Intake Pipe and Filter
A high-flow K&N (or similar) intake filter will help throttle response and help all future mods achieve maximum performance. A hard intake pipe will also improve air flow over the factory rubber snorkel. This, along with the MAS modifications should take care of your intake needs at this stage.
Aftermarket Boost Gauge
The factory boost gauge is simply not accurate and shouldn't be trusted when raising boost levels. To avoid severe engine damage when increasing boost levels, an aftermarket unit is mandatory. You'll find many brands to choose from, including Autometer, Greddy, Apex'i, Blitz, HKS, Defi, and many more.
In order to raise boost levels, you'll need a boost controller. Whether it be a manual or electronic unit will depend on your budget. Electronic boost controllers are cool looking, but most have been known to be unreliable at over 25psi. At this stage, you won't be turning the boost up past 16psi, so it won't be a problem. But if you plan on moving up to the next stage, a manual boost controller may be better.
Cat-back Exhaust System
A 2.5" or 3" high flow exhaust system is needed to gain more power. You can choose between aluminized steel or stainless steel depending on your budget. If your power goals are over 400hp, you'll want a full 3" system from the turbo all the way back. A "cat-back" system replaces the exhaust components from the catalytic convertor to the exhaust tip. A "turbo-back" system replaces the entire exhaust system, and is made up of a "cat-back" plus a downpipe and high-flow cat/straight pipe.
High Flow Cat/Straight Pipe
Replace the factory catalytic converter with a larger, higher flowing unit. For off road use, simply bolt up a straight pipe in it's place - of course, this is obviously not street legal.
High Flow Downpipe
The factory downpipe is very restrictive and should be replaced with a larger, better flowing unit. For Stage 1, a 2.5" downpipe will do. If future plans involve 400+ horsepower, consider a 3" downpipe. These are available in press-bent aluminized steel for those on a budget, or mandrel-bent stainless steel for those with a little more cash to spend.
The exhaust manifold on 1G DSMs are notorious for cracking and leaking. The solution is to replace it with a ported factory 95-99 exhaust manifold or aftermarket tubular header. If you're planning on going with a full Garrett turbo, you'll need a manifold that is made to bolt up to a Garret turbo.
O2 Sensor Housing
In between the turbo and the downpipe is a 90 degree elbow called the O2 Sensor Housing. The O2 sensor screws into this housing. This is the last restriction in the exhaust system. The stock unit can be ported out, or you can simply buy a better flowing unit. If you choose to go with a "turbo-back" exhaust system, this housing will automatically be replaced.
The stock fuel pump will not be able to supply the needed fuel for anything over 16psi of boost. Though you won't need to go past that in this stage, it's still cheap insurance to put in at least a 190Lph fuel pump to ensure you have enough fuel in case you want to raise the boost with race gas in the tank. It's also a good idea to re-wire the fuel pump for consistent fuel pressure.
The rubber intercooler piping should be replaced with better flowing solid pipes. Some replacements will come with a larger throttle body elbow (which is needed also). The turbo outlet elbow should be swapped out for a larger unit as well. Please note that if a Stage 2 setup is in the plans, this upgrade may not be necessary as the intercooler piping will likely be routed differently with a front mount intercooler.
With more power on tap, the stock clutch probably won't last that long. It will start to slip, especially in the higher gears. This means it's time for a high performance clutch that can handle the increased horsepower.
If you have a 5-speed tranny, you'll notice that the shifting is likely less-than-desirable on the 90-94 cars. These transmissions tend to be notchy from the factory to begin with. The best solution short of a rebuild is to put in some good tranny fluid and replace the rubber bushings in the system with some metal bushings. This should help with grinding and missed shifts as well.
Limited Slip Differentials
If you have a front wheel drive DSM, you'll probably need an aftermarket LSD to keep the tires from spinning - especially if you plan on making more than the stage 1 power levels. FWD cars will have a more difficult time achieving fast 1/4 mile times compared to the AWD cars due to traction. Tranny rebuilds are the most opportune time to install LSDs, since the tranny is already being taken apart.
Auto Tranny Upgrades
Though you won't find as many turbocharged automatic transmission DSMs around as 5-speeds, there are modifications available to help improve shifting, help handle increased horsepower, and just make driving more enjoyable.
Stage 2 Upgrade Path
Requires all of the Stage 1 Mods, minus the Upper Intercooler Pipe upgrade.
Before you can safely begin making fuel adjustments, you'll need to know what's going on inside the engine. Logging software will give you this data and will help you decide what fuel delivery changes are needed, how much change is needed, and at what RPM point. Though datalogging is mainly for tuning, it's also a critical tool to prevent major engine damage as well.
Along with a datalogging solution, you'll want some gauges to help monitor the engine. While dataloggers allow you to analyze data after doing some test runs, guages are easier to read while driving. An EGT (exhaust gas temperature) gauge and probe will allow you to monitor the exhaust temperature exiting the engine. Extremely high temps generally translate into a lean condition and can be catastrophic. You can also go with a Wideband O2 system that will allow you to monitor the air/fuel ration of your car while driving. This will tell you exactly how rich or how lean the car is running, allowing you to adjust the fuel curve accordingly.
» Shop for Gauges
Fuel Controller/Engine Management
In order to control the flow of fuel of the larger injectors, you'll need some form of a fuel management system. The factory system simply cannot control the changes you've made/are going to make. There are a variety of ways you can go here. The most common route is a piggy-back style computer which still uses the factory MAS in combination with a reprogrammed EPROM chip that plugs into the factory ECU. The drawback to this is that some cars didn't come with an ECU that utilizes an EPROM, making an ECU swap necessary. Another option is a stand alone fuel system which completely replaces the MAS system altogether. Stand alone systems give you much greater tuning capability, but may require more knowledge of fuel tuning than the average weekend warrior may possess. Either route you choose will allow you to run larger-than-stock fuel injectors - which will be needed very soon.
Fuel Injector Upgrade
To supply the amount of fuel needed for a larger turbo, the stock fuel injectors will need to be replaced with higher flowing units. The larger the turbo you plan on upgrading to, the larger the injectors you'll likely need. If swapping in a 16G turbo, you can get away with 550cc injectors. Most T3/T4 combos will require at least 650cc and up.
Front Mount Intercooler Kit
The stock side mount intercooler will not be able to cool the amount of air flow needed to produce 400hp or more. Thus, it will need to be swapped out for a Front Mount Intercooler. This install is pretty involved, especially if you choose a kit that routes the piping around the sides of the radiator. Most kits require some trimming of the steel bumper and some dremeling of the plastic bumper cover. Most "street" kits will do for this stage. But if more than 400hp is desired, you might want to consider a "race" kit which will have a larger core for better cooling characteristics.
At this point, the stock 14b turbo is running at it's max. In order to increase boost, a larger turbo is necessary. You'll need something larger than a 16G or 18G in order to achieve power levels above the 375hp mark. A Mitsubishi 20G, a Mitsu-Garrett hybrid, or a full Garrett setup will be needed for higher horsepower levels. Keep in mind that any non-Mitsu or non-Mitsu hybrid turbo (like a full Garrett) will require a special exhaust manifold, external wastegate and O2 sensor housing. This is the point where you'll have to decide what your car will be used for. The larger turbos are great for drag strip action, but not so great for road race and autocross use - something to keep in mind.