Why a 6psi boost switch?
Digifant has two main maps stored in the EPROM, fuel and ignition timing. These contain the core information that enables the engine to run. However, there are several additional tables/maps in the code that Digifant uses to apply correction factors to the information stored in these core maps.
One of these additional maps is a fuelling correction factor which varies with rpm - this factor is used whenever Digifant sees the car is on full-throttle - it looks up the fuelling value in the main fuel table, and then applies the correction factor on top.
For a supercharged (G-lader) application, whenever you hit full-throttle you're also getting lots of boost instantly, so you need lots of additional fuel. At say 60% throttle you'll be getting some boost, but you're likely to just be cruising along the motorway, so you don't need lots of additional fuel as the engine's not under a heavy load. However, full-throttle is different - you're at maximum load then and need the additional (rich mixture) fuel, so this is why the full-throttle fuelling correction factor is used.
This all goes to pot when you turbo the car. Full-throttle on a turbo doesn't mean you get lots of instant boost. As the turbo spools you can have no boost or vacuum until the turbo's wound up. This means that Digifant sees full-throttle and whacks a lot of extra fuel in - but you're not actually making any boost because your turbo's not spooled-up yet, and your engine's not under a huge amount of load. This extra fuel actually causes the car to bog down at low rpm on full-throttle, so makes the car feel very laggy.
So we use the boost switch to trick Digifant. We need the fuelling correction under high load, but not before the turbo's spooled - even if we are on full-throttle. So on a turbo application a good approximation to this is to use a boost switch set to come in at 6psi. This means Digifant won't massively richen up the mixture until it sees 6psi of boost - which works well on a quick spooling K03.
Why do I need a different chip if I don't run a boost switch?
If you don't run a boost switch and want to retain the existing Digifant setup, we need to do our best to reduce the massively rich mixture you can get on full-throttle at low rpm with no boost. This means editing the main core fuel map to remove a lot of fuel.
However, this is a compromise. The core fuel map is based on rpm and manifold pressure (MAP), so we take out a lot of fuel from the low rpm, no boost areas of the map to lean-out the mixture a bit when you're on full-throttle. But you might also need to use that bit of the map when you're not on full-throttle, when the additional full-throttle fuelling will not be applied, so under partial throttle you end up being too lean!
Put another way, say at one rpm/MAP point the ideal fuelling number is 50 - so you put 50 into the core fuel map. All is well, until you drive through that bit of the core map on full-throttle. Digifant will now apply a full-throttle correction, say 50 x 1.5 = 75. So you're far too rich. If you then wind the core fuel map figure down to 33, on full throttle you have 33 x 1.4 = 49.5 - so your fuelling is correct on full-throttle, but on partial throttle you're too lean. Hence the compromise with this setup.
dub-disaster
Your current chip will not work properly with a boost switch, so you'd need to change to our map designed to be used with one so that your partial throttle fuelling is correct. With the boost switch and chip to suit you'll find that partial throttle driveability is much improved.
You need a normally-open pressure switch, which should be set to close at around 6psi.
