Custom Street/Import Tuner

Page: 1

Custom Street/Import

Although not technically built to a regulation as part of a racing series, custom street or import cars have become very similar to touring cars or GT cars in their performance.

A custom street/import car generally starts as a 2/4-door Asian, European or North American performance sedan or hatchback. The engines are typically the top performing 4 or 6 cylinder units offered in that model, although swaps are common as well. Many are stock turbocharged.

The cars are then designed and built to higher level of handling and power.

Cars may race in Autocross events where class regulations permit the modifications made to the car. Racing is generally on parking lots, kart tracks, and dedicated race circuits and is widespread throughout the Americas, UK and Australia.

Power and Weight Stats
Horsepower (Typical Range) 125-700
Race Weight (Typical Range) 1135-1816 kg
2500-4000 lb

Design and Construction

Race Car Models of This Type

Production vehicles which are modified with racing/performance parts and are still considered "Streetable". Honda Civic, Honda Prelude, Toyota MR2, Toyota Supra, Toyota Celica, Scion FR-S, Nissan Altima, Nissan 350Z, Nissan Skyline, BMW 3 series, and Mitsubishi Evo are but several of the models used as a base.

Build Your Own Custom Street/Import Car

Knowledge Level

If you are building your own custom street/import car, it is useful to have a working to advanced knowledge of all aspects of the vehicle (Handling/operation, suspension, chassis, powertrain, aerodynamics, and safety).

A working knowledge level will enable you to build using examples of others as your guide, and it provides a reasonable learning curve to begin with. An advanced knowledge level will enable you to be a trendsetter with your vehicle designs because not only can they be aesthetically pleasing, but they can have solid racing performance underpinning the good looks.

The greater the knowledge of these systems, the better the car you can make. For instance, knowledge of composite design and fabrication will enable you to create custom bodywork that is unique to your car. Many street custom body kits that are sold today evolved from the personal projects of builders.

Design Challenges

Suspension: Vehicle suspension modifications that can result in significant performance gains:

  1. Increasing tire contact patch and grip through wider wheels and wider street-legal racing tires.
  2. Lowering the ride height (with corresponding changes to the suspension geometry to ensure maximum tire contact patch on the road). Reducing the height of the center of gravity produces less weight transfer and increases cornering speed, braking and acceleration.
  3. Changing springs, anti-roll bars and damping to manage weight transfer and help the tires follow the surface of the road better.
  4. Increasing stopping power through bigger brakes

Chassis: Safety modifications such as a roll structure are recommended and can provide additional chassis torsional rigidity which makes handling more consistent. Removing unnecessary weight from the vehicle will give better acceleration and better braking.

Lightening of the body through replacement of body panels with composite fiberglass or carbon fiber can significantly increase performance. A double benefit is that if you create custom panels, the panels are one-piece instead of stock panels with attached custom pieces.

Powertrain: The goals of changes to the powertrain should be to increase horsepower and engine efficiency. This can be accomplished through replacing stock components for increased internal flow (Intake/exhaust), improved combustion, reducing friction and removing/replacing power robbing accessories.

Turbocharging and supercharging will enable a large power increases. In terms of final drive, any modifications that increase power delivery (ie. Limited slip differentials) to the road could be considered.

Aerodynamic: Spoilers, airdams, wings and diffusers can provide serious increases in grip if properly designed. Using underbody tunnels in conjunction with rear diffusers can provide a great deal of downforce with a limited penalty in drag.

Safety: Adding a roll cage provides a measure of safety that a stock vehicle does not provide. It will also add a measure of confidence for the driver if any high performance driving is expected. A racing seat with a multi-point harness should also be considered to enable the driver to feel the car more accurately, maintain seating position in cornering and secure the driver fully in the case of a rollover.

If a roll structure is not available off-the-shelf, however, it will need to be designed and fabricated, which will require some steel design and welding work (This can be done by speed or metal fabrication shops)

If you intend to race under a sanctioning body, always read and understand the regulations of your chosen racing class before designing or building any race vehicle.

Design Resources

Learn the basics about race cars and race car design from our free online knowledge series

Download our free race car design aids to assist you designing your race vehicle.

In-depth books and learning resources we recommend for custom street/import car design.

Join our forum to ask and find answers to your custom street/import car design/construction questions.

Construction Challenges

Many components used to customize/tune an import vehicle are available as bolt-on type components and therefore are within the mechanical skills of most people. Challenges include the number of components which are changeable, and the difficulty in changing them (i.e. Things like engine removal), which may require specialized workshop tools.

Composite fabrication is a skill that can be learned from a number of good books.

For roll cages that must be custom fabricated, there is the additional requirement of fitting it into a production vehicle and of locating mounting points that do not compromise the uni-body chassis.

Build Costs

It is quite easy to spend a significant sum of money customizing your car. However, with adequate knowledge, it is possible to select areas of modification that provide maximum value and keep costs to a minimum. Doing modifications and fabrication yourself can also reduce costs and allow you the freedom to design your own bodywork, interior, and aerodynamics components.

Build Effort

Build effort depends on the number of areas that are modified. If your modifications are performed a little at a time, the effort will also be manageable. Custom fabrication does take more time, but the results are genuinely your own.

Racing Cost

If you do choose to go racing with your car, the greatest regular expense will likely come from the purchase of street-legal racing tires.

Transportation and Support Equipment

The vehicle can be driven to the track and support equipment can be carried in the vehicle.

Page: 1