It must be a perverse connection between pre-ignition within the fuel system and the oil specification within the lubrication system, normally kept apart.

Not really, oil is spread by the oil control rings onto the cylinder walls as the piston rises and falls. If that oil atomises under compression - especially with high compression pressures in the cylinder - there's a risk of pre-ignition.

Very little oil gets into the cylinder - otherwise oil consumption would be excessive.

What has the stresses in the various subcomponents of the engine got to do with the combined swept volume of the cylinders? Nothing would be my opinion. A 1.0 turbo engine where the conrods, gudgeon pins, piston crowns, crank shaft etc etc are sized correctly to the anticipated loading is no more stressed than any other engine regardless of displacement.

Back to the topic at hand, An engine may be designed to “tolerate” RON95, which is not to say that it runs optimally on it. In my own experience, I feel that the low speed operation of my engine improves, when running on 98 vs 95 (Mazda Skyactiv G 165, admittedly an NA engine, but one with high compression), just has that little bit more urgency down low. I wish this was simply placebo effect, as I would rather like to spend less on fuel, but the butt dyno doesn’t lie :-)

Brake mean effective pressure inside the engine cylinder is typically around 150 psi for a N/A petrol engine. I remember reading previously that Fords 1.0 Ecoboost for example, could run BMEP at around 350 psi. The cylinder pressure is higher, but the surface area of the piston crown is smaller vs an equivalent power larger displacement NA engine, which has lower in cylinder pressure, but a larger surface area to react it on. The outcome Is that the force transmitted into the conrods below may not be all that different.

Steel conrods (assuming low alloy steel AISI 4340) would have an allowable tensile/compressive yield strengths of over 200,000 psi, three whole orders of magnitude higher than the gas pressures inside the cylinders. Unless the con rod cross section is really really tiny, stresses across it due to gas pressure loads are not huge. Inertia loads as the piston is accelerated up and down the cylinder are much more significant.

Steel, like titanium alloys, have an endurance limit below which an infinite number of stress cycles can be applied without causing fatigue damage (typically around half of tensile strength for steels). Conrods are almost certainly designed so that this is the case. Engines can expect to go through 100’s of millions of revolutions over their operating lifespan without fatiguing to pieces. Aluminium doesn’t regrettably have an endurance limit so alloy conrods are only really suitable for race engines, where the lighter weight matters more than longevity.

I guess the point I’ve laboured on is that provided that an engine is designed correctly (and I trust my colleagues over in this industry), a turbo’d engine is no more stressed (and by stress, I mean mechanical) than a larger N/A engine, its all in how the bits are sized for their loads. Now thermal loading and cooling is another kettle of fish and one that I’m not particularly au fait with, maybe someone else on here could comment?

Many thanks for the discussion.