include only the amount of fuel left in the vehicle upon completion of timed laps, however it is at the event stewards' discretion to weigh vehicles prior to competition. In this case, vehicle weight may include fuel as ballast. In most cases, the actual weight of the car is unimportant as long as the vehicle is known to be at or above the minimum weight of the car's class.

Cars will have their race weight signed-off in their log book by a tech inspector, in order to prevent having to reweigh the vehicle at every event. It is the driver's responsibility to maintain the weight of the vehicle at or above the signed-off weight, or have the vehicle reweighed and signed-off if the weight is changed. It may be advantageous to record the race weight lower than the actual weight as long as it is at or above the class minimum in order to prevent reweighing the vehicle.

If scales aren't available at an event for classification use, the factory specified curb weight of the vehicle plus 100 lbs may be used for classification, or an alternative weighing of the vehicle (such as the results of a certified public scale) may be accepted at the event stewards' discretion. For unmodified cars that should be close to their factory weight, event stewards may elect to use the curb weight plus 100 lbs even if scales are available at an event in order to streamline the classification process, though it is recommended that cars close to their class weight boundary be weighed.

Weight Adjustment:

The purpose of the weight adjustment is to account for the fact that lighter cars will corner and brake better than heavier cars even when both cars have identical power to weight ratios.

The weight adjustment is the difference between the race weight and the baseline weight of multiplied by the weight factor of , and rounded to the next largest pound. The weight adjustment will be negative for cars lighter than the baseline weight (a penalty) and positive for cars heavier than the baseline weight (a handicap).

[weight adjustment] = ([race weight] - [baseline weight]) * [weight factor]

Weight Penalty:

Common modifications that improve performance but are not reflected in the power/weight ratio of the classification formula have been assigned a penalty weight. A vehicle's weight penalty is the sum of all penalties from the table below that apply to the vehicle.

Actual Displacement:

The actual displacement of the vehicle is the displacement of the vehicle's engine as raced, rounded up to the nearest 0.1 liter. Vehicles with modified or swapped engines must use the displacement of the engine in the car, rather than the displacement of the original engine.

Displacement Factor:

The displacement factor scales the actual displacement of the vehicle based on the feature's of the vehicle's engine. The values assigned to modifiers are not intended to specifically quantify the performance gain from that feature, rather they are based on the idea that cars with certain features tend to have similar performance potential, and each modifier is weighted to draw similar cars towards an appropriate class, rather than to explicitly quantify the performance of a car.

The displacement factor is the sum of all the modifiers from the table below that apply to the vehicle.

Classification Lookup:

Use the calculated PPI as an index into the classification table below to determine the vehicle's classification. The lower the PPI, the more potential performance the vehicle has, the higher the class it competes in. Each class is available for both Track Trials (TT) and Club Trials (CT) levels based on the level of preparation per the SCCA Time Trials program rules and the Reno SCCA Time Trials Supplemental Regulations.

Classify a Car

Use the following form to classify a car for competition.

Classification Examples

The following are some classification examples to demonstrate the algorithm as well as to give a sense of the type of cars expected in each class. Cars noted as (stock) or (modified) are based on curb weight + 100 lbs. Other examples are based on actual submitted car information from past seasons.

Classification Grid

The following table shows classes for weights from 1000 to 4000 lbs, and displacements from 0.5 to 10.0 L in order to visualize the way PRI is affected by penalties and modifiers. Hover the mouse over the table for specifics about each cell. Use the following check boxes to apply penalties and modifiers.