Warning: Do not turn up the boost pressure unless supporting mods are in place.  At the very minimum, an aftermarket cat-back exhaust and low restriction air intake filter are needed.  Even then, the boost should only be turned up a pound or two of psi over the stock level of 9.5 psi.  Any higher than that and the  ECU chip would need the fuel maps reprogrammed. 

The highest psi I would recommend on stock turbos with an aftermarket intake, exhaust, and chip would be 14 psi.  Installing boost jets is the most basic and simple way to add boost pressure, which will increase horsepower, and create a very noticable improvement in performance.  I'm talking the kind of improvement that puts a smile on yor face and a "wow!" in your brain.  Now would be a good time to refer to this site's Disclaimer.

One boost jet is the same thing as a "MIG contact welding tip," which can be bought at any Sears, Harbor Freight, or welding supply store.  At least two matching tips are needed, but in reality a range of sizes will have to be tried.  A good, safe size to start with would be .035" or larger.  Four different sizes should be good enough.  Boost jets work well with stock turbochargers. 

Other more expensive and complicated ways to control boost pressure is with a manual boost controller (MBC), or an electronic boost controller (EBC).  Boost jets cost a few dollars.  MBC's between $20-$100.  EBC's can run several hundred dollars, and give the tightest control of boost pressure along with many other options. 

The engine does not distinguish how boost is generated.  Boost is boost.  As long as the results are reliable then the engine will perform the same with any of the above controllers.  Two of the main reasons given for only using EBC's are boost creep and boost spikes.  The way I get around these pitfalls is to not run my boost pressure so close to the edge of the upper limit.  I try to build in a safety margin to any mod I do on my car.

Adding aftermarket intercoolers and bigger fuel injectors can allow for another couple of pounds of boost to be tried, but I would not push it past 15-16 psi on stock turbos no matter what.  The consequence of running too high of boost pressure is engine self-destruction due to detonation.  Detonation can be sudden, loud and catastrophic, or silent and insidious.

Running the engine too close to the edge of its boost pressure limits even without detonation is not safe due to variations in fuel quality and ambient temperature conditions.  Always run 92-93 octane fuel as a minimum.  Octane is simply a measure of an engine's reisistance to detonation. 

Size of the welding tip is measured at the diameter of the tiny hole going through the center.  These welding tips are very "small" at .024" but I had aftermarket turbos, downpipes, injectors, intercoolers, chip . . .

I could not increase the  boost pressure of my new JWT 530 BB turbochargers  even with these small boost jets, so I switched over to a manual boost controller.  Boost jets work fine for stock turbos.

Driver side,   front of car to right, nipple on the black intercooler hardpipe to which the wastegate hose from the driver side turbo attaches.  Fusebox can be seen in the left upper corner.

Same pipe as above under my finger.  Arrow shows where the red intercooler hose was removed. 

For some reason, folks seem to get confused as to where the wastegate hose is located.  Here is the simplest rule: the wastegate hose is always connected to the black hardpipe under the intercooler hose that goes to the throttle body, shown by the top yellow arrow.   Just work backwards from the throttle body on each side and you will not miss. 

Angle between the radiator shroud and the hardpipe shows the driver side wastegate hose. 

Different angle, driver side wastegate hose disconnected, clamp off.

Squeezed out my old boost jets with padded pair of pliers.  Squirt WD-40 into hose end first.  Always put the smooth, butt end of the welding tip in the hose first, mainly for ease of removal. 

The old .030" welding tips I removed.  The measurement describes the tiny hole in the center.  The length of the welding tip doesn't matter.

Ready to reattach the wastegate hose with a smaller boost jet inside.  I like to put a 1/4" hose clamp around the boost jet to make sure that no air leaks around  the boost jet. 

I hook up a temporary aftermarket boost gauge to get an accurate reading of the boost pressure. 

On stock turbos, the .030" above welding tips gave me 15-16 psi, which is the maximum I would recommend on stock turbos IF the car has all the other necessary supporting mods.  On my new JWT 530 BB turbos, the same diameter welding tip only made 10-11 psi.  The boost pressure did not go up even when I dropped the boost jet size to .026".

There is not a formula for what size boost jet yields how much boost pressure.  The main variable seems to be the integritity of the various hoses and clamps that are under boost pressure.  Setting boost pressure with boost jets requires trial and error.  This involves installing the boost jets, making WOT runs in second and third gear, and recording the boost pressure with an aftermarket gauge. 

If the car sputters, cuts out, knocks, pings, or in any way runs like crap after a boost jet install, then the first assumption should be that the boost pressure is too high and that you engine is about to blow up.  Back off and try again.