Intercooler (minimal HP gains) - $750-1300  Intercooler Pipes (minimal HP gains) - $50-120

Intercoolers (IC's) usually do not give large power increases but rather allows you to run higher boost levels without problems such as detonation or pinging/knocking. However, by cooling the intake charge with a larger IC HP gains of approx. 1% per 10 degrees, minus whatever boost losses you get. If the IC flows the same as stock and drops the charge temp 50 degrees on a chipped motor you would get about 10hp. BUT usually a larger IC does not flow the same as stock and you will lose some boost due to the larger volume, as well as longer intake pipes (depending on style of intercooler). An intercooler upgrade is not needed for cars that have just a chip. Intercooler Pipes   Replacing your stock lower intercooler "pancake" pipe will increase the airflow of your intake. This is similar to using a big bore intake, it just lets air flow faster through it. Gains to be seen here are really not peak HP but more power in the upper rpms. The lower "pancake" pipe is the only IC pipe that needs to be replaced due to its restrictive flow. The picture here shows a comparison of the "pancake" tube and the larger one. Tech Tips - Misting - Fender liner - Test DATA

Do I need a bigger intercooler if I'm running a high boost chip?
  NO, the majority of people with chipped 1.8t's have made no intercooler mods.  The stock IC does a great job of cooling the intake charge.  Some people have modified the plastic liner behind the IC to allow heat to dissipate better much like the Audi TT has.  A larger intercooler may be necessary when using a larger ko4 turbo at higher boost levels.  But at 1 bar the stock IC does a good job.

Where is the intercooler located?
  The intercooler for all mk4 Golf and Jetta 1.8t's is located behind the passenger side front bumper.  Looking through the lower left bumper opening you can see it!!  

Misting
  The idea of misting the IC with a fluid is to help cool the intake even more.  Usually water is used as the mist but in some cases nitrous oxide has been used for optimal cooling compared to that of water.  However the cost of using nitrous to cool your IC may be too much for most people so water is the usual.  Check out this page for more info on intercooler misting.  More info here

Fender Liner
  Modification to the fender liner behind the IC will allow heat to dissipate more readily much like the Audi TT does.  So why would this be better...  with the stock fender liner the air that goes to your IC has no where to go but up into the engine bay and back down and out under the bumper. With vents in the fender liner behind the IC this gives the air a straight path to follow through which allows more air to flow through the IC which keeps the intake air cooler.  Heres the how-to on modifying your fender liner as performed by Jim Wilson.
     1. Remove wheel and fender liner from car
     2. make 6 8" horizontal cuts w/razor in front of where IC sits
     3. get at least 1500W hair dryer and heat where you want to make a permenant bend
     4. hold dryer as close as possible to plastic without over heating dryer, while holding dryer with one hand use needle nose(or whatever you have) to twist the plastic. Use your feet to keep the rest of the liner from bending.
     5. keep holding the piece you're bending (bend further than you need because it will go back slightly) and switch the dryer to no heat (or just wait till it cools)
     6. once cool it will stay in that position. keep doing this til your fender looks vented
     7. takes about 45min start to finish.

If doing this mod sounds like too much for you then Audi has done it for you.  Simply go to your local Audi parts dealer and order a TT fender liner, its got the vents already, and you just have to cut out the vents and mate it up to your mk4 golf/jetta liner, only $40.

Forced Performance
The following is from an email received from Forced Performance about their intercooler:

   The improved intercooler has been flow bench and field tested with great results. On the flow bench the FPIC showed a 46% increase in flow at the same 1.5 psi pressure drop from the inlet to outlet tank. In car testing of the FPIC matched HP for HP the performance of a custom built front mount intercooler while giving a slight advantage in midrange torque. This test car was a K04 equipped Golf putting 219 to the tires. This same test car has since replaced the K04 with a turbonetics T3 manifold and Garrett T3 super 60 turbo and is currently over 290 hp at the wheels, still using the FPIC. Below is a quote from the owner of the test car, Billy Tylaska.

