Quote:
Originally Posted by Alan Smithee
Unlike gasoline turbos, BMW has been making turbo diesels continuously for years. I am no expert on the subject, but diesels typically use lower revs, lower boost, and therefore generate less heat. Heat is what will shorten the life of everything in the engine compartment.
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Since you bring up revs, let's consider two high output BMW engines.
The V10 S85 in the M5 had 5 litres, and 500 hp. Specific output of 100 hp/litre.
My N54 in the 535 had 3 litres, and 300 hp. Specific output of 100 hp/litre.
Those two engines were offered in the same 5 series chassis (with some mods for the M5, obviously, but it was the same physical space). Which do you consider would be more reliable, the V10 turning at 8250 rpm redline, with 10 sets of piston/conrod/etc, or the turbocharged 3 litre engine with peak torque reached at 1200 rpm, and only six sets of pistons/conrods/etc?
Obviously BMW could have built a 5 litre turbocharged engine to get 500 hp (and they did a few years later) but comparing these two in terms of specific power output, since they were available during the same time period, I would take the N54 over the S85 every time for reliability. And for fuel consumption. And for lower cost. Turbochargers and all.
If I wanted incredible engineering-as-art in my engine, I would buy an S85. I have huge respect for it. I just wouldn't expect above average reliability.
Sidenote: diesels typically have higher compression ratios in order to ignite the air-fuel mixture, achieved with higher boost. Gasoline turbos are stressed less, as peak cylinder pressures are lower.