turbo charged engines.reliabilty?

[Alan Smithee]

Originally Posted by JCL

BMW introduced a production turbocharged vehicle in 1973, almost 40 years ago. This isn't exactly a new concept.

...and up until the twin-turbo 3.0 N54, they only built one other turbocharged gasoline model (the 745i, in very limited numbers, in the early 1980s). A new concept? No. Has BMW proven long-term reliability with gasoline-powered turbos? Hardly.

[JCL]

Originally Posted by Alan Smithee

...and up until the twin-turbo 3.0 N54, they only built one other turbocharged gasoline model (the 745i, in very limited numbers, in the early 1980s). A new concept? No. Has BMW proven long-term reliability with gasoline-powered turbos? Hardly.

Fair enough. How about the early '80s BMW turbo diesel? Turbos are a pretty simple concept, and they aren't different between gasoline and diesel engines. Do you see the turbo as the failure point, or the resulting specific power output of the engine?

I just don't think the turbos are a reliability or durability issue. Sure they have only been around for four years. So has the E70 platform, so it isn't any more proven. How about the new 8 speed ZF transmissions? One year now. The modified x-drive? Since 2010. All more prone to failure than a turbocharger.

[Alan Smithee]

Originally Posted by JCL

Fair enough. How about the early '80s BMW turbo diesel? Turbos are a pretty simple concept, and they aren't different between gasoline and diesel engines. Do you see the turbo as the failure point, or the resulting specific power output of the engine?

I just don't think the turbos are a reliability or durability issue. Sure they have only been around for four years. So has the E70 platform, so it isn't any more proven. How about the new 8 speed ZF transmissions? One year now. The modified x-drive? Since 2010. All more prone to failure than a turbocharger.

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.

[JCL]

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.

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.

[Alan Smithee]

Originally Posted by JCL

Since you bring up revs, let's consider two high output BMW engines...

Going back to the original post in reference to the new N20 2.0 4-cylinder turbo, I think a better comparison is to the engine it is replacing, which is what my initial comments were based on. Certainly you wouldn't bet against the N52/N53 lasting longer without major work, would you?

Originally Posted by JCL

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.

Of course diesels have higher compression ratios, but to take from one of your previous posts, diesel engines are designed for higher compression, so they are not more stressed than a gasoline engine.

Again, it is the additional heat in a gasoline turbo that makes it a poor long-term proposition.

[JCL]

Originally Posted by Alan Smithee

Going back to the original post in reference to the new N20 2.0 4-cylinder turbo, I think a better comparison is to the engine it is replacing, which is what my initial comments were based on. Certainly you wouldn't bet against the N52/N53 lasting longer without major work, would you?

Lasting longer isn't an indication of reliability. It is an indication of durability.

If one engine breaks down every month, but runs for 300,000 miles with regular interventions, and another engine never breaks down, but runs for 200,000 miles before requiring an overhaul, which is more reliable? IMO, it is the one that never breaks down.

How many late model BMWs get traded in because the engines are worn out (rings, cylinders, valves, etc) vs those that get traded in because they are always leaving their owners on the side of the road?

I would bet that the N52/N53 would be as reliable as the N20, once both are past their typical first-year-of-production build issues.

In terms of durability, I don't have data to suggest that one is more durable than the other, but I would bet that other systems on the car would be the determining factor, not the presence of a single turbocharger.

[JCL]

Originally Posted by Alan Smithee

Again, it is the additional heat in a gasoline turbo that makes it a poor long-term proposition.

OK, let's compare a naturally aspirated 240 hp inline 6 (previous generation) against a turbocharged 240 hp N20. I don't know which model produced exactly that hp, but there were a number of engine models close to it.

At a high level, the N20 has a higher thermal efficiency. It converts more of the fuel to useful hp. That is why it has better fuel efficiency. That comes from having less friction, and heat recovery from the exhaust, as well as other design features. All the energy that doesn't make useful power is dispersed as heat. So, the turbocharger engine produces less heat at the same hp than the NA one. We can argue that the heat is more localized, but there is less of it produced.

If you compare a naturally aspirated engine to the same engine with a turbocharger, the turbocharged engine produces more heat. But it is also producing more power. If you have two engines of the same power, there is less heat from the turbocharged one.

[Alan Smithee]

Originally Posted by JCL

We can argue that the heat is more localized...

If by that you mean more heat is retained under the hood with a turbo, that is the point I am making. With all of the plastic parts under a modern BMW hood, because of sustained high temperatures inherent to an efficient turbo, I sincerely doubt the ancillaries will last as long as those in a non-turbo (not to mention the turbos themselves). If they have been re-engineered, it is likely for recycle-ability rather than 200,000 mile operation. Anyway, time will tell.