MRC Chips for the Audi urS-cars

VMAP Choice
MRC chips for the AAN engine are designed with the assumption that fuel octane ratings of 93 or better [(R + M)/2] will be used. Fuels with this rating are not readily available in some areas. If 91 octane fuel is used there is a possibility of knocking. The remedy for this is to enrich the fuel mixture just a little and to retard the timing a bit at high power levels. This can be accomplished in at least two ways. The first way is to rework the code in the chips to permit use of 91 octane fuel. Chips are available for this situation. A second option is to lie to the Fuel Chip software by telling the software that the boost level is a little bit higher than it actually is. This will cause the fuel chip to provide just a little more fuel to the engine. This technique does not change the timing at all.

VMAPs come in two basic flavors.
1. The less expensive VMAP is the 2.5/3.0 bar VMAP. This unit is adjustable to be used as either a 2.5 bar or a 3.0 bar unit. It comes set for 3.0 bar operation and it is well suited for 93+ octane fuel and is the unit regularly supplied with our chipsets.

2. The 2.5/4.0 bar VMAP is jumper adjustable in 0.1 bar steps between 2.5 and 4.0 bar. It also comes set for 3.0 bar operation and likewise is well suited for 93+ octane fuel. It is an extra cost option for our chipsets.

Telling lies.
The fuel chip expects that a full-scale signal from the VMAP to correspond to a MAP of exactly 3.0 bar. If the VMAP is adjusted to a setting where full scale is 2.9 bar then at 2.9 bar the VMAP will send out a full-scale signal to the fuel chip. The fuel chip “thinks” that the MAP is a bit higher than it actually is and therefore provides slightly more fuel than it would if the pressure were reported truthfully. This enriches the fuel mixture slightly in order to reduce the tendency for knocking with poor fuel.

Choices:
If you frequently drive in a region where fuel quality is suspect then the 2.5/4.0 bar VMAP makes sense. If you very rarely enter such regions then the less expensive 2.5/3.0 bar unit is quite suitable.

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