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bazhart
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PostPosted: Mon Oct 11, 2010 10:25 am    Post subject: 997 cylinder scoring Reply with quote

“Does anyone know why cylinder 6 is frequently scoring on the 997 3.6 engines” was asked on the Internet many months ago and since we have repaired increasing numbers with that very problem and written at length on the subject to discuss the causes and solutions.

I had hoped to wait until actual temperature tests were completed to finalise our conclusions but with so much correspondence going on since - have decided to reveal the theoretical explanation first.

Traditionally engines were designed so that all the coolant went into one part of the cylinder block and passed through it and into the cylinder head before going to the radiator. The greatest cooling is achieved by the coolest coolant – so as it heated up on it’s way through the engine – it was progressively less able to remove heat and the cylinders all ran at slightly different temperatures with the head receiving the warmed up coolant last and therefore being the hottest. The cylinders have to do a difficult job of keeping the piston face sufficiently cool to allow the oil to stop the piston seizing (or “picking up”) made worse as power increases and were traditionally cooled by 100% of the coolant first. The cylinder head (valves, camshafts etc) have a much easier life because how much power the cylinder is making makes little difference to how they work reliably (unless cavitation occurs through just running to hot).

Because the cylinders used to run so cool – these temperature differences were usually no problem unless the output was significantly increased and tuners frequently often found that after turbo or super charging standard engines – they had to alter the coolant flow proportions to balance those temperatures and avoid hot spots that limited reliability.

More recently engines have become more sophisticated and exploited the extra surface area in the cylinder jacket by reducing the flow or coolant depth in there, increasing it to the cylinder head and even flowing the coolant through a header pipe to individual cylinders - changing the amount in each cylinder and head to get a better balance – driven on partially by the increase in cylinder temperatures resulting form reduced emission regulations to avoid detonation.

The M96 Boxster and 996 engines up to 3.4 litres were typical of a newer regime with coolant flow controlled by different hole sizes in the cylinder casting and head gaskets. Because the cylinder heads are fitted opposite ways round on each side of the engine (the front on one side being the back on the other side) the gaskets had to be “handed” for each bank with different sized holes feeding different cylinders. It meant that if cylinder head gaskets were fitted to the wrong side it would reverse that balance and make the least coolant go to the hottest cylinder – but as long as they were fitted correctly the engine seemed well balanced thermally.

With the coolant pump being on the bank 1 side (cylinders 1, 2 and 3) feeding straight into cylinder 1 and bank 2 (cylinders 4, 5 and 6 which unlike bank 1 also have the piston thrust face on the hotter side – a longer coolant channel and an oil cooler) being some distance away - you would have expected the least coolant flow to bank 1 and cylinder 1 and the most flow to bank 2 and especially cylinder 6.

This is exactly what we found. Using the area of each feed hole as an approximation of the coolant flow proportions - the % of the total coolant in each cylinder of a 996 3.4 was as follows (cylinder sequence 1-6) 3%, 3%, 3%, 2.7%, 3%, 4% and each head as follows 8.9%, 11.3%, 15%, 8%, 11%, 27%. Overall head and cylinder combined the result is 12%, 14.4%, 18%, 10.7%, 14.3% 30.6% and although personally – overall - I would like to see a greater proportion of the coolant in the cylinders (and less in the heads) - no particular seizing (or “picking up”) problems emerged unless some other fault occurred like cracked liners or heads or coolant failures.

Changes to this system were found in the 996 and 997 3.6 and 3.8 engines. The size of the holes in cylinder block (feeding each cylinder) were altered slightly and the holes in the head gasket were made the same size for all cylinders (and the gasket could therefore now be fitted to either bank). With these engines producing more power and more thrust on the piston face I would have expected the proportions of the coolant going to the cylinders to be increased – but instead the size of the holes in the cylinder head gasket were made bigger – increasing the % flow to the head and reducing it to the cylinders – which will therefore run hotter with cylinder 6 being the most vulnerable.

The result is that now the % of coolant in each cylinder is as follows (3.6 cylinder sequence 1-6) 1.65%, 1.63%, 1.61%, 1.64%, 1.89%, 2.43% and in each head is as follows , 14.9%, 14.9%, 14.9%, 14.9%, 14.9%, 14.9% and the combined result in each cylinder is 16.5%, 16.5%, 16.5%, 16.5%, 16.75%, 17.29%.

