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bazhart
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PostPosted: Tue Aug 09, 2011 9:25 am    Post subject: How to possibly avoid cylinder scoring Reply with quote

Although our low temperature thermostat is something that we think could make these engines less likely to score bores - I have also been trying hard to work out what driving conditions might be more likely to lead to scored bores in the first place – to be able to offer some advice on how best to avoid the consequences. Feedback suggests it is not people driving around in track situations that are particularly any more likely to experience this problem than anyone else – so it does not seem to be purely down to revs and hard driving entirely.

Furthermore – for those only ever driving on public roads - for some to fail so soon while others never seem to – or when some owners never have the problem while others do – there must be some differences - and if we could identify them (and back them up with technical/scientific support) it could possibly help avoid the consequences altogether.

I think I may have worked out a typical scenario – best avoided – here is the evidence and connected technical issues.

Scoring was more commonly caused by full piston seizures in which the piston grew too big to fit in the bore with some clearance and the resulting additional force between the piston and cylinder wall squeezed out the oil film resulting in metal to metal contact – the increased friction causing local melting of the piston surface and scoring both piston and bore on both sides. Dynamometer tests I have previously carried out to destruction (to measure temperatures etc) showed this cycle took about 4-5 seconds.

The odd thing about this current problem is that one side of the piston and bore looks exactly like a fully scored seized piston but the other side is perfectly OK and untouched. It is therefore clear that when the scoring occurs – one side of the piston is sufficiently cool to lubricate the piston/bore face and support the piston whereas the other side has become excessively hot and the oil film cannot support the loads applied – which result in metal to metal contact – overheating of the surface and scoring the bores and piston on one side only.

It is always the thrust side that scores (proving the additional load between the piston and cylinder wall is the problem) but it cannot be the piston temperature overall – or the other side would also seize. Although the thermostat controls overall coolant speed (from the radiator efficiency) the coolant flow in individual parts of these blocks is much slower than traditional engine designs (because it is split into 12 separate flows streams instead of one (each therefore at least 1/12 of the speed) – the 6 through the block being very much slower than the 6 through the cylinder heads and the individual speed past any one cylinder many times slower) so the temperature of the coolant as it slowly passes over the outer cylinder wall increases more giving a greater temperature rise in any one area. It is almost always bank 2 (because on that bank the thrust face is on the hottest side of the block – as the coolant is about to leave the cylinder having picked up all the heat on its travel through the block - whereas on bank 1 the coolest coolant enters on the thrust side).

For the cylinder wall to get so hot on one side that the engine piston scores must require (IMHO) some bubbling of the coolant on the surface of the outer cylinder wall and this usually only occurs when coolant speeds would be very slow indeed and bubbles would naturally rise to the top (the thrust side on bank 2 – not on bank 1). But it would also need high piston to cylinder wall forces as well to squeeze out the oil film and make hard contact between the piston and Lokasil bore.

Centrifugal coolant pumps have relatively low pumping speeds until they reach a critical speed after which they suddenly increase flow dramatically.

Radiators cool more when the road speed is higher (when their efficiency increases).

The engines in question ( 3.4 Cayman S, 996 and 997 3.6 and 3.8 ) produce more torque at low revs than their predecessors.

There is a piston cooling affect from inlet charge when the throttle is open momentarily lost as the throttle is closed.

Spray jets in the block lubricate the cylinder wall but have ball valves that require some oil pressure to open and the oil pressure and delivery is low at low revs – rising until the pressure relief valve opens when it is constant at much higher pressure and delivery providing much better cylinder wall lubrication.

Crankshaft speed also distributes more cylinder oil spray at higher speeds.

The relatively long distance from the engine to the radiators reduces cooling reaction time.

The thermostat is on the way back to the engine so if the engine is ticking over after a run the cooler coolant that has just come from the radiator will slow down as it reaches the thermostat and for a while – will not encourage it to open more – in fact it would probably start to shut a little.

Putting all this together – if we had a scenario in which the engine was under quite high load and road speed but then it suddenly went to tickover speed for a short while – at this point the piston would be very hot (no inlet charge), the coolant speed would be very low (pump slow), the heat passing from the cylinder outer wall to the coolant would still be high (probably leading to bubbling which is very poor for heat dissipation), the oil spray cylinder wall jets output and crankshaft rotational speed would be very low (so reduced cylinder wall lubrication) and while the coolant in the block would be very hot the coolant near the thermostat would be quite cool.
If just then the car was driven off under high torque/full or nearly full throttle (not necessarily at high revs) the pistons would instantaneously heat up, the dynamic forces between the piston and the cylinder wall would be at their highest, the cylinder wall and oil film temperature would be very high, the thermostat would not be fully open, the car would not yet be at a high enough road speed for the radiator to cool the coolant effectively and the oil spray jet and crankshaft speed splash lubrication would not yet be good.

