This topic has been covered a number of times and by spending some time searching you would have found all the answers to any 'mod/performance' either tried or discussed. That being said since you are new, here is a quick list of possiblities based how much labor and cost it would be to you:
1) Ported throttlebody/spacer ~ Get someone with a lathe and it can be done cheap, pay someone to do it for you and you're looking around a couple hundred. Otherwise easy bolt on, also good for a few HP (Check
www.caddyinfo.com for the numbers)
2) High stall torque converter ~ This is the best bang for your buck, period. This keeps the vehicle in the powerband and the Guru felt it could knock as much as a second off your 1/4 mile times. It isn't easy to install, requires removal of the tranny and disassembly, but the part is a few hundred, Yank makes them for our application as I recall. No HP to speak of, but it will help maintain the engine in the powerband. Also changes the true daily driveability of the car.
3) Custom exhaust headers ~ The most effective at opening up potential ponies but requires radical plumbing change. The biggest restriction is the exhaust and it is NOT from the cat back, it is from the block to the cat. If you check a schematic, or crawl under your ride you will notice the front exhaust is in a log format that all bottlenecks down to a circular pipe around 2 inches in diameter, then flattens to an ellipse to pass under the cradle and tranny and up BEHIND the oil pan to where it meets at a y-junction with the rear log manifold. If you were to plumb something up you would get the most benefit here but it would take a lot of custom welding and time, also would help to have the engine out to install/uninstall . Again, the cost is really in your ability to custom weld pipe, if you cannot then don't plan on this being economically feasible. HP numbers are unknown as no one has done this without forced induction being tacked on (see #9).
4) Limited-slip differential (LSD), helps put the power down more evenly (helps remove torque steer) and might help acceleration but would not increase horespower. Mark99STS has this on his STS and it helps put the power down more evenly and should increase the service life of his tranny during high rpm dumps, I can't remember off hand who makes them but they are custom order as I recall.
5) Computer tuners ~ To date I believe CadillacETC1997 had the only known prototype 'jet' chip that has provided gains for a 97+ OBD-II Northstar and he paid upwards of $900 for it. Unfortunatley this chip has not been duplicated though I'm sure it could be. Also, there is a thread here and over on the HPTuners.com website about potentially making a tool to tune the 96-99 N* but this has happened many times and nothing has really ever come of it. Horsepower is unknown on this mod as well but if we had some dyno pre an post we could have an answer. The problem with 97+ Northstars is the OBD-II computer system which no tuner to date has produced a custom tuner. Having this done as a cutom job is a massive undertaking, just ask Mark99STS (see #9)
---Mods below this point should require #6 as a FIRST STEP---
6) Timeserting bock or install headstuds ~ Running higher power is going to put more stress on the engine, overtime this can lead to threading the main head bolt holes causing the head to lift (especially in the case of nitrous or forced induction) because the Northstar's main head bolt holes are tapped DIRECTLY into the softer aluminum block with no reinforcement. Timesert kits run in the hundreds and are labor intensive as the engine needs to be pulled out, heads pulled, and all main bolt holes drilled and tapped. No HP created here but will make other big mods possible without having to worry about lifting the heads.
7) Nitrous ~ ShadowLvr400 ran a 125HP shot on stock internals but he also managed to lift the stock heads due to not having timeserted/headstuds installed (see #6) before he wrecked his Eldo. After the timsert job he had no problems. N2O kits are several hundred to a few thousand and are not hard to install but require alot of layout and testing, espeically to ensure a symmetric jet pattern between cylinders. Shadow had a relativley inexpensive bolt on kit and he didn't have problems with throwing codes, but he did manage to lift his heads. More recently, wydopnthrtl has a very nice nitrous setup that was very well laid out and was much 'safer' than Shadow's bolt on kit. WY's setup is more expensive but definatley provides much greater protection against asymmetrical injection patterns and melting the pistons as the nitrous feed opens gradually. The nice thing about WY's is it uses less power than Shadow did and gets better results, prooving the old addage, more isn't necessarily better!
8) Cams, lifters, springs ~ Can be had by a few engine builders/hotrodders (CHRFAB and other N* rebuilders) with more aggressive grinds, full set can cost in the hundreds to around a thousand. Will also need new retainers and springs which will add several hundred. Can boost power by up to 50HP in some applications but throwing codes is possible (a bad thing), and it moves your power band into the higher RPMs beyond where your stock redline/shift points are. Difficult to install as the heads require removal which means the engine needs to come out, may as well timesert the block or put in headstuds at that point (see #6), you may also need custom tune, see #5.
9) Forced induction ~ To date Mark99STS is the only person to suscessfully convert his OBD-II equipped 99 STS over using a turbo but it required a tremendous amount of custom work, lets just say well over $10k. This included forged pistons and rods which are available, CHRFab.com makes the pistons for sand rails but they are expensive, last time I checked they wanted $1400 for the set. The rods can be had for less as the design of the CHRFabs was duplicated by Eagle I believe and can be had for a few hundred. Again, this is a huge job including custom exhaust, mounting of the turbo, the turbo itself, scavange pump for the oil from the turbo due to height constraints, tranny rebuild with high stall torque converter and LSD, computer simulator that piggybacked off the ECU to prevent codes from being set as well as a full engine rebuild, upgraded fuel pump, regulator, and rails, and a manual shift controller install. Mark was well over 400 WHP and was closing in on 500 with a bit more tweaking.
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