Bank 2 now receives 9% less coolant than it did before and cylinder 6 (the one likely to potentially run the hottest) now receives the least % of coolant and overall receives 40% less than it did before in the 3.4 engine.

The 3.8 is similar but has 45% less coolant in the number 6 cylinder than a 996 3.4 and has 44% proportionally less coolant flow in the cylinders. I didn’t check the flow areas in the Cayman S we recently rebuilt but I think it will prove to be the same and I will check next time.

I could of course be entirely wrong about all this and it could be that adding more coolant to the cylinder heads overall makes the engine run cooler and there is some other explanation for the failures. However whenever in the last 45 years I have come across an engine engineering problem and the evidence points in the same direction as the technical analysis – it has always proven to be right.

To review it in very simple terms – engines used to have 100% of the coolant passing into the blocks first and these engines have reduced that to about 10% of the total (3.Cool, 8.5% (3.6) and 15% (3.4 996) as the engines have increased in power output and torque – at the same time as this balance of the amount of coolant passing through the block and cooling the pistons has reduced - some pistons are seizing (or if you prefer “picking up”) on the thrust face on the side worse effected by the changes and in the greatest need of cooler cylinders.

It seems to me that either some gaskets were found to have been fitted the wrong way round (and this change was thought to be a solution to that) – or the engineers went in entirely the wrong direction when altering the design (which I find unlikely) or accountants over-ruled engineers for the miniscule benefits of cost reduction through one standard rather than two head gaskets being used.

Whatever the reasoning, economics or politics – there is no doubt to me that the increased incidence of cylinder to piston face failures that causes cylinder scoring/piston seizing (or “picking up”) – is directly related to the these technical and logistical alterations – pushing the cylinder temperatures on all cylinders higher and bank two and cylinder 6 in particular.

There are not many options available to correct this. We can at present only alter the areas feeding individual cylinders during a full strip and rebuild. New, handed, head gaskets with different feed-hole sizes to each cylinder could also improve the balance and coolant volumes (cylinders to cylinder and cylinders to heads) but would require a lot of work to change as a preventative measure and since there is a small change in the hole position of one bolt – the older 3.4 gaskets cannot easily be used for the 3.6 (same bore) and modified ones are not available.

A lower running temperature thermostat – would be the least expensive option – because despite not altering the overall balance – it would at least reduce the actual cylinder wall temperatures of all the cylinders - and will be available from us soon.

Unfortunately – for us to prove the above analysis and test out the results of all these options by fitting temperature sensors to different parts of different engines and compiling the results – is a major undertaking (which is in progress) but IMHO the need for something to be done (and for owners to be more informed) is taking precedence over awaiting that confirmation and the above evidence is so compelling that I feel it serves a useful purpose to reveal and explain it all now.

My only purpose in doing this is to empower owners who may have been mislead, explain that improvements are undertaken @ Hartech when rebuilding engines, that a low cost improvement will shortly be available from us and that full actual test results will follow to hopefully verify this analysis. My reason for conveying all this now before those results are available is just because they will still unfortunately be a long way off completion.

The numbers failing are exceptionally low and in my view the cars are great cars and a reliable warranty should protect owners from this unlikely occurence.

Baz
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bluenose
Monza


Joined: 14 Aug 2010
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PostPosted: Mon Oct 11, 2010 11:04 am    Post subject: Cylinder scoring. Reply with quote

Hi Baz,

I am sure I speak for all when I say how reassuring it is to hear from experts like yourselves that are exposed to these engines in a manner not many of the forum users ever will be, regarding your experiences and research. It is much appreciated. You could IMHO continue to fix broken engines without involving your time and effort enlightening everyone else.
I understand what you are saying about the imbalance of the system and how you legitimise it. I had looked at a previous thread on the subject and had thought about lowering the overal temp by increasing the capacity of the system with an extra rad as in the x51 set up? I would be interested in the thermostat option when it is available.
I realise at some point it becomes difficult to compensate for intrinsic flaws in a system and as someone who already owns a vehicle with potential shortcomings I am very interested in being best informed about how to prevent problems. As a point of interest what is your opinion/ experience if any on the coolant additive "wetter water" that apparently has the ability to lower temps through some chemical mechanism related to reducing surface tension?
 