So – put simply – I think that if the owner of one of these models – on a spirited drive - could not then resist the temptation to floor the car away from standstill (at say lights or junctions) - then I think we could have the worst possible scenario for piston/cylinder scoring at that precise moment.

This would also explain why – once the models are up to temperature and having had the speed gradually increased around say a circuit or long unrestricted roads (say in Germany) they seem to be OK despite the extra strain put on the engines (because thermostats are more open, radiator cooling is good, engine speed is high so oil spray etc is good et)c.

I agree such a proposition is almost madness – I can almost hear you all shouting “what the bl**dy hell do you think I bought it for” - why would anyone buy such a car if they cannot at least feel that acceleration and enjoy that performance (not to mention the satisfaction of showing everyone just how quick the car is when in reality none of us can safely or legally demonstrate anything like the cars top speeds anywhere in the UK – but we can out accelerate most others). Surely that exact scenario is partly why we bought it and high on the list of the pleasure we get from driving it and possibly the only occasion we can stuff it to other drivers! I am not against that – in fact I also love it myself (truth be known).

So I am not proposing you all drive around like old men in a Ford Fiesta – (I am of that age – but still not a slow driver either)but I am just suggesting that – it makes sense to me – that weighing up all the evidence and connecting it to solid technical analysis - this is the most likely moment of failure and therefore – it could be that if you could simply accelerate from a stationary rest - a little more gently – under slightly less throttle opening -for a few seconds until the coolant speed, oil pressure and thermostat opening times and the balance of the oil and lubrication system have equalised again – it may just save the expense and disappointment of an engine failure.

Baz
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apw2007
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Joined: 29 Apr 2008
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PostPosted: Tue Aug 09, 2011 9:47 am    Post subject: Reply with quote

Finally a post that makes sense (to me at least) - I assume the launch control button should be left well alone?

Can you explain (briefly) why the same does not happen in the GT1 engine?
 
  
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spectraluk
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PostPosted: Tue Aug 09, 2011 10:38 am    Post subject: Reply with quote

I assume also therefore that after a 'blat' and increased temperatures, the best scenario would be to come to a complete stop, switch the car off and let it cool naturally, rather than continue to drive, at a slower pace?
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Maxie
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Joined: 11 Mar 2007
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Location: London


PostPosted: Tue Aug 09, 2011 11:18 am    Post subject: Reply with quote

spectraluk wrote:
I assume also therefore that after a 'blat' and increased temperatures, the best scenario would be to come to a complete stop, switch the car off and let it cool naturally, rather than continue to drive, at a slower pace?


Surprised That's exactly the opposite of what I do......After a blat I keep the engine running for about 90 secs before turning it off. Always thought that was the best thing to do....no?

Otherwise, great post Baz.

~ Maxie thumbsup
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spectraluk
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2004 Porsche 996 Carrera 4S

PostPosted: Tue Aug 09, 2011 11:21 am    Post subject: Reply with quote

lol, me too generally Maxie, but after Baz's post - I'm not so sure now??
doesn't appear to be a good thing to be driving the car whilst very hot, at slow speeds...??
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JohnB
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Joined: 10 Apr 2009
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Location: Suffolk


PostPosted: Tue Aug 09, 2011 3:56 pm    Post subject: Reply with quote

I thinks it's down to interpretation, after reading Baz's post, I though the gist was if you gunned the engine hard i.e accelerating away from lights then came to a rapid halt with the engine ticking over you were putting the engine at risk as the cooling / lubrication systems would not be operating efficiently and would not be capable of dissipating the heat etc.

i.e warm the engine progressively before giving it the beans, then permit the engine to cool progressively before turning it off.

After a blatt, on the run home I take the the last 3-4 miles easy to let the engine wind down. A bit like after having a session in the gym and stretching down afterwards. Or am I completely barking.
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spectraluk
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2004 Porsche 996 Carrera 4S

PostPosted: Tue Aug 09, 2011 4:06 pm    Post subject: Reply with quote

Can't comment on whether or not you are completely barking.. we've never met... puh

get your gist though...
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PeterS
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Joined: 01 Nov 2009
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Location: Solihull

2003 Porsche 996 Carrera 2

PostPosted: Tue Aug 09, 2011 4:07 pm    Post subject: Reply with quote

I think what Baz is trying to say is that you should treat the car like a race car, (or an athlete), let it warm up by driving it gently, and the same with the warm down.

