Kev.K there is a simple short answer to the question about the reliability of our oversized engines – which is – that they are more reliable.
If you want more detail - you are absolutely right krispe – the engine was designed originally as a 2.5 with Lokasil bores and ferrous coated pistons – and apart from a mistake over the IMS bearing design (that was much later resolved using a bigger bearing) –was otherwise pretty well bullet proof.
The 2.7 increased the stroke which would normally be followed by making the cylinder block higher by half the stroke increase - but wasn't and neither was it increased when the stroke was increased again for the 3.6 and 3.8. This means that the rod length and the position of the gudgeon pin to piston crown become compromised slightly from the original 'best design" – but any small reduction in performance resulting was more than made up for by the extra capacity.
The 3.2 Boxster s then increased the bore as well as the stroke but the space between the cylinders remained the same and hence the space for coolant was reduced, although the cylinders were made thicker to maintain their strength.
When the 3.4 996 increased the bore again the wall thickness of the 3.2 was not increased but used the same casting – so the cylinder wall thickness became thinner and weaker and they started to migrate oval under the increased power and some eventually cracked.
Increasing the bore once again for the 3.8 reduced coolant volumes even more and with the same cylinder wall thickness but even more power to resist – also went oval but lasted longer due to a change in the Lokasil mixture using larger particles – but then at the same time the original ferrous piston coating was replaced with a plastic coating that resulted in bore scoring when the larger particles became de-bonded from the cylinder wall - and hence the reputation we are all used to.
The original basic design – having been gradually increased by 50% was still OK apart from the piston coating, the cylinder bore material, the cylinder stiffness, the reduced coolant volume and the IMS bearing (in the original versions with the smaller bearings).
Hartech rebuilt engines address all these remaining problems. The cylinder bore material we use is Nikasil (recognised as the current best proven finish for high performance sports car engines with large pistons), the cylinders are stiffer (being aerospace alloy – solid liners – that completely replace the whole of the original cylinders and are supported at the top to prevent distortion converting the engine to a closed deck design), the cooling system is now wet liners providing increased thermal cooling and are ribbed at the top to increase the surface area (and all this is unique to Hartech cylinders) and we alter the proportion of coolant flowing around the cylinders by modifying the inlet to (and outlet from) the cylinder block which combined means the increased thermal cooling efficiency negates any reduction in coolant volume.
Meanwhile we can provide an IMS that houses the new reliable larger bearing for both the original roller chain and newer Hivo chain drives to the camshaft drive. All weaknesses are therefore addressed (and in most cases more than exceed the level of improvement needed).
One good aspect of the gradual increase in capacity of the same basic original design is that (apart from the weaknesses we have overcome) we know it works perfectly well right up to 3.8 and - because our changes more than compensate for the original short comings and result in a much more reliable and stronger engine – it can easily handle another small additional increase in cylinder diameter of 1mm (compared to a 3.8).
The biggest benefit that Porsche achieved while they gradually increased the capacity was in the improved mid-range acceleration and throttle response resulting from better mid-range torque and better top end power - the same benefit applies to our capacity increases.
This is most noticeable in the Cayman S (from 3.4 to 3.9) because it is a 500cc increase and a lighter car than the 911 variants, but the 3.4 to 3.7 and 3.6 to 3.9 produce much better performance and with several years development behind us (including testing different pistons and profiles) result in a inexpensive way to obtain better performance –if you have to have the engine rebuilt anyway. In the case of the Cayman S – the bigger performance increase cost may well be justifiable even if the engine is still OK. Inevitably a 3.8 to 3.9 conversion only shows a relatively small increase in mid range performance (being only a 2.6% increase and 100 ccs).
To follow - the 3.2 to 3.7 Boxster S should be quite some car with 500ccs extra and light weight.
The Gen 2 9A1 engines were never conceived as a 2.5 and gradually stretched bigger- but I would say were designed as a 3,8 and slimmed back to a 3.6 and smaller – so already have the basic design parameters to be stretched further (as we expect our 4.0 litre Gen 2 9A1 will demonstrate in the future).
They have a closed deck block and Alusil bores and seem extremely reliable in most cases. We have experienced some failures of the bores caused mainly by age related cylinder creep distortion that is working in the opposite direction to the M96/7 engines by closing in on the piston over a long period of time and seizing it (rather than increasing the clearances) but this problem is removed when we fit our typical Nikasil cylinders as they are re-machined after the block distortion has stabilised and fitted with a compensating clearance where required.
Baz