Porsche engine over Revs
Rev Range Activity….a much discussed and often misunderstood topic striking fear into the hearts of those exploring Porsche ownership or those looking to renew or activate insurance backed warranty schemes on their vehicles.
We embraced the importance of Rev Range activity many years ago and our contribution towards a universal clarity has been significant. In theory, the principles behind Rev Range activity are simple, it’s the variation in data interpretation that causes issues.
Engines are designed to operate up to a maximum rotational speed. The manufacturer uses a rev limiter to prevent engine speeds exceeding a pre-determined point of safety. At full throttle and whilst pushing and holding the accelerator pedal down indefinitely the engine can’t and won’t exceed the rotational speed of the limiter. However, when a driver selects too low a gear relative to the speed of travel, the momentum of the car can mechanically drive the engine past a point of safety.
Rev Range data is stored in the cars ECU (engine control unit). The ECU is able to record and store engine ignitions at or past the car’s limiter. On a 6 cylinder engine, there are 3 ignition pulses per engine revolution. Ignitions are recorded as a cumulative total throughout a cars life.
- 986 and 996 models have 2 Rev Ranges. Rev Range 1 records ignitions at the rev limiter whilst Rev Range 2 records ignitions beyond the limiter
- 987, 997, 981 and 991 models have the ability to record ignitions in 6 different ranges. Rev Range 1 and 2 record ignitions at or just below the limiter. Ranges 3 through 6 represent increasingly higher engine speeds
Shown below are the engine speed ranges for selected models within the Porsche line-up in revolutions per minute.
Until a few years ago ignitions recorded in ranges 3 and 4 were considered by the Manufacturer to be safe with such vehicles still eligible for Porsche Extended warranty. Ignitions recorded in Ranges 5 and 6 would not be considered for warranty with existing policies invalidated at the point of over rev. Current Porsche warranty procedure ( as of 2020) considers activity in ranges 3 and 4 to be potentially problematic with compression tests and / or oil checks deemed necessary irrespective of whether or not incursions occurred several hundred operating hours prior to the check. However, in a pragmatic relaxation of previous protocol vehicles with activity in ranges, 5 and 6 will be considered for the extended warranty provided the over rev activity occurred more than 200 operating hours ago. The warranty provision is at the discretion of the importer and subject to the aforementioned compression/oil checks.
*** Porsche dealers are advised by the manufacturer to disregard over rev data on all PDK and Tiptronic transmission cars when assessing a car for warranty provision. Consequently, a PDK / Tiptronic vehicle with activity recorded as high as Range 6, assuming all other mechanical checks are passed, will be applicable for warranty*** – Accurate as of 2020.
An operating hour log captures the exact moment when the most recent activity within a given rev range is recorded. The timing of the most recently recorded over rev is as significant as the over-rev itself.
When the engine rev limiter is exceeded the engine can fail or be damaged leaving it susceptible to future failure. Instant failure would normally be caused by valves hitting a piston. Subsequent or delayed failure is often caused by a compromised timing chain which can be weakened or stretched at high engine speeds. If a timing chain were to be excessively stretched failure is possible due to the abrasive effect of a slack chain against the sprocket teeth on which it runs.
Industry experts would suggest a 50 operating hour window of danger after the point of an over rev where chains or associated inter-gear can fail or develop problems leading to costly engine work / engine rebuilds.
At 911Virgin our purchase policy prevents us from considering a car with engine over rev activity beyond the rev limiter within the last 50 operating hours.
We are regularly approached to help interpret Rev Range Data. The ability to separate genuine incident from an erroneous recording is critical. In general terms we would consider the registering of less than 10 ignitions within a given range to be no cause for concern. 10 ignitions represents fractions of a second, insufficient time, in our experience to cause damage. Porsche take a different view and consider the registering of a single ignition (one third of an engine revolution) to be relevant. A single ignition recorded in rev range 3 or higher within a 200 operating hour period means that an engine compression test is deemed necessary.
The registering and recording of a single ignition at such high engine speed, in our view, is just not possible in the same way that a single ignition in, for example, Rev Range 4 can’t follow a single ignition in the previous rev range. In order to pass in to a higher bracket of engine speed the engine must complete at least a full engine revolution, a minimum of 3 ignitions.
We are regularly asked if Tiptronic and PDK equipped Porsches can record legitimate engine ignitions past the limiter; Due to the manner in which the gearbox automatically changes up a gear when the engine speed reaches a certain point it is unlikely but not impossible. We recall a 996 Tiptronic Turbo that we travelled to purchase. On the surface a beautiful car but with 313 ignitions recorded in Rev Range 2, the most recent of which recorded within the last 3 operating hours, not a car we were prepared to buy. A wasted journey, but with the benefit of hindsight a narrow escape. Two weeks later we were contacted by the owner of the car who, after our viewing, had managed to find a private buyer for his vehicle. The buyers pre-purchase inspection picked up on an engine rattle, he pulled out of the sale narrowly avoiding a large bill because a week later the inter-gear failed and the engine needed to be rebuilt. We eventually got to the bottom of how ignitions were recorded past the red line. Whilst on a final blast before attempting to sell to ourselves, a moment of indiscretion racing a group of motorbikes saw the limiter exceeded whilst at full throttle down a long steep hill. The incline caused the car to travel faster than it otherwise would have done, the cars momentum taking the engine past its limiter damaging it in the process.
The image below shows the Rev Range print out from the car in question
The print out shows 7904 ignitions recorded at the limiter (Rev range 1). The figure to the right shows when the last incident occurred. In this instance 2262.8 hours.
313 ignitions have been recorded past the limiter (Rev range 2) with the last incident also occurring at 2262.8 hours.
The significance of any over rev can only be fully understood by considering it in relation to the total operating hours for the vehicle. In this instance the car has been used for a total of 2265.7 hours meaning the last rev range 2 activity occurred just 2.9 operating hours ago well inside our suggested 50 hour danger window. We are not able to determine the number of ignitions recorded at this moment as 313 is a cumulative total from new. It could be a single ignition, it could be significantly more but in our mind it would not be worth taking the risk.
Calculating the period of time that a car has spent at the rev limiter can also be of use. Using the example of the 996 Turbo above with a rev range 1 threshold of 6750rpm the following calculation can be used
Number of ignitions / 3 = number of engine revolutions
Number of revolutions / by rev limiter = minutes spent at limiter
Minutes spent at limiter x 60 = seconds spent at limiter.
Consequently using the above data.
7904 / 3 = 2634.7
2634.7 / 6750 = 0.39 minutes
0.39 x 60 = 23.4 seconds spent at the limiter as a cumulative total from new
It is not possible to accurately calculate the exact time spent past the limiter in range 2 due to us not knowing the exact engine speed reached on each occasion. We know it to be higher than 6750rpm but we can’t be any more accurate than that.
The maximum number of ignitions that can be recorded is 65,535.
An understanding of the significance of Rev Range activity is important but so too is a pragmatic approach. A lack of consistency from the manufacturer has turned the subject topic in to something of a hot potato, all smoke, mirrors and mystique. Despite all this the principles are straightforward and simple to understand. A don’t panic but don’t dismiss approach best sums up the methodology in respect of our buying process.
We hope this overview has been of some use.
Tom Harris – 911Virgin