1. they decrease the force of friction
   they counteract the force of gravity
2. they're lighter

Since well-inflated tires have less contact with the road, they encounter less friction and the engine doesn't have to work as hard to move the car.

1. It heats the air.
   It adds special chemicals to the air.
2. It improves air flow.

An upgraded air filter improves airflow into the engine, allowing better combustion.

1. headers
   footers
2. mufflers

Headers improve engine performance by making the exhaust stroke less power intensive.

1. better fuel economy
   louder exhaust sounds
2. more impressive revving noises

Not all performance improvements boost horsepower or increase speed. Fuel economy is one such sign of a better-performing engine.

1. adds more memory to the car’s computer
   helps the engine run without as much gas
2. alters the engine programming in the car’s computer

Most cars have computer chips that control engine function, and thus, engine power. Performance chips change the factory settings to improve power.

1. It improves the air-fuel mixture.
   It helps the engine spin faster.
2. It takes pressure off the radiator.

Cold air is denser than warm air, so it can be mixed with more fuel.

1. subframe
   transmission
2. wheels

Switching to lightweight alloy wheels can help improve performance as well as style.

1. the tires
2. the catalytic converter

The catalytic converter can only be replaced when necessary.

1. exhaust manifolds
   back pressure
2. exhaust headers

Engines tend to waste power during the exhaust stroke when they come up against the resistance of back pressure.

1. bad fuel
   lack of compression
2. worn-out ball bearings

Although thousands of minor things can create engine problems, the big three include a bad fuel mix, a lack of compression and a lack of spark.

1. The pipes are curved.
   The pipes are straighter.
2. The pipes are insulated with cat fur.

A cat-back exhaust has straighter pipes, which improve exhaust flow.

1. bearings
   air filters
2. nitrous injectors

Air filters can easily be removed and washed to keep the engine’s air clean.

1. nitrogen
   oxygen
2. hydrogen

When nitrous oxide is heated, it splits into nitrogen and oxygen, making more oxygen available to the engine during combustion.

1. hitting the gas so hard that the driveshaft cracks
   hitting the gas so hard that your head snaps back
2. wasted torque from natural engine movement

Drive line lash is the torque that’s wasted as the engine shifts around.

1. a buildup of crud on the engine
   a radio that was removed to save weight
2. the perfect mix of air and fuel

One source of poor performance is engine deposit buildup caused by old or dirty oil.

1. Glass windows are lighter than acrylic windows.
   Disc brakes are lighter than drum brakes.
2. Original body panels are lighter than carbon fiber body panels.

Disc brakes are a popular upgrade, in part because they are lighter than drum brakes.

1. more nitrous oxide and a better turbocharger
   less air and more fuel
2. more air and more fuel

To become more powerful, the engine needs to be modified to accept more air and more fuel.

1. Install firmer motor mounts.
   Install softer motor mounts.
2. Install racing harnesses.

Most factory motor mounts are soft, which increases ride comfort but also wastes some of the engine’s energy. Performance motor mounts, which are firmer, help reduce wasted torque.

1. installing a turbocharger
   installing a hood scoop
2. installing a performance chip

Installing a performance chip is a simple plug-and-play project.

1. take out the headliner and interior door panels to save weight
   make a spoiler out of junkyard scraps
2. trade your car for someone else’s

Weight reduction can be as simple and inexpensive as removing unnecessary parts, such as upholstery and trim, but it’s only recommended for drivers who don’t really care about comfort.

1. just two
   at least four
2. six or more

Dual overhead camshafts (DOHC) produce more power and can run at higher speeds since they allow an engine to have at least four valves per cylinder.

1. 10 times faster
   20 times faster
2. 30 times faster

Using the exhaust flow from the engine, turbines in a turbocharger can spin the air about 30 times faster than a regular car engine can -- at speeds of up to 150,000 rpm.

1. 25 percent more
   50 percent more
2. 200 more horsepower

A supercharger improves a car’s power output by about 50 percent.

1. installing a supercharger
   machining the valves
2. performing a drivetrain swap

Installing a supercharger is a fairly straightforward bolt-on procedure requiring basic tools.

1. a cleaner-burning fuel
   a cooling effect
2. a quieter performance

When it vaporizes, nitrous oxide provides a cooling effect on the intake air, which increases the air's density and provides even more oxygen inside the cylinder.

1. false, always
   true, always
2. true, in older cars

Most car manufacturers actually advise against flushing modern engines, but it may help engines in older cars.

1. it's difficult to find
   it's expensive
2. it's fairly bulky

Due to the bulk of nitrous oxide, it takes up a significant amount of space even when compressed into a liquid. Drivers tend to carry only a small amount and use it selectively by pushing a button.

1. 10 percent
   25 to 50 horsepower
2. 5 to 7 horsepower

A cold air intake adds about 5 to 7 horsepower.

1. one
   two
2. four

Exhaust headers eliminate back pressure by giving each cylinder its own exhaust pipe.

1. 25,000 rpm
   50,000 rpm
2. 150,000 rpm

A supercharger spins at at least 50,000 rpm.