)I only have pics for the spi engines, but it's not that hard on the mpi's too, so I'll describe it as best as I can lateron in this thread.
Some remarks:
If you choose to do this, it's not my bad if it turns out wrong, just follow the instructions and/or tips, and you'll be fine. And tweeking your fuelpressure will decrease your mileage a bit. Also don't expect miracles on the performance, sometimes you won't even notice it, it has the most effect on (slightly) modded engines (no, a backbox IS NOT an engine mod)
Basic info on fuelpressure:
Your engine has a fuelpump in it, which supplies your engine of fuel (duuh). This runs from the tank to the injector(s) and the not used fuel is directed back to the tank again. And in this redirection of non-used fuel lies the trick. The pump generates pressure (in Polo's commenly around 3,5 Bar), but not all of this pressure is used, since there is still fuel being redirected. So the harder you make it to redirect the fuel, the more pressure is used to inject the fuel into your engine by the injector(s).
And that's the basic info for increasing your fuel pressure, all that is needed is to make it harder for the fuel to go back to the tank again if not used.
SPI-engines:
Now we known the basics, it's time to get to the actual trick.
For SPI engines, take of the airfilter, so that you see your throttle, injector and FUELPRESSURE REGULATOR (which is indicated below by the red arrow, the black round thing abow is the housing for the injector, and underneath the injector is the throttle):
The trick is to take out the pressure regulator, caution, there is a little spring underneath it, so don't just unscrew it's 3 bolts (indicated below):
And here's the spring:
Keep the spring and the red film in place, just don't touch it, since it should stay in place perfectly.
Now to increase your fuel pressure, you get a little piece of small/narrow metal piping which fits into the shallow part of the regulator (as can be seen in the picture below) Don't use a screwdriver or something like that, since then you'll have the chance of hitting through the regulator and f*ck it up:
The area I mean is in the directmiddle of the round top of the regulator.
Put your piece of pipe in it, and give it some slight taps with a hammer (make sure you do this on a secure surface, and don't soil the inside of the regulator).
DON'T HIT IT HARD!!!! or else you can either go through it and f*ck it up, or you max. out the pressure, which isn't good (more is not always better).
No place everything back and your done!
My tip is to just tap the pipe with the hammer just ones or twice, with minor taps. Then mount it all back and drive round a while. Since the procedure can always be repeated if you are not pleased with the result (so you can tweek it further if you like, but you can't untweek it so to speak, so don't give it a good ram at the first attempt)
How far to tap it with the hammer? Depends, I did this on a mates mk4 1.3, who had a different cam and inletfilter, due to the cam it needed more then I would do to a "standard" engine, DON'T OVERDO IT!!!
MPI engines:
Like I said, I don't have pics for this, but it's not that hard. First you'll have to find your fuelrail, which is a long metal rail, with a round "thingy" on one of the ends. This is your FUEL REGULATOR.
It's mounted on the rail with a clip, which you can take of, now you can take of your regulator, fuel from the inside of the rail will probably
drip out, but not taht much, since your engine is off and therefor the fuelpump isn't working/building pressure in the rail.
Now follow the procedure described above on tapping with the hamer and metal small/narrow pipe. Mount it all back and give it a try.
Voila, you just tweeked your fuelpressure.
