Upgrades

You didn't think that you were going to be able to run a stock car down here and win, did you? No, my friend, you're going to have to reinvest your winnings into upgrades.

Upgrades come in two forms: Visual upgrades, which boost your Reputation, which, in turn, leads to more style points for all those slick maneuvers you're going to be pulling off, and Performance mods, which give your car better acceleration, speed, handling, or what have you. Performance mods are, obviously, the more important of the two, but don't neglect to buy a few Visual upgrades as you proceed; some races will have a Reputation requirement for entering, which can only be boosted by outfitting your car with stylish new parts.

Performance

The cars in Need For Speed: Underground are apparently Plug N Play compatible; you don't have to tweak any gear ratios or adjust your brake camber for maximum performance or anything like that. All you need to do to upgrade your car is buy and install an upgrade; your car's performance will automatically be adjusted to account for the new parts.

All of these parts are linear, in that a Level 3 part will always be better than a Level 2 part, and so on. Of course, you'll have to unlock the different levels in the Underground mode before you can install them. Most of these parts will have "brand packages" that you can choose from; these have no gameplay effect at all, so if you don't particularly care who's making the parts that go into your car, you can default to the first selection after every upgrade.

Engine / Exhaust Package

Engine / Exhaust packages can refit or replace almost every part of your engine, eliminating power bottlenecks and freeing up your car's hidden potential. This increases your potential acceleration and top speed.

Tier 1: Street Performance
Cost: 500
Unlocked: Race #06

Tier 2: Pro Performance
Cost: 1000
Unlocked: Race #30

Tier 3: Extreme Performance
Cost: 2000
Unlocked: Race #66

Drivetrain Packages

Drivetrain mods will replace your flywheels and clutch, giving you more power from each gear; level three kits even add a sixth gear to cars that don't already have one. Going from stock parts to a level 3 kit will more than double most cars' top speeds, and lower-level kits provide big bonuses, as well.

Tier 1: Street Performance
Cost: 500
Unlocked: Race #06

Tier 2: Pro Performance
Cost: 1000
Unlocked: Race #30

Tier 3: Extreme Performance
Cost: 2000
Unlocked: Race #66

Tires

Stock tires aren't going to last long once you hit the mean streets. They may be fine for everyday driving conditions, but to deal with the dirt, water, and other surfaces that you're going to be coming across, you'll want to upgrade to tires with better traction. Doing so will improve your car's handling and let you take turns more tightly at the same speed as with your stock tires.

Tier 1: Street Performance
Cost: 500
Unlocked: Race #06

Tier 2: Pro Performance
Cost: 1000
Unlocked: Race #30

Tier 3: Extreme Performance
Cost: 2000
Unlocked: Race #66

E.C.U. & Fuel System

Most modern cars have an Engine Control Unit; a chip that controls such things as valve timings, fuel delivery, turbocharge boosting, and so forth. Factory E.C.U.'s can be overwritten or replaced, which will result in more horsepower and faster acceleration.

Tier 1: Street Performance
Cost: 1000
Unlocked: Race #09

Tier 2: Pro Performance
Cost: 2000
Unlocked: Race #38

Tier 3: Extreme Performance
Cost: 4000
Unlocked: Race #80

Turbo Package

Turbochargers are used to drastically increase an engine's performance without affecting its weight. They do this by compressing the air that's sent into an engine's cylinders, which in turn allows each cylinder to burn more fuel with each explosion. Turbocharging your ride will result in much faster acceleration and higher top speeds.

Tier 1: Street Performance
Cost: 1000
Unlocked: Race #09

Tier 2: Pro Performance
Cost: 2000
Unlocked: Race #38

Tier 3: Extreme Performance
Cost: 4000
Unlocked: Race #80

Brake Kits

Although you may not think it when you first start playing Need For Speed: Underground, high-performance brakes will become an integral part of your car's arsenal in the later, more difficult races. Hairpin turns and right-angle corners will prevent you from exercising your lead foot and force you to use technical braking to prevent your car from slamming into the wall. Better brakes will reduce your braking distance, allowing you to start accelerating sooner and letting you be more accurate in your position as you come out of turns.

Tier 1: Street Performance
Cost: 1000
Unlocked: Race #09

Tier 2: Pro Performance
Cost: 2000
Unlocked: Race #38

Tier 3: Extreme Performance
Cost: 4000
Unlocked: Race #80

Weight Reduction Kits

An engine's power only tells half the story of a car's performance: the weight of the car will also have a huge impact on its overall racing ability. Heavier cars will have reduced acceleration and poorer handling in turns, but these kits will help you strip down your car to the bare essentials, by ripping out the back seats, replacing the windows with "lightweight polycarbonate", and so on.

Tier 1: Street Performance
Cost: 750
Unlocked: Race #14

Tier 2: Pro Performance
Cost: 1500
Unlocked: Race #48

Tier 3: Extreme Performance
Cost: 3000
Unlocked: Race #91

Suspension Package

A new suspension package will replace your shocks and perform upgrades on the other parts of your car's suspension, resulting in a smoother ride and tighter handling.

Tier 1: Street Performance
Cost: 750
Unlocked: Race #14

Tier 2: Pro Performance
Cost: 1500
Unlocked: Race #48

Tier 3: Extreme Performance
Cost: 3000
Unlocked: Race #91

Nitrous Oxide

Installing a nitrous-oxide system on your car can greatly improve performance, if only for a short period of time. When you pump nitrous oxide into your engine, it breaks into its component parts; roughly one-third of the liquid's weight will be converted into pure oxygen, which allows much more fuel to be burned. The nitrogen in the mixture also acts as a buffer against increased cylinder pressures.

In NFS: Underground, successive levels of Nitrous Oxide will allow for longer periods of injection.

Tier 1: Street Performance
Cost: 750
Unlocked: Race #14

Tier 2: Pro Performance
Cost: 1500
Unlocked: Race #48

Tier 3: Extreme Performance
Cost: 3000
Unlocked: Race #91