Every water-cooled 911, Boxster and Cayman from the 996 and 986 on keeps a record of over-revs in the engine computer. Air-cooled cars don't record anything. Buyer usually gets this report as a printout or phone photo from the seller, ranges with numbers, no explanation. Most people read it wrong, both ways. Some walk away from good car because of range 1. Some buy hurt engine because numbers in high ranges look small.
First what the numbers are. DME does not count events. It counts ignitions in each rpm range, from rev limiter up. Six cylinder fires three times every revolution, so at 8,000 rpm it is 400 ignitions every second. 20 counts in a high range is about a twentieth of a second. 500 counts is more than a second. Counters stop at 65,535.
Ranges are different for every engine. The 996 and 986 have two. The 997, 987, 991 and 981 have six. The 992 has three. On the 718 and GT cars it depends on model and year. Rpm of each range changes from engine to engine too, so never read report of one model with numbers of another model.
Range 1 is the rev limiter. Every time engine touches the limiter, it counts there. This is normal, engine is made for it, and most cars have some range 1. Older car with zero in every range, I look twice, is this the original DME.
High ranges are different story. Limiter cuts fuel, but it can not stop the engine when the wheels are turning it. Let's say you go for fourth and catch second on a manual. Engine spins way past the limiter and computer simply can not do anything. This is mechanical over-rev. It lasts fraction of a second, so the count is small, but the range is high. Small number in range 5 is worse than big number in range 1. Porsche itself wants their technical hotline involved for range 2 on a 996 or 986, and for ranges 4 to 6 on a 997 or 987.
Each range also saves engine hours of its last over-rev. For me this is the most important number on the report. I compare it with engine hours now and I know how long ago it happened. Porsche uses 50 engine hours as a guide. When engine fails within 50 hours after high range over-rev, Porsche calls the link highly probable. When two or three ranges show same hour stamp, usually it was one event. And no range can show last over-rev at more hours than engine has now. If it does, DME was swapped, hours were changed, or data is bad. On a 992 the stamp can be in minutes, so check the unit.
DME writes the counts in time steps. So high range can show counts while the range under it shows zero. Porsche says so in their own papers, report is not broken.
Tune with raised rev limiter changes the picture. Ranges don't move with the limiter, so normal limiter hits now land in range 2 or 3. Range 1 full at 65,535 with a lot in range 2 is typical sign. This is why I read the DME software too. Software that doesn't fit the car is a question for the seller.
PDK and Tiptronic cars over-rev too. Wheelspin over a crest, limiter held in manual mode, car landing after it left the ground on track. Porsche's own over-rev bulletins make no exception for them, so as a buyer I would not ignore it.
Data is not perfect. Report lives in the DME, not in the engine. After engine swap it still shows the old engine, and after DME swap it shows another car. Jump start or voltage spike can leave wrong data too. Report does not clear with fault codes, and idling does not clean it. Idling only adds engine hours, so old over-rev looks older. One more reason I divide miles by engine hours. Normal mixed driving gives average around 25 to 40 mph. Far under that means a lot of idling, or question about the odometer.
After engine replacement or rebuild in my shop, I can reset the over-rev memory. But best practice is to leave it as it is and document it. Before old engine comes out, I save the whole report with hour stamps and engine hours. It goes on the work order with date, mileage and both engine numbers, and owner gets a copy for the car file. Anything with hour stamp after the swap happened on the new engine. Reset with no papers behind it, for me it looks like somebody hiding a mistake.
When high ranges have counts, report alone is not enough. Porsche's own list of over-rev damage is long: damaged valves, pistons and cylinder walls, rod bearings, loose rod bolts, loose crank pulley and cam gear bolts, cracked timing chains, cam timing that moved, clutch and synchro damage. So engine gets compression and leak-down, cam timing gets read live, cylinders get the borescope, and on manual car I feel the synchros on the drive.
If you buy from far away, ask for the report with hour stamps and engine hours now, not only counts. Photo of the tester screen is better than numbers written by hand.
Range 1 is normal. High range, for sure somebody made a mistake, and then engine gets checked properly.
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