As a bit of background, my objectives with water meth are mainly: (1) reduce air intake temps, which RS4 appears particularly sensitive to, and (2) provide cleaning effect to combat carbon buildup. There are a few other debatable benefits e.g an *effective* octane boost if using low octane fuel - but they don't really interest me much.
The installation roughly breaks down into 4 chunks:
1) Wiring: connect and route the various inputs/output for the control unit (which I'm locating in the glove box), mainly from glove box to engine bay, but also one cable from boot to glove box.
2) Running the water meth line from boot up to engine bay.
3) Engine bay work: split and relocate MAF/IAT sensors, fit "fast acting valve" and the actual jet into the intake pipe.
4) Fitting tank and pump in boot.
The wiring work is probably the bit I expected to be trickiest so I wanted to crack it first...
The Aquamist HFS-2 uses fuel injector duty cycle (F-IDC) as the main sensor input, which is what attracted me to that product really, especially for our N/A vehicles where we don't have boost to probe. As an aside, some of the cheaper systems use things like MAF voltage to control flow rate but I'm not convinced that's the best way tbh. With this Aquamist kit, your water meth injection system is acting in a very similar way to what the fuel injectors are doing.
There is also a new "aux" input that isn't currently documented, which adds another layer of accuracy to the injection - but I don't think it's been publicly announced yet (but I'll update this as soon as it is).
So, here's the basic wiring diagram...
(The HFS-2 kit doesn't include the dash gauge (just a button instead) or the flow sensor)
The first cable I ran was the "trunk" cable from the boot to the glove box, which deals with pump relay switching and the tank level sensor. The best route is to follow the battery cables left out of the spare tyre well into the side of the boot, then round the left hand side of the rear bench, and then follow the path of some existing looms into the channelling which runs from rear left passenger footwell straight forward underneath the left of the passenger seat up to the glove box area.
This means removing the left boot trim, removing the rear bench and seat backs, removing the passenger seat, inner sill trim and glove box. Phew. None of it is particularly tricky but I think it is quicker just to rip it all out and get proper access to run the cable through the proper channelling than to try and fiddle it.
Boot trim and rear seats removed:
After routing the trunk cable:
If you bought your car second-hand (or maybe if you didn't) - be prepared for a shock when you remove the rear bench. I found this little collection!...
Passenger seat and sill trim removed (which enables the carpet to be pulled back revealing the cable trunking)...
This trunking ends up behind the carpet on the bottom left of the glove box. So you can sent the wire all the way along this channel and up behind the glove box. I used copious amounts of cable ties and cloth tape to limit the chances of snagging/chafing.
I then needed to find a suitable path from the engine bay through to the passenger compartment. I found a very suitable unused grommet underneath the pollen filter housing where the cables from the engine bay can be safely routed through.
I removed the pollen/cabin filter housing to give a bit more space and then very carefully punctured a small hole in the rubbery inner lining so the cables could be fished through to passenger compartment. To do this, I used a stiff piece of earth cable and taped each connector to the cable and pulled them through one-by-one. To prevent snagging, I taped all round the connectors so there was nothing to catch and damage the connector when pulling through.
I'm going to locate the FAV in the left hand-side of the engine bay, so for now I cable-tied it in place to stop it bashing around and fed the cable through the large grommet nearby into the plenum chamber and then, like the others, through the grommet near the pollen filter housing into the passenger compartment.
Next up, getting access to the ECU connector wires...
