Helmsman wrote but simply curiosity of your S65 vs S85 view and why the latter seem to fail more.
Cheers
Nik
Hi Nik...I can give you a view but as I haven't really followed the S85 story that much, I can't say how well it will stand up to scrutiny. But anyway...
IIRC BMW built the S85 to try and cash on its F1 programme by building an engine loosely based on its F1 V10 race engine....and I have the impression they may have packaged it a little too tightly...and the original design would have benefitted from a wider rod bearing crank journal.
In any case they went with a rod bearing clearance that is popular in mainstream car manufacturing (although mostly in conjunction with a lower viscosity oil).
As always, warranty replaced engines were returned to the factory for disassembly to discover what, if anything, could be done to reduce the cost of warranty engine replacements by redesigning components or changing clearances.
So when it came to designing the S65 they had a quandary...they knew that the S85 engine was susceptible to certain failures but by using a cut down version of it, it saved a fortune in R&D costs compared to a whole new V8 engine design.
So BMW worked on the weaknesses they could, changing the high pressure Vanos and upgrading the ion sensing anti knock hardware. The marginal ability of the rod bearing oil film to resist high combustion pressures (showing as accelerated rod bearing wear) however couldn't be resolved, without a costly engine redesign so they settled with optimising what they had (improving the anti knock system).
They already had a bearing clearance they were happy with, increasing it would not make the bearing wear rate any better...increasing bearing clearance reduces the area of the oil film area that is under load. And they needed to keep the Castrol 10W60 oil because it has a robust oil film strength under high load/temp.
The downside of 10W60 oil is that the engine must be warmed up before subjecting the engine to high loads..so BMW designed a warning system into the rev counter in the E9x M3s...something some owners likely ignored.
So back to the question...why do S85 seem to fail more.
Obviously they are an older engine, more miles more hours.
I believe the high pressure vanos was problematic.
As were injector problems.
And I think the ion sensing anti knock system (unpopular in mainstream car manufacture) was far from perfect in the S85 design.
Logically BMW optimised that design for the S65 as best they could at the time and as a result the S65 does seem to be more reliable.
I think its likely that the S65 engine failure rate (due rod bearings) will slow down now as the majority of cars that left the factory with out of tolerance rod bearing clearance due manufacturing inadequences have already failed (by 60k miles it seems) and then as cars get older and acquire more miles the engine fail rate will eventually start to pick up again on cars with original bearings.
I didn't track S85 engine failures but it wouldn't surprise me if it followed a similar pattern.
It is my belief that the changes in the hardware of the ion sensing anti knock system had benefits especially when combined with the various software updates issued by BMW. The principle benefit being better combustion control and therefore less variance in the load on the rod bearing oil film.
I believe the (now dropped by BMW) ion sensing anti knock system was in the early years not especially comfortable with changes in fuel octane rating and volumes of ethanol...but it did seem to be better after the mid 2012 software update. That the EU enforces quite strict control over fuel in its markets, particularly octane and ethanol levels (compared to the USA) may account for the lower incidence of engine failures in the UK.
It could also account for why engines that have had replacement OEM clearance spec bearings fitted, look fine when later dismantled...although that assumes they benefited from running the later software (and that made a difference).
In any case both the S85 and the S65 seem to have a marginal ability of the rod bearing oil film to resist high cylinder combustion pressures...and BMW with all the data, decided increasing the clearance was not going to help that.