"I did back to back dyno testing with the FP IC and the stock IC. The FP upper and lower IC pipes were also on the car, but I only swapped out the IC. Cars that upgrade from the stock setup to the complete FP setup(upper, lower and IC) will see huge gains. On each run, I recorded the intake temp, before, during and after, with both ICs. With the stock IC, the intake temp was before was 67 degrees, rose to 96 during, and sat at 85 after. Each time I made a pass, the temps rose about 3-4 degrees at each point. A total of 5 passes was made. My peak hp with the stock IC was 207, while torque was at 270ish. These numbers are at the wheels. I then put the FP IC on, and ran the same tests. Intake temp was about the same, but during and after temps were about 5 degrees less, and stayed the same thru each of the 5 pulls. Peak hp was 219, while torque jumped up to 297 at the wheels. Around town, throttle response was lightning quick, and the car pulled all the way to 7K. Instal is pretty straight forward as the FP IC bolts right up to all of the factory mounting points, no fabrication needed. Good mod for somebody that wants to upgrade, but doesn't want the hassle or expense of building a front mount... trust me, I did that too. And one more thing, I am almost making about 300 hp to the wheels thru the FP IC. That should speak volumes..."

ECS Tuning - Dual Intercooler TESTs (many thanks goes to Ari @ cosmicmotorsports.com)

Disclaimer: Before you read this please take note that this testing was done by Ari and a friend whom are NOT affiliated with ECS Tuning or any other specific tuner or 18turbo.com. We are a customer to these tuners, just like all of you. Furthermore, the testing was done in the fashion explained below. Please utilize this information as you see fit, but lets keep this thread clean and productive. Thank you!

FINAL RESULTS : Stock Intercooler vs Dual-Intercooler Testing!

Test Method : Driving on the freeway only, at speeds never below 50MPH, we attemped to heat soak the intercoolers. Testing was to be done at 55º F or better, and was to be a non-stop boost/brake/boost/brake routine. We ran from 50MPH to 120MPH (average, see data), slammed the brakes, and repeat. The goal was to get the IC to drop in overall efficiency.

Location, Etc : The testing was done in Akron, Ohio which is about 45 mins south of Cleveland. Testing was done in my 2001 VW GTI GLS 1.8T (for a full list of modifications the car has please refer to the link in my signature). This is all with a stock AWW-turbo from my car. The ECU on the car is from APR in Auburn, AL. The largest spikes we saw were in the 1.3x bar range, with boost holding nice at roughly .98-1.00 bar through the range, and dropping off up near redline. This is the 93-octane v1.1 program.

STOCK Intercooler Results : The stock intercooler had an average efficiency of 70.29% when driving. This is truly not bad at all. For those of you who are not sure if you want to spend the cash, I'd dare say the stock IC is doing you justice. Anything over 70% is not bad at all, and initially I was REALLY worried the dual-IC wouldnt be much better than this. The stock IC, however, soaked nicely towards the end of our runs (see data link). In a road-race condition, the stock IC would start to be "less than ideal" I felt.

DUAL-INTERCOOLER Results : The dual-IC was installed by ECS Tuning and is a kit put together by EuroSpecSport of which ECS is a dealer for them. The kit consists of a 2nd IC, Manifold, Etc. from a TT225 which uses two IC's in series. This setup can be found on any 225HP version of the Audi TT, so the parts are OEM in essence. The Dual-IC really suprised me because I felt you would get 80-85% efficiency. Instead, however the dual-intercooler setup was constantly over 90% efficiency, with an overall average of 93.49% !!!

Conclusions : Rather than bore you all with more rambling, I'll let you scroll down to click on the test data link. See it for yourself, it is all there in black and white. The dual-IC works, and works VERY well! Sometimes when on the freeway just cruising for long times, the IC works at 100% and its just amazing. And when pushed, the dual-IC works nicely too.

If you want to upgrade your intercooler this kit works! There is probably some pressure drop, but we didnt see the turbo putting out exceedingly high temps and it was hotter the 2nd time, so lets face it, all the talk about pressure drop and whatnot, its negligable. We saw next to no boost lag, and the turbo didn't appear to be working to the point of destruction. If it was good enough for Audi for the 225HP TT, its good enough for me. And I dare say, good enough for you too! Cheers everyone!