  
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Maxie
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PostPosted: Mon Oct 11, 2010 11:38 am    Post subject: Reply with quote

Hi Baz - thanks for this. I actually had to print out your post to read it properly! What I am understanding in reference to the 3.8L engine as found in the 997 is:

- a more uniformed coolant flow to the cylinder head (1-6 of 14.9% each) rather than the varied percentages of the 3.4L. Might be good, might not be good because in your opinion, there should be a greater flow of coolant to the actual cylinder and not the head?

- an even less percentage coolant flow to the cylinder (ranging from 1.61% to 2.43% as opposed to the earlier 2.7% to 4%).

Because of less coolant flow (in percentage terms), does this mean that a 3.8L engine is more flawed than a 3.4L engine (I'm aware you have provided stats for the 3.4L and not the 3.8L)? If the 3.8L has a bigger coolant resrvoir than the 3.4L, should this not eliminate any concerns with cylinder no.6?

I'm no engineer but any further explanations on this would be great.

Many thanks,

~ Maxie
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bazhart
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PostPosted: Mon Oct 11, 2010 1:24 pm    Post subject: Reply with quote

As the engine temperature is controlled by the thermostat a bigger radiator would make no difference to the flow rate as it would simply reduce the flow until the temperature equalled that of the thermostat.

A lower temperature thermostat would lower all the temperatures inside the engine.

My calcualtions are based on averages of castings etc so you have to allow alittle leeway over precise figures - but the point is that the proportion of the coolant going into the heads has been increased so the amount going into the cylinders (where the problem is) has been reduced.

Since the thermostat controlls the average temperature when the two coolant routes join together again - it follows that a lower flow in the cylinders will result in the temperature of the coolant coming out of the cylinders being higher and that coming out of the cylinders would be lower than say a 3.4.

With higher output the cylinders should be getting hotter and the pistons will be pushing harder against the cylinder wall - so IMHO it would have been better to make the cylinders take more coolant and less to the head (but perhaps there are other issues and I could be wrong).

The piston coating seems to be coming off as well - again through too high a temperature IMHO.

We didn't see many 3.4's with this particular problem and anyway not as early in their life - so it seems likely that there will be more 3.6's and 3.8's going this way (although total numbers will still be very small) so I expect the incidence to be higher with these later cars - hence my posting (and the 3.4 CaymanS will probably be similar).

Baz
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spyderman
Suzuka


Joined: 26 Jul 2009
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PostPosted: Mon Oct 11, 2010 3:06 pm    Post subject: Reply with quote

Are they seeing significantly more issues of failure of this nature in places like Dubai and Abu Dhabi? They do like their supercars over there. And what about places like LA where the ambient temperature is much higher and the cars end up stationary in traffic jams all the time?
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bazhart
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PostPosted: Mon Oct 11, 2010 7:21 pm    Post subject: Reply with quote

Don't know about Dubai etc but I do know that in the USA drivers tend not to rag the cars like they do over here and that there are a lot of failures reported - in fact I am sure that is where the original question came from!

Baz
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Ocean Blue
Kyalami


Joined: 04 Jul 2009
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PostPosted: Mon Oct 11, 2010 8:19 pm    Post subject: Reply with quote

Hi Baz,
My background is industrial and I found your read very informative and ask can I clarify some points with you.

Cavitation
I'm pretty good on cavitation, but fail to see how it occurs in the cooling system as I have no knowledge on engine cooling systems. The fact that they size the orifice on the gasket was facinating and I could see cavitation here but would need pressure, temperature and flow rates (volumetric) to work out if it was a weak spot, but on reflection the gasket orifice damage would be visible on dis-assembly? Would really like an insight here if you would please. I always imagined they would spot something like this and move the vena contracta further downstream.

Temperature
When your talking temperature measurement around the engine I take it your talking about placing sensors on the external part of the engine? Again while I'm not bad with PT100 RTD's and bringing the measurement out into a readable form and trending them, even using them for alarm and executive actions, again out of self interest do you measure the temp differential across each cyclinder or the inlet or outlet temps.

All good stuff Baz.

regards
 
  
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bazhart
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PostPosted: Tue Oct 12, 2010 9:09 am    Post subject: Reply with quote

That's interesting and reveals that I have probasbly used the wrong terminology.

What can occur is local bubbling or boiling just at the surface of the heat source (in this case the cylinders) resulting in bubles of air due to the relatively slow coolant speed through the block. As we use air bubles to insulate things it obviously is a poor thermal conductor and so it results in a hot spot.

Often with tuned turbos this occurs until the flow is altered to balance out the cooling more evenly around the cylinders or head.

