Quote:
Originally Posted by clarence
I think they're pushing this N20 less than their NA engines in terms of specific power outputs, for example, the 3.0 N53 pushes out 90.7ps/litre & the N52 at 86ps/litre. So I think this engine is relatively less stressed than a lot of their NA engines.
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Peak power per liter probably isn't the best measure of how stressed an engine is. The higher spec N52/N53 engines just have induction systems tuned to still deliver a lot of torque at higher RPMs. A better look at engine "stress" is from the BMEP figures (brake mean effective pressure). It's a bit more complicated than this because you have to factor in compression ratio, but you can get rough figures from the torque per liter numbers.
N52: 220lb-ft or 300Nm / 3.0L = 100.0 Nm/L
N53: 230lb-ft or 310Nm / 3.0L = 103.3 Nm/L
N55: 300lb-ft or 400Nm / 3.0L = 133.3 Nm/L
N20: 260lb-ft or 350Nm / 2.0L = 175.0 Nm/L
Forced induction engines are definitely more stressed than most any NA engine. They need extra strength to handle the power which is why you drop the magnesium engine block for aluminum, or aluminum in favor of iron. I think the VAG 2.0T engine is still using an iron block, for example. You also need to pay a whole lot more attention to cooling as well, in terms of water and oil passages in the engine block and eliminating hot spots. You've got a whole lot more heat to remove from a 2.0L engine making up to 260 lb-ft or 350Nm of torque and employed in a heavier 4000lb 5er class vehicle where its average load will be heavier. The turbochargers themselves get incredibly hot and require special cooling attention as well, and spin at up to 100,000rpm. The more goodies in the engine, the more that can go wrong. Personally I wouldn't expect any of these new turbo engines from BMW to be able to last to 100,000 miles without needing some sort of major overhaul. Hopefully they'll have learned a thing or two from all the issues on the N54 and this N20 will be better, but BMW has never been a company that places a big focus on reliability like the Japanese.
Subaru turbo engines seem to be about the best out there as far as reliability, but a) they're Japanese, b) they have a whole lot of experience with turbo engines, and c) they're not pushing the tech envelope nearly as hard as BMW and the Europeans in general are trying to do. The latest EJ25T in the Subaru Legacy has similar power figures to the BMW N20, but from a 2.5L engine rather than a 2.0L, and it's also running a much lower 8.4:1 compression ratio rather than 10.3 I think on the BMWs, and port injection. Will result in higher fuel consumption and also more turbo lag, but better reliability. No worries about intake deposit issues on a port injected engine, but you can't push the compression ratio nearly as high without direct injection. Tough to have it all at once. The Honda/Acura turbo engine in the RDX is similar with a much lower 8.x:1 compression ratio and port injection. Not all that efficient, but I bet it will be more reliable than the BMW engines.