"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 (3.8), 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.
Baz
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 (3.8), 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.
Baz
