Nerdy engine efficiency thread – N/A versus Turbocharged

RickMM

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Firstly, this is not about complaining of the cost of running an AMG, whether diesels are the spawn of Satan, or whether the future of the planet is doomed without eco electric vehicles.

I’ve just swopped in an M156 6.2 C63 after 3 years and nearly 40k miles for the 4.0tt model. I do a weekly commute where the traffic/roads often restrict progress and I regularly got 25mpg on A-roads from the old normally aspirated 6.2. This was actually better than my previous 4.0 V8 BMW M3 (although that did have the manual gearbox). Now the official mpg figures for the new 4.0tt AMG lump are a lot better than for the old M156, presumably because it’s a smaller engine, less friction, better engine management, etc. etc. plus it’s also turbocharged. As we all know, in theory the turbos make use of the free energy flowing out of the exhaust pipes to ram more air mass (as it’s pressurised) down the inlets. (Of course it’s not actually completely “free” because you have to impede the exhaust gas flow to spin the impeller, but it should still be a more efficient system overall).

Anyway, doing the same routes and type of driving I’m finding the fuel consumption on the new car to be only marginally better than on the old 6.2L. I was a bit surprised at this (again, not complaining about the cost of running a high performance car). Now when I keep an eye on the turbo pressure indicator on the AMG screen (which I use most of the time) for the vast majority of normal road driving conditions there is hardly any boost pressure at all (because you can't plant the loud pedal without ramming the car in front). So effectively the engine is behaving like a normally aspirated lump, and a low-compression one at that. So it’s no wonder it’s no more efficient a lot of the time :rolleyes:

Perhaps I just need to get out more...
 
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RickMM

RickMM

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Having donned the flame-proof overalls ;) I have to say that the new car is better in just about every way. The whole package is more modern (as you'd expect), the tech is better, it's got more toys and options to play with, and it feels faster (grin factor) with the extra low-down torque. In fact I even prefer the sound of the 4.0tt as it bangs through the gears compared to the N/A 6.2. As my only car, I really like it.
 
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The fuel difference might be down to your right foot - I’ve always found N/A engines to be more relaxed when driving “steady” whereas a forced induction is like a dog with two dicks - can’t be still.

A supercharged engine in particular just wants to be booted..
 

alabbasi

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It's probably down to your driving. As you had indicated, it's a smaller engine which means it has less torque until the turbo gets on it so it's likely that you're squirting the throttle a lot more than you would in the old 6.2 NA car. It's the squirting that's causing your fuel mileage to drop.

Stop looking at the turbo gauge, it's going to ruin your driving. I would do that with the economy gauge in my old E30 BMW back when i was 21 and would find myself rocking the pedal back and forth to get the best fuel mileage. A bad habit that I still fall into every no and then.
 

E55BOF

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The 5.5 tt is quite a bit more fuel-efficient than the 55K, and I rather think more so than the n/a 6.2 as well. Perhaps it falls into a sweet spot between the n/a and the 4.0 forced induction?

"There's no substitute for cubes"; add forced induction, and you have both...
 

st13phil

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Where's @Mactech when you need him?

The reality is that the thermal efficiency of ic engines doesn't actually vary that much. Forced induction allows you to increase volumetric efficiency, but doesn't really change thermal efficiency. Thermal efficiency is the major influence on fuel consumption.
 

190

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Maximum torque thermal efficiency doesn't vary that much but specific fuel consumption per unit of power output varies enormously at lower loads and different revs. The specific fuel consumption maps for the two cars are bound to be different and each car would have to be driven differently to get the best MPG.
 

Mactech

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So your new turbo car is faster, more modern and preferred to your larger capacity 6.3 C63. And slightly more economical?
No real surprise that the improvement is only small, manufactures have been working on making cars more efficient (even AMG's!) for quite some time now and real gains are hard to come by.
190 is correct regarding thermal efficiency, and if you are diving the same route the same way then the real problems will be overcoming the inertia of stop/starting and all the inherent drag that cars suffer. It's just plain physics.
The way the car is driven and the route you drive (and when!) will have a much larger effect on the consumption than anything the car makers can do, and yes, you may find that driven slightly differently the turbo car can be more economical.....but that's probably not why you bought an AMG.

Engine efficiency is just a small part of a much bigger picture of automotive efficiency and it takes lots of continual improvement in all areas (aero, transmissions, tyres, lubricants, weight, etc) for you to see a substantial difference.

The biggest single change you could make is the 'nut behind the wheel':)
 

abar121

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Turbo'd cars are much better at scamming the EU MPG and emissions figures ;) Let's hope the WLTP testing improves matters somewhat.

I can't see enough data to be meaningful on Fuelly for M156s, but for M3s, you can clearly see that since dumping the V8s, the latest M3 with the turbo'd six is back to similar MPG to the S54 models. Much faster for sure, but zero character in the M4 i drove. The only petrol BMW six that sounded soulless to me.

I'm surprised that you were able to average 25mpg with the M156. I think mine would only do that on a strictly law abiding motorway cruise with no towns at either side!

I averaged 22mpg across Europe driven normally. Usually it's around 20mpg.
 

tonyg1987

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The 5.5 tt is quite a bit more fuel-efficient than the 55K, and I rather think more so than the n/a 6.2 as well. Perhaps it falls into a sweet spot between the n/a and the 4.0 forced induction?

"There's no substitute for cubes"; add forced induction, and you have both...


c63 na i got 11mpg
town driving, cls63 5.5 lump i get 18mpg, c63 i got 23mpg long distance cls63 i got 28mpg.

i find the turbo amg better for mpg consumption
 
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RickMM

RickMM

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The traffic on my regular weekly commute dictates that I remain law abiding for a large chunk of the journey. That said, I am still "playing" a bit more in the new car at the moment so I'd expect the mpg to improve over the next few months.
 

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