I make a contribution to this post with some trepidation that it will be misunderstood – but it is important for owners to recognise and accept that there are many influences on engine life that are outside of their control and may result in an early failure, a very long life or – for most – an 'average expectancy". This means that while there will be reports that many engines have covered huge mileages there are also others that never managed to get past 20K – the why's and wherefores are difficult to quantify or provide reliable correlations for or generalised advice.
What is for sure is that there are more 'weaker areas" than some older Porsche models and that means that the car's previous use (before the present owner) will have more of an influence of life expectancy as will the general quality of some of the original build and components.
Although older models were almost bullet proof, cars that have been respected with owners with mechanical sympathy who always warmed up their cars properly and had good quality maintenance carried out and then were driven reasonably within legal limits, will on average last much longer than those whose first owners thrashed them from cold knowing they would not be the owners when the consequences materialised (or who afforded the Porsche warranty – so were not too bothered about long term consequences anyway).
The difference in engine loads, wear rates and thermal stresses magnify exponentially with throttle opening, revs and top speeds – so unless you know for sure the total driver history including the driver's attitudes from new – it is impossible to assess the likely lifespan.
With the crankshaft (in common with most recent engine designs from all manufacturers) having thinner bearings (between 57% and 70% of the 944 size for example) and the crankshaft only 1/3 as stiff – and with the flywheel overhanging by 3 times the length - much more wear on the bearings will result – 'IF" the car has been driven really hard – especially from new. Because of the older models 'over engineering" the magnifying affect of hard driving was not so relevant – but although these newer engines driven with respect will still last a very long time but those driven very aggressively will not.
The thrust loads on the cylinders (earlier 3.4 engines) and the wear on piston coatings (later models) all increase exponentially with revs and throttle opening leading to premature cylinder cracking or bore scoring while the temperature gradients inside the engine react with increased stresses on a lot of components.
If an engine is over engineered it can stand the stresses of aggressive driving much longer than one designed to a finer tolerance level – so it is not unusual with more modern engines for there to be some with short lives and some with very long lives and the only advice experienced experts can provide is the 'average" they work with. This is in itself flawed because they tend to only see the engines that have so far failed and therefore their experience is focussed on a relatively small and select group. It is most likely however that those they do repair had experienced a harder life than those that survive longer.
The IMS bearing was probably one of the few areas that this reality did not apply to – being much more random – but it is not a common failure and the higher the mileage it has survived the less likely it is to fail – making the most common failure now being 3.8 bore scoring. However neither bore scoring nor even cracked cylinders in earlier models are too catastrophic and rarely wreck the engine – usually making a post failure rebuild viable.
In contrast the crankshaft bearings are critical because if they wear prematurely the result can wreck the entire engine.
As engines wear the clearances between the crankshaft bearings and the piston to bore increase while piston coatings reduce and the hard silicon cylinder particle loss increases – so in both cases changing to a thicker oil viscosity helps prolong lifespan. Cars used lightly may very well still cover very high mileages on the original oils but those driven more spiritedly will benefit from an oil upgrade and the sooner that was done the longer they are likely to last. Similarly the fitting of a LTT, cleaning radiator blockages etc all prolong potential lifespans.
If at any time in the car's life - it suffered a failed coolant pump, blocked or leaking radiator, hose or header tank, failed radiator fan, low coolant, low oil level, extended oil change period etc – that all eventually influences long term life expectancy.
Owners buying older cars (on average and I accept there are always exceptions) tend not to abuse them as much as some previous owners and often do all the right things to preserve their car but this cannot completely repair damage and wear that has already taken place.
This all means that if you are not sure about repair history, previous owners care or driving style - your engine could be nearing the point at which the crankshaft, or cylinders may fail and if you intend to keep the car it could cost you less to have a rebuild at the right time than risk a more serious failure - providing a reliable car for your enjoyment and benefitting re-sale values as they appreciate with time.
Since this post has been about 100K cars – can I respectfully advise that despite the reported exceptions - this is a mileage at which the average will have worn crankshaft bearings, oval cylinder bores, piston coating loss and yet can still be rebuilt to a reliable standard at moderate costs and result in a brilliant car to own and drive. Around and over this mileage we are beginning to experience serious failures and the need to replace expensive parts that would have been Ok if rebuilt slightly earlier.
My concern is that too much confidence generated because some cars have managed to cover high mileages may result in more owners pushing their boundary of expectations towards higher rebuild costs for those that leave their own engine too long before rebuilding it.
Like all risk assessment it is impossible to be sure of the outcome until failures occur but also like all good and reliable preventative maintenance – averages and experience provide the best guide to the right mileages to act. Lower oil pressures on tickover, increased oil consumption and reduced performance can all help but ultimately you always have 2 choices, rebuild before failure or afterwards and at over 100K – my advice is to seriously consider a reliable pre-emptive rebuild that also improves the mechanical specification from the original.
These fantastic cars still look and drive brilliantly after nearly 20 years (and are usually in better overall condition than the older examples) so there is no reason not to expect them to eventually enjoy the appreciation in values that help justify engine work (and other contributors are quite right to point out that even the older cars had various issues needing engine attention eventually) it will just be better with this model range to get it done earlier rather than later.
Can I also add that this advice is not in any way going to increase our business turnover or profitability. We are flat out all the time rebuilding engines that have failed and those that have more serious failures cost more to fix and actually increase our turnover and labour content. With all existing M96/7 cars covering ever increasing mileages the numbers needing repair will continue to rise and with oversized engines coming on line, increased numbers of competition engine rebuilds being requested (as a result of our continued success on track) and developments under test for 9A1 (Gen 2) engines – we are already at maximum capacity with no desire to expand any further.
But we make our living from Porsche cars and prefer for our customers to still hold the marquee in the same awe as we do for fantastic engineering of superb cars that look and perform brilliantly at ages and mileages that most other sports cars have been scrapped at and to provide a service that benefits them by rebuilding for the long term at the right time rather than be involved in a complete disaster that destroys confidence and ultimately damages reputations.
Over to you - I'm off to the bomb shelter!
Baz