Speed limiter removal in ECU software (VMAX OFF)

VMAX OFF raises the factory top speed limit in the ECU software: 250 km/h or whatever figure the car stops accelerating at while the engine still has more to give. Power stays the same. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.

How the ECU holds top speed

There is no mechanical stop. As a rule, the engine ECU receives all four wheel speeds from the ABS/ESP unit over the bus and works out vehicle speed itself. That figure is compared with the limit stored in the calibration. As speed approaches the limit, the speed limiting function trims engine torque and the car simply stops gaining speed.

On a petrol engine torque comes off through the throttle and ignition timing, and if needed by cutting fuel to individual cylinders. On a diesel the software converts torque into injection quantity, so limiting means less fuel. Saab's factory documentation for the Bosch Motronic ME9 system puts the rule simply: the ECU takes the highest torque request and the lowest of all limitations. The speed limit is one of those limitations.

A modern calibration usually holds several speed limits. In a Ford calibration on Bosch MG1, for example, the main 250 km/h limit sits next to separate limits for reverse, low range (80 km/h), the MyKey key, hot weather and transmission oil temperature, low urea level on diesel versions, and the limiter the driver switches on. There is even a driveshaft speed limit to protect the shaft. When one of these limits kicks in, it moves at a set rate rather than in one step.

Where 250 km/h and other figures come from

No law requires the 250 km/h cap; it comes from a voluntary agreement. In 1987 the big German carmakers, BMW, Mercedes and Volkswagen among them, agreed with the German government to limit even their most powerful models to about 250 km/h (155 mph). BMW regards the 1988 750iL with its V12 as the first car fitted with that governor. That is why so many powerful cars stop at the same number, and why factory packages such as the M Driver's Package raise the limit of the BMW M3 and M4 to 280 km/h.

On other cars the figure depends on the engine, market, trim and use. Commercial vans are often limited to suit fleet requirements, while trucks and buses are limited by law, more on that below. For Toyota and Lexus with Denso GEN 3 and GEN 4 ECUs the same job is a separate option, SPL OFF.

Where else the limit can live

Usually the limit is calibration data in the engine ECU flash, and changing that one file is enough. There are exceptions, though, and it pays to know them before flashing:

• EEPROM and adaptations. On a VW Crafter with EDC16, speed limits are set with a dealer scan tool, and the "legal" one is stored in the ECU's external EEPROM. Editing the flash does not touch it. Many commercial vehicles get their limit coded after sale, at the request of a leasing company or fleet. A simple rule of thumb: if the van never went faster even before any remap, look for coding rather than a map in the flash.
• Other modules. On a Peugeot Boxer or Citroën Jumper with Delphi DCM7.1, the limit is written both to the ECU EEPROM and to the body control module (BCM) EEPROM. Three reads are needed: the flash, the engine EEPROM and the BCM EEPROM.
• Gearbox ECU. Sometimes the transmission control unit has a limit of its own (such solutions exist for the DQ500 DSG, for example). Then the gearbox file changes too.

When VMAX OFF adds speed and when it does not

Removing the limit adds no power. The car only goes faster if the engine has power to spare. Aerodynamic drag grows quickly: the power needed to overcome it rises with the cube of speed. To go from 250 km/h to 270, a typical saloon needs roughly a quarter more power at the wheels.

Logs show what the car is actually running into. If speed stops dead on a round number with the pedal floored and revs below the limiter, that is the speed limiter. If revs reach the red line in top gear, the rev limiter is in the way. And if speed creeps up ever more slowly and settles on an odd figure, the engine has simply run out of power.

What changes in the file

The rest of the calibration is unchanged, checksums are recalculated and the file is ready to flash. VMAX OFF can go into the same file as Stage 1 and other options.

Tyres, brakes and the law

Every tyre is rated for a maximum speed. The rating sits on the sidewall after the load index: H means up to 210 km/h, V up to 240, and tyres for speeds above 240 km/h carry a Z in the size designation. Under European rules the tyres must match the vehicle's maximum speed, and winter tyres with a lower rating need a sticker showing their limit in the driver's field of view. After VMAX OFF the car's top speed goes up while the tyres stay the same, so check the rating before the car goes back to the customer.

Brakes, suspension and cooling were also designed around the factory top speed. In Germany, for example, ADAC points out that modifications without approval void the car's operating permit, and insurance problems can follow if tuning increases the risk.

Trucks and buses are a different matter. In the EU a speed limiter is mandatory on buses (categories M2 and M3, no more than 100 km/h) and on goods vehicles over 3.5 tonnes (N2 and N3, no more than 90 km/h). In the UK these limiters are also sealed so they cannot be readjusted. Removing or altering one is illegal, and VMAX OFF is not meant for that. Keep in mind that a van over 3.5 tonnes gross weight is already N2, even if it looks like an ordinary Boxer or Crafter.

Do not confuse the limiter with ISA, which is now mandatory on new cars in the EU. It warns the driver when the posted limit is exceeded, can be switched off and does not stop the car from going faster. VMAX OFF does not touch it.

How to order

1. Read the original file from the engine ECU via OBD or on the bench. On commercial vehicles, read the EEPROM as well.
2. Upload it to the portal and select VMAX OFF. In the comment, note the speed the car currently tops out at.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file and check the result on a closed course only.

Driving above the factory limit requires tyres with a suitable speed rating, sound brakes and suspension, and is only acceptable where the rules allow it: on a track or a closed course. Removing or readjusting the mandatory limiters of trucks and buses is illegal, and the service is not intended for that. In some countries software changes need official approval for the car to stay road legal. Responsibility for using the file lies with the vehicle owner.