ECU Reflashing and Car Tuning Training – BMW, Mercedes and Others

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An ECU (Engine Control Unit) upgrade is one of the few power upgrades that gives you the most bang for your buck. The beauty of a reflash or EPROM swap is that you get all of this performance while keeping your car’s stock look.
An EPROM swap or reflash is difficult to beat in terms of value and stealthiness.

Reflashes and swaps
Reflashes and EPROM swaps usually are the best solution if your car :

Would you like to make a quick “plug-and-play” upgrade?
You don’t want to make any changes while driving.
Have a warranty from the manufacturer.
Even if your car’s “bumper-to-bumper” warranty has run out, it may be covered by an emission compliance warranty (which is required by law). If you break the emission warranty with a modification, you may be responsible for the entire cost of getting your car’s breath back to squeaky clean.
Would you like to use the factory diagnostic system?
Plan to sell your car in factory-stock condition at some point.
Nothing irritates buyers more than a jumbled wiring harness or evidence of an aftermarket piggyback ECU controller.

Controllers that are both stand-alone and tunable
If you want to avoid reflashing or an EPROM swap, consider a full standalone or tunable Engine Controller Unit.

  • Want to make some changes while driving?
  • You don’t have to be concerned about warranty coverage.
  • I don’t care if the factory diagnostic system is used.
  • You don’t need to sell your car in factory-new condition.

Factory ECU reprogramming
Both reflashes and EPROM swaps take place within the confines of the original ECU. New mapping data is reprogrammed into the ECU, including :

  • Fuel
  • Ignition timing
  • Turbo boost maps


Because the ECU case looks completely stock (at least from the outside), and dealership service technicians can retrieve OBDI diagnostic codes from the tuned ECU, cars with reflashes and EPROMs often fly under the radar for routine dealer service and warranty work. Car manufacturers, on the other hand, are hesitant to pay for warranty repairs caused by unapproved tuning. To detect tuned ECUs, service centers use one of two methods:

  • Scanning with a special diagnostic tool
  • Test driving

Running the engine to the factory rev limit is a simple test. The service center opens the ECU and checks whether the EPROM has been swapped if the engine can rev past the factory rev limit.

Flashing ECU (OBDII)
OBDII-compliant automobiles (mostly 1996 and newer) can be flash programmed. Map parameters that have been re-calibrated are saved to flash memory, which is similar to the memory found in digital cameras and MP3 players.
Flash data uploads are a mixed blessing from the standpoint of the car h4cker:

  • A reprogrammed OBDII system is harder for service technicians to identify than a swapped EPROM in an OBDI system. That means warranty coverage is less likely to be cancelled simply because of being reprogramming.
  • You can’t simply swap stock and tuned chips in an OBDII car. Only a small fraction of flash tuners offer the necessary remote programming module for OBDII.

If your car’s battery is disconnected, OBDII reflashes do not go away. The learning parameters are reset to the ECU’s default when the battery is removed, but the tuned maps remain.

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