After reading this thread Im going to make a few suggestions based on my experiences of getting power out of the YD25.
The biggest single gain in power you will get is from a chip, these cost anywhere from $300 for an ebay special all the way up to $2000 for a top of the line Unichip. You get what you pay for. There are also mixed result from ECU re-flashes but I don't have any experience on this so cannot comment.
Second biggest gain is boost pressure, a YD25 runs between 15 - 16psi stock. The stock intercooler is good till about 19- 20psi before an aftermarket unit is required. By running higher boost it will allow the turbo to spool quicker resulting in you reaching what is called WOT (wide open throttle) at lower rpm. It is also pretty cheap ($50) to buy and fit.
Third biggest gain is an intercooler. A large front mount or W2A will give you best results. I found that running a top mount with a thermo fan did bugger all. Maybe a slightly quicker spool time at low rpm. These can range from $200 for an ebay special to $3000 for a top of the line W2A from Laminova. Keeping intake temps cool is critical when running higher boost
Other mods that help:
Aftermarket turbo: the most common upgrade is the Forefront turbo. Due to its billet design and superior inducer wheel it will spool quicker than the stock turbo. As I have not purchased one I cannot comment on its performance. There are a few individuals running VNT or twin turbo setups however their results are not well documented.
EGR blocking/removal: Blocking this line will allow the turbo to spool at about 200rpm earlier. Complete removal, especially at the intake port will give slightly better results. Pretty cheap for $20 - $50 bucks.
Aftermarket exhaust: 2.5" - 3" is pretty common. biggest gain will be the dump pipe, can cost anywhere from $700 - $1500.
Airbox, Snorkel and Intercooler piping upgrade. Will help with turbo spool and reduce stock piping restriction, prices will vary depending what you get done.
Diff ratios: changing to a higher diff ratio will help you get off the line quicker at the expense of top end speed. 4.6's are ideal for turning 33's
Tyre Size: 33's are a lot harder to turn than 30's due to their weight and higher inertia.
Weight of the Nav: It can add up pretty quick. Dual batts 30kgs, steel front bar 50kgs, winch 30kgs, sliders 30kgs, rear bar 25kgs, draws 20kgs, canopy 15kgs, aftermarket fuel/water tank - anywhere between 50 - 150kgs. Tools and gear - up to 400kgs.
Carrying all this extra weight everyday adds up.
As you can see there are many factors which affect turbo lag and no 2 navs will be the same. Having a budget and goal on what you want out of your YD25 is the best place to start
