Petrol RS8's all use FSI (direct injection) engines.
To convert direct injection engines using any slave type LPG system (this includes all the usual type vapour injection, and the liquid injection type - any system which uses a set of dedicated LPG injectors), the electronics have to be available to suit the exact engine code. The situation regards Di engines is very different to the generic systems that are available for port injected engines. Two reasons for that - A. because direct injection engines pulse petrol injectors in a very different way, they may use multiple injector pulses per induction (even compression) cycle where port injectors generally use a single (at least main) pulse per induction cycle - and any slave LPG system needs to interpret those pulses to derive a single pulse per induction cycle (particularly regards vapour injection LPG systems), B. because direct injection system injectors use injectors with different electrical characteristics to 'normal' (port) injectors and all slave type LPG systems have to emulate the electrical characteristics of the original injectors.
Last check (by me at least), the electronics were not available for anything except 4 cylinder FSI engines... Though (as I've said before), I would bet 2 x 4 cylinder FSI systems could be fitted to a V8 FSI and work well. To confirm, though, I would need at least one 4 cylinder FSI vehicle here at some point to check injector pulses on an oscilloscope, and the R8, also to put on an oscilloscope, and then compare injector pulses... If frequency and duration of direct injector pulses looked very similar then I expect this approach would work. But would work out a relatively expensive conversion, as LPG ECUs for direct injection engines are generally more expensive than the usual port injection counterparts and the R8 would need 2 of them.
I have been keeping an eye on Vialle LPG liquid injection systems recently. Before we start raising eyebrows in anticipation too much, most of these are designed to suit port injected petrol engines... But Vialle also make 'fuel shunting' systems to suit direct injection engines which employ tanks with a high pressure LPG pump and fuel diverting valves. The diverting valves in effect simply switch fuels (petrol / LPG) that are supplied to the direct injected vehicle's set of standard (direct) injectors. This type of system at first glance looks like a 'fit all' solution for converting direct injection engines but the actuality is far from that. They really would work on every vehicle (including the RS8) if the driver remembered to switch back to petrol for a few minutes before turning the engine off, but if the engine is stopped while running on LPG, engine start problems occur on some models - Audi FSI's included so far included - I have checked regards Audi's! This type of system is in it's infancy. Prins make a similar system, which must almost inevitably reap all the same advantages and reap all the same consequences- we are, after all all, only talking about diversion valves. The 'swap fuel supplied to standard direct injectors' approach works on all direct injected engines, except engine starting problems occur on some models...
I recently repaired a Subaru flat 6 (in a, erm weird SUV type Subaru vehicle with a forgettable name) with a Vialle liquid injection system 'factory fitted' but this fuel system was not the type you would be concerned with... This Subaru engine features 'normal' port injection, this Vialle system was the port injection type and I wasn't overly impressed with the nature of the system - weird seeing LPG injector pipes freeze up due to the liquid in the pipes between injectors and manifold frostbitten, getting cold due to LPG evaporating inside the injector to manifold feed pipe. To me, that evaporation should have taken place within the inlet manifold for a truly liquid injection system (like of a Ford Transit Connect).
None of this bit is relevant to the Rs8, but I am impressed by the fact Vialle liquid injection (port) injectors use petrol spec injectors. Petrol (liquid) injectors are much faster than the fastest (opening and closing) LPG vapour injectors because the mechanics don't have to move as far. The petrol spec liquid LPG injectors use (which are an early 1990's petrol injector spec) will withstand the high pressure of liquid LPG. The Vialle system with it's methodology and injectors may make for a good jump in LPG port injection systems, but I think if only they could make a system with individual injectors (to allow short pipe runs to all cylinders) rather than a block of 4 injectors (those piped closest might receive liquid LPG, those pipe furthest might receive vapour which evaporated from liquid in the piping between injector and manifold).
To confuse matters, Vialle also produce a slave type system for direct injection engines but this is more like the usual (vapour) injection systems for direct injection engines, because it still uses a dedicated set of LPG injectors and is still slave (to the petrol ECU) type... and again, they don't produce a version of this system for the Audi FSI models.
The fuel shunting type is not a slave type at all, it simply shunts between petrol/LPG to supply the selected fuel to the vehicle standard set of petrol injectors. Since LPG and petrol are so similar, matching the fuel pressure of LPG to the fuel pressure of petrol, the system requires no injection pulse fiddle factors (the fiddle factors are the main reason slave type LPG systems require an ECU).
I am apparently the first installer in the UK to have training booked with Vialle for these systems - which I think are promising for DI engines, but I haven't been given a date to attend training yet! That said, I can't see how much training would be necessary - as this type of system pretty much requires not much more than plumbing skills and the ability to set tank output pressure to match standard (direct injection - much higher than port injection) petrol pressure.
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