Hi,
saaber1 - You said this:
"Depends what specific engine you are referring to. Porsche Boxsters and 911's in particular have failure rates sometimes quoted as high in 1 in 5 due to intermediate shaft failure, cracked piston liners, and slipped liners. This is only for about 1997-2003ish water cooled flat sixes. There are tons of them that failed within the first 30k miles even. That is why 911s of this vintage are so cheap. There are a few good articles on these failures online and some aftermarket companies have come up with remanufacture fixes for both problems. Older air cooled ones are pretty bulletproof."
Sadly this is not the correct situation - and certainly not a major failure rate of 25%. With the M96 engine family there very few IS failures and a number of engines were replaced by Porsche. This engine was a radical new design from Porsche and a departure away from air cooling. These were random failures and a number of reasons were given. It was more prominent on Manual vehicles and particulary on the bored "S" - and not too dissimilar to some issues with the 928! The leaking RMS was often thrown into the mix - this was a relatively minor matter and again random in nature. These were corrected under Warranty. Some cracked and chipped liners (ends) were also noted. There were variously attributed to contaminated fuel and suspect manufacturing-machining processes. Slipped liners.........?
You also said:
"Older air cooled ones are pretty bulletproof."
This is simply not the case and Owners will speak openly about valve component, heads and other expensive "issues". One such expensive thing is the main web and case caused by distortion over time and use. This is a common VW and Porsche issue going back to the 1950s and the 356 is the case of Porsche. Other were chain tensioners, rapid valve guide wear and head studs and top end sealing matters
No engine design is perfect! By and large boxer engines are a great design and everybody will have an opinion of course - this is mine, laced with some other data
If I was to design an engine for today's environment from scratch for a mass production small/mid car it would be a boxer! IMO Subaru's new diesel range is a great starting point
The German Auto Industry's "public notaries" - Horsch, Porsche, Benz, Maybach and etc all flirted with boxer engines
Porsche's design criteria in the 1930s for a "Volkswagen" casued most German Auto Manufacturers to submit examples with horizontally opposed engines. That the VW emerged after WW2 to become a "universal" vehicle says a lot for the design of the engine - as well as the rest of it
It's not really simple to compare modern boxer engines from Subaru and Porsche with those of even two decades ago. Today's engines are more reliable and longliving. They can have excellent fuel economy and typically they have much longer lives to first overhaul. M96 engines have been taken aprt with 300k miles on them and no measurable liner wear
My first involvement with VW engines occurred in the 1950s, Porsche in the 1960s and Subaru in the 1970s. One of my mechanics (in New Zealand) successfully raced one of the first Subaru series cars - we both gained a lot of knowledge from that!!