EDIT Sorry, starting writing this before JohnBs post, then the phone rang, I got sidetracked, and now it looks like I am copying him!
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infrasilver
Fast & Furious
Fast & Furious


Joined: 04 Oct 2010
Posts: 8496
Location: East Midlands

2001 Porsche 996 Targa

PostPosted: Tue Aug 09, 2011 4:37 pm    Post subject: Reply with quote

I have been having a think about this since I have had my car and experimented myself with temperature, whether to let it cool or turn it off.
My conclusion was switching the engine off whilst it is still very hot, the heat will still be localised to a particular area but the piston will not be moving up and down in the bore so is unable to score anywhere.

I don't like the idea that having a high performance sports car that I'm not allowed to drive fast from a standstill or drive hard then get stuck in traffic without taking the risk that I may be damaging my engine.

My idea has been to fit an inline oil cooler located at the rear of the car somewhere and have something like a take off plate to feed the cooler, I think this could be a solution along with the thermostat modification.

Or is this just the ramblings of a mad man. Dont know
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bazhart
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PostPosted: Tue Aug 09, 2011 6:03 pm    Post subject: Reply with quote

I didn't suggest the engine scores while it is idle - but when after idling following a spirited drive you then go flat out again from a standstill rather than work it up to speed again a little more progressively.

I like the analogy with an athelete/sportsman - warm up - then perform - then cool down progressivley - the trouble is you cannot avoid stopping when the lights are red, the junction is busy or the traffic in fron just stops - but you can avoid flooring it imediatley it the congestion free's up - just a suggestion - I am actually better off if as many people get scored bores as possible - but I am trying to help them avoid it.

Baz
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apw2007
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PostPosted: Tue Aug 09, 2011 6:35 pm    Post subject: Reply with quote

That would appear to be a fundamental flaw .... wonder how it would stand up against 'fit for purpose' wack
 
  
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Wattie
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PostPosted: Tue Aug 09, 2011 7:29 pm    Post subject: Reply with quote

I think this is a 'characteristic' of the boxer engine design to be honest and the prominence in water cooled cars are due to the water cooling actually causing the temperature changes that Baz speaks about.

I believe the worst scoring is on the side of the engine where gravity is adding to the thrust forces on the side of the cylinder? In a vertical or narrow-V engine, the gravity effect on the side walls is massively reduced (and actually helps the thrust forces to produce engine output - unlike in a boxer engine, where it works against it by creating sidewall friction.)

I wonder if it would be possible to devize a more sophisticated method of controlling engine temperature than a cheap, old-fashioned, thermostat? How about electronic control of the cooling system to stop the bores from cooling too much when idling with a hot engine? The alternative, of course, is to sit there blipping the throttle to 5,000 rpm to keep the oil pressure up...


(That's my excuse and I'm sticking to it... Thumb )


PS. The slightly worrying thing about Baz's theory is that this would make 996 and 997 Turbo cars more susceptible to scored cylinders?
 
  
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bazhart
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PostPosted: Tue Aug 09, 2011 7:56 pm    Post subject: Reply with quote

No turbos etc and gT3's have entirely different cooling flow, cylinders and balance.

Baz
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Chaddy100
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Joined: 04 Nov 2010
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Location: Stamford


PostPosted: Tue Aug 09, 2011 9:36 pm    Post subject: Reply with quote

Speaking as a new owner of a 997 3.8 2005 car I think Baz's views make a lot of sense and its good to hear from someone who is clearly well placed to comment on this worrying subject.

If only we knew just how prolific this problem really is! Anyone care to take a calculated guess? 10%, 20%, 30%? Dont know

No wonder Porsche are keeping stum on this issue - bet they're loving the profits from the warranty money nooo
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MisterCorn
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2004 Porsche 996 Turbo

PostPosted: Tue Aug 09, 2011 9:42 pm    Post subject: Reply with quote

So to boil it down to the basics. Don't thrash it from idle (such as after sitting at traffic lights) IF the engine is hot. Good description and makes perfect sense. thumbsup

MC
 
  
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GT4
Nordschleife
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PostPosted: Tue Aug 09, 2011 10:13 pm    Post subject: Reply with quote

Yet another reason not to buy a London car!
 
  
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MisterCorn
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PostPosted: Tue Aug 09, 2011 10:15 pm    Post subject: Reply with quote

GT4 wrote:
Yet another reason not to buy a London car!


Yet another reason to buy a turbo / GT3

MC
 
  
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infrasilver
Fast & Furious
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PostPosted: Tue Aug 09, 2011 10:16 pm    Post subject: Reply with quote

MisterCorn wrote:
GT4 wrote:
Yet another reason not to buy a London car!


Yet another reason to buy a turbo / GT3

MC


So very true.
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GT4
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PostPosted: Tue Aug 09, 2011 10:20 pm    Post subject: Reply with quote

So a rural GT3 it is!
 
  
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apw2007
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PostPosted: Tue Aug 09, 2011 10:24 pm    Post subject: Reply with quote

GT4 wrote:
So a rural GT3 it is!


Let me have another look at that Ferrari wack
 
  
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