These days, cars are so optimized that it's usually not possible to improve over the factory designs. We said "usually"—we've found 10 relatively simple bolt-on components that give you true performance increases without permanent modifications to your car.
[Related: Six ways to ruin your ride with terrible add-ons]
Wider Wheels and Tires
Advantages: Improved road-holding, turn-in precision, and braking.
Trade-offs: The extra weight can overwhelm shock damping and springs, so loose-surface tracking stability is reduced.
The easiest way to improve handling is to "plus-size" your wheels and tires (a term coined by The Tire Rack). It means proportionally reducing sidewall height, increasing wheel diameter, and increasing width to ensure your wider tires keep the same diameter. Prices start at about $1300 for plus-sizing a Ford Focus.
High-Performance Shocks/Springs/Coil-Over Springs
Advantages: Tires stay in contact with the road on bumps and at higher speeds.
Trade-offs: Ride is stiffer, steering feels rougher.
The B12 Pro-Kit offered by spring-maker Eibach and shock-maker Bilstein combines springs and shocks that will slightly lower your car's ride height and are tuned to work together. A full setup for a new Scion FR-S would run about $1500.
Larger Anti-Roll Bars
Advantages: Car stays level with the road, optimizing tire contact patch.
Trade-offs: Bumps on one side are transferred to the other side, which makes for a rougher ride.
Front and rear anti-roll bars come in kits. One from Eibach for a Mustang GT runs less than $400.
Adjustable Top Strut Mounts, Chassis Braces
Advantages: Greater alignment range allows fine-tuning for better road-holding.
Trade-offs: More noise from the road comes through to the car body.
Along with wider tires and lowered springs, the best way to fine-tune your alignment for cornering is to add adjustable top strut mounts, also called camber plates, in place of your front suspension's fixed rubber mounts. A pair of adjustable mounts would cost about $400 for a BMW 3-series.
Larger Diameter Catalyst-Back Exhaust
Advantages: More efficient high-speed engine operation
Trade-offs: More noise, potentially.
High-tech advances in exhaust design mean that opening up your pipes doesn't have to be noisier—that's what APR's "Reflective Sound Cancellation" system promises. It doesn't promise to be cheap, however. A catalyst-back RSC system for an Audi S4, for example, runs $1169.
Stiffer Motor Mounts
Advantages: Better engine response and handling precision.
Trade-offs: More engine noise and driveline vibration.
Nissan struggled with driveline mounts when it first introduced the 2003 350Z, trying to balance vibration versus unwanted engine movement. But if you don't mind the occasional buzz, stiff urethane mounts from Z1 Motorsports cost $198.
Advantages: Greater body support for better steering and pedal control.
Trade-offs: Bumps that jiggle your insides.
We've found that firm racing-style seats are surprisingly comfortable if they support you in the right places, but they also have to hold you in place as your car subjects you to g-forces, which means the ride could get a bit uncomfortable. A Recaro Cross Sportster CS comes with optional electric heat and a side airbag for $1949.
High-Temperature Brake Pads
Advantages: Less fade during heavy brake use and faster deceleration.
Trade-offs: Poorer brake performance when cold or wet, and squealing noise.
Racing brake pad maker Carbotech offers a high-performance version called the 1521 for street cars. It'll cost you about $250 for a complete set on a Corvette C6. Carbotech also makes a varied array of race and autocross high-temperature pads, but warns of excess noise when they're used on the street.
Less Restrictive Cold-Air Intake
Advantages: More efficient engine operation, especially at higher speeds.
Trade-offs: Intake noise and maintenance needs increase.
While the factory air intakes on most cars are designed to keep the engine clean and free-breathing in all conditions, opening up the path with a foam filter that doesn't have a stock bug-catcher airbox lets cooler ambient air help the engine produce slightly more power. A K&N 69 Series Typhoon system for a VW GTI runs $480, though you can catch it on sale.
Stainless-Steel Braided Brake Lines
Advantages: Quicker brake response, less chance of ruptured line.
Trade-offs: It could be pricey.
By using stainless-steel braided brake lines, more of the pressure you put on the pedal is transferred to the brake caliper than with a rubber brake line. A set of lines from Stoptech for a Lotus Elise costs about $100.