Porsche Engine Rev Limiting Systems
Engine Rev Limiter systems started in the early Porsche 911’s as a mechanical sliding ignition rotor whose sole job was to cut power in the event of an engine RPM over-rev. A section of the ignition rotor would slide outwards driven by centrifugal force depending on the engines RPM speed and would eventually make contact with a ground terminal, shutting the engine off until the engine reached a safe RPM range. The simplicity of the past is now gone and replaced with an extremely accurate electronic control system. The rev limiter for the 9X6’s and 9X7’s utilize a fuel/ignition cut off system operated by the DME (Digital Motor Electronics) controller that engages when the engines RPM reaches a model specific maximum limit.
In addition to the current rev-limiter controls, today the DME keeps a record of all engine over-revs. The over-rev capture capability should be checked during a PPI (Pre-Purchase Inspection) because these can be a valid data point in determining if a used 9X6 or 9X7 model Porsche is a good one to purchase or not. This information can be utilized to tell if an engine has been over-revved, and just as importantly, if a car has possibly been raced or at least tracked. Certain over-rev values will usually void the manufacturer’s new car warranty.
The 9X6’s are the first Porsche’s that have this RPM over-rev data capture capability. The over-revs are broken down into Range #1 and Range #2 segments. The 9X7’s have a more refined over-rev data acquisition system, breaking the previous range #1 into 3 segments known as ranges #1-3 and the previous range #2 also into 3 segments known as Ranges #4-6.
Range #1 over-revs are OK: these are benign and within the rev limiter’s control. These occur when the engine power is reduced by means of a DME random cylinder cut out via fuel and ignition interruption. In the 997, this range is broken into three finer segments known as ranges #1-3.
Range #2 over-revs are not OK: these are dangerous and are known as a mechanical over-rev. These occur above and beyond the control of the DME’s rev-limiter. The only way to have a range #2 over rev is to put the car into an incorrect lower gear at high speed and/or a few select very dynamic driving situations. In the 997, this range is broken into three finer segments known as ranges #4-6.
It is not possible to over rev the engine in a Tiptronic (automatic) transmission equipped model. Note, the engine over-revs are counted in ignitions or spark plug firings. An important thing to watch for is extremely high number counts in the lower benign ranges, i.e. Range #1 in the 9X6, or Ranges #1-3 in the 9X7. This generally means that the car was driven very aggressively or it was tracked and the driver was using the rev limiter as a shift point indicator.
Tony Callas
Tom Prine