Originally posted by deevergote
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First, they produce much more detrimental quantities of particulate matter. We are talking the sort of microscopic particulates that cause smog and other health concerns. It is to the point where they are talking about forcing DI gas engines to have a particulate trap like diesels currently feature... Then you have the reality that these engines tend to have reliability issues due to not only the fuel pressures they run (things like HPFP's fail more often) but also the carbon build up issues most of them are experiencing (with most manufacturers claiming to have "fixed" this issue). Yet, you are seeing major OEM's like VW and Toyota increase the use of dual injection to solve the cylinder pressure/deposit issue. Couple that with a particulate trap and you are looking at quite a cost increase over the long run of the engine.
Then, you have the issue that by and large, you aren't getting vastly better MPG out of these engines anywhere but on the EPA test. My buddy has a turbo 4 328i with the four banger and his MPG is about 1MPG better than my ~300HP NA, port fuel injected V6, both with manuals, both with similar weight, etc. It does get slightly better MPG than his previous E92 328i, but who knows what some minor improvements would have brought? In either case, you are talking about MAYBE $200 a year in operating cost savings, and I can tell you from experience that NONE of these little turbo fours have the refinement of either that BMW straight six or Honda's own J series V6.
To top if off, even the European governments that drive a lot of the technology you find in BMW and Audi are starting to realize the downsides to their course are fairly significant and are starting to push away from diesels and possibly small turbo engines.
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