It is difficult to measure because the coolant a little further away from the cylinder wall (where the bubbles may be) picks up less heat and so doesn't warm up as much and consequently it can fool the test results if you don't see very hot coolant temperatures but never the less have recurring seizure points (or picking up) in the same areas that you later cure by increasing coolant flow in that area. The best clue here is a lower expansion tank coolant height but seizures are so quick it doesn't take much to invoke one and can be difficult to diagnose if bubbling (or local boiling) has occured.

I think I was just imagiming that bubble cavity similar to a vacuum induced cavity which is actually different and so my description was probably misleading - sorry.

Yes the temperature sensors are fittied through the outer cylinder and head jackets into the coolant areas (with htreads and spot face location) - picking up coolant temperatures directly and both on the inlet and outlet of different cylinders. It will prove things one way or the other but you can imagine the logistics and work involved (not the least to have to own several different cars and modify them all).

Baz
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wizard993
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PostPosted: Tue Oct 12, 2010 9:53 am    Post subject: Reply with quote

Baz,

What a great report/insight to this potential problem for a minority of drivers, mainly from hot countries Thumb

My concern would be when the 997's start to age and the cooling channels start to get restricted, even by a 1-5% that the scoring issue increases.

I have always been a fan of cooling system flushes every 1-2 years but on the 977 3.8 I am coming to the conclusion that it should be done each year.

Are there any revised thermostats in the market yet that could be fitted to the 997 C4S 3.8?
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Last edited by wizard993 on Tue Oct 12, 2010 10:36 am; edited 2 times in total
 
  
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Maxie
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PostPosted: Tue Oct 12, 2010 9:57 am    Post subject: Reply with quote

wizard993 wrote:
Are there any revised thermostats in the market yet that could be fitted to the 997 C4S 3.6?


Wizard - I think if your 997 is an S model, it will have the 3.8L engine, not the 3.6L. I'm sure it was just a typo.

~ Maxie Wink
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wizard993
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PostPosted: Tue Oct 12, 2010 10:23 am    Post subject: Reply with quote

Yip Thumb

..sorted(before the Vulture Squadron swoop in hard and fast) Grin
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bazhart
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PostPosted: Tue Oct 12, 2010 12:04 pm    Post subject: Reply with quote

Yes changing the coolant is a good preventative measure and might explain why they seem to fail after 20 to 30K? (although we have had a failure at much less)

Baz
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PostPosted: Tue Oct 12, 2010 12:20 pm    Post subject: Reply with quote

wizard993 wrote:
I have always been a fan of cooling system flushes every 1-2 years but on the 977 3.8 I am coming to the conclusion that it should be done each year.


Is this a DIY job by any chance? If this is a reasonable preventative measure of cylinder scoring, how can I do it?

~ Maxie
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bazhart
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PostPosted: Tue Oct 12, 2010 3:26 pm    Post subject: Reply with quote

I am always in favour of owners doing things themselves if they can - but on this occassion I think it best to get it done at a specialist because it is very difficult to bleed the system if it has been properly emptied first and the last thing you want is air pockets in it.

Furthermore we still don't know for sure if there are any other contributory factors in the problem which a diagnostic tester may reveal.

We have now just stripped another 3.6 to find several valve guides have simply slid down the head until they sit at the bottom of the valve, and the valves have jumped around hitting the head (there is no flange on the guides to stop this happening) = scrap head. Cause - ? - temperature too high orguides fitted too loose originally - i.e. tolerances not giving sufficient grip.

Another (but a 3.4) also today - has a cracked head and the core plug has slipped out - obviously ran hot (still stripping but bronze on the oil filter looks omminous).

Also this week another 3.6 seized (or picked up) and another rather wierd one with some additive or another having been put in the oil that allowed all the piston coating to wear off and all the white metal in the shells to wear off - but was a good enough lubricant to not damage the crank or the bores - very odd indeed!

I guess for us seeing so many it seems like a lot are failing but the numbers must still be very low overall - but enough to make a reliable warranty worth seriously considering.

Baz
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wizard993
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PostPosted: Tue Oct 12, 2010 3:33 pm    Post subject: Reply with quote

Hi Baz,

Are the 3.6's you are discussing/seeing with issues, 996 or 997's?
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bazhart
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PostPosted: Tue Oct 12, 2010 5:50 pm    Post subject: Reply with quote

Both
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