When you consider price and performance, the Motronic system (also called the Digital Motor Electronics, or DME) is hands down the best overall fuel injection system to use. Ignition timing and fuel delivery are all controlled by a digital map that is recorded in a flash memory chip located within the main fuel injection (DME) computer. The computer takes input from a variety of sensors that are located on the engine: cylinder head temperature, crank angle, throttle position, exhaust gas oxygen (mixture), ambient air temperature, and mass airflow. The DME is programmed from the factory with certain performance characteristics (mostly conservative) so that the engine will react well under a host of varying conditions. Major changes to the engine (increased displacement, the addition of different camshafts, etc.) require an updated map to take full advantage of these modifications. Failure to update the Motronic system may actually result in a decrease in performance, as the original system is finely tuned to supply the correct timing and fuel injection values for a stock engine configuration. To gain the maximum benefit from engine modifications, you need to either update the flash software in your DME (easy) or install a programmable aftermarket engine management system (not so easy).
The Motronic system is generally very reliable. Its main failure points are the sensors that send data back to the DME computer. Although I haven't seen it on the modern Carreras just yet, another odd failure point on other older Porsches appears to be the DME relay. Corroded contacts appear to cause this mission-critical part to fail somewhat intermittently. While this is a very rare issue with the Carreras I do recommend that you carry a spare one, as a failure can potentially leave you stranded on the side of the road. Also vulnerable is the fuel pump relay above the front fuse panel (see Figure 2 of Pelican Technical Article: Replacing the Porsche 911 Carrera Fuel Pump).
If you are running a stock engine with the Motronic injection, one of the best upgrades you can perform is the installation of aftermarket performance software. As stated previously, the factory programmed the original software to compensate for a wide variety of driving characteristics. These days, you can find software maps that will elevate the rev-limiter, advance your timing, and generally run the engine with less conservatism than the factory programs. The only downside to running a more aggressive map is that sometimes the timing curves are a bit too advanced and may cause detonation on low-octane pump gas (as it is here in California). The 911 Carreras have a knock sensor that will reduce detonation if the timing is too far advanced, and you can also get maps that are specifically tailored to your region if lower octane fuel is the only type available.
One downside to installing a performance software map is that you basically need to run premium fuel with the chip installed. Whereas the stock chip is designed and mapped to provide good performance across a wide variety of operating conditions, the performance chips are typically mapped to assume that you are running high-octane gasoline. If you run low-octane fuel, the knock sensor will generally prevent detonation, but in general you will not fully utilize the performance improvements of the chip.
There are many manufacturers out there selling various variations of performance software for the 911. One company that has differentiated itself from the pack is Softronic, founded by former PCA tech guru Scott Slauson. The Softronic software upgrade kit has a few innovative features not found in other DME programming products. The software upgrade is installed using a Softronic cable attached to a typical Windows computer (others often require you to ship them your DME for programming). The DME is updated and reprogrammed using the OBD II port (see Pelican Technical Article: Reading Porsche 911 Carrera Fuel Injection Fault Codes), which allows you the freedom to update your software and then also convert it back to stock when needed. This is particularly useful when you need to bring your car in for emissions testing and the updated software in the computer might make it difficult to pass. The Softronic software makes an exact backup copy of the software on your DME prior to installing the updated maps.
Installation of the software is a snap. Simply install the Softronic software onto your computer and then connect it via the supplied cable to the ODB II port on your Carrera. The software will perform a variety of pre-programming checks, create a full backup on your hard drive, and then install the performance software into your car. Just sit back, drink your coffee, and let the computer do all the work. The total programming time is about five minutes or so.
Performance software is required if you are upgrading your engine to a 3.6 or 3.8 liter. I used the Softronic performance software to install a higher output Porsche 996 program into my stock 3.4-liter engine. The increase in horsepower went from 300 to 322, and torque increased from 258 to 271--quite an improvement. In addition, Softronic was able to custom design our program to fit our particular engine installation profile (engine/intake/exhaust).
It really is as easy as it looks. Here is a screenshot from Softronic's software. Simply plug in the cable into the OBD II port on the Carrera and run the software. It will create a full backup of your existing configuration and install the performance software update in less time than it takes to brew a cup of coffee.
Here is all that is involved with upgrading your software once you have the Softtronic's software on your computer