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Fuel Consumption Comparisons

2K views 22 replies 12 participants last post by  Tony Rome 
#1 ·
This has been bugging me for some time. OK, the 166 may not be new anymore but I still find this amazing:

Alfa Romeo 166 3.2 V6, 240PS, 12,5l/100km (22.6mpg)
BMW 550i 5 litre V8, 367PS, 10,9l/100km (25.9mpg)
BMW Alpina B5 4.4 litre V8, 500PS, 12,3l/100km (23mpg)

How the hell is it possible that the consumption of a 3 litre V6 is worse than a 5 litre V8, let alone a 500PS monster like the Alpina, when the cars are very similar in size and weight?! In fact, I think you'll find the 5 series is a bit heavier!
 
#3 ·
The big V8 is probably just ticking over and handles the weight of the big car even beter than the V6.
 
#4 ·
Did you get those figures from the "official" fuel consumption database? If so, those figures are taken during the fuel consumption test to EC Directive 80/1268. This has a standard "Euro" drive-cycle that the car has to do. You start up, idle for "X" seconds, into first, accelerate up to "Y" speed in "Z" seconds, hold it, change gear, go up, go down, etc etc. It has an "Urban" cycle and an "extra-urban" (motorway) cycle. The problem is that those figures need taking with a pinch of salt. It's the same test for everything from a Smart car to a Ferrari so it means that the Smart car is virtually "foot-to-the boards" to reach the required speeds within the required times whereas the Ferrari will do it almost all with a virtually closed throttle. I guess that with the cars you mention, the Alfa is using more throttle than the others to achieve the same drive cycle. Obviously, this can't explain ALL the differences but I think it might be a significant part. Similarly, depending on the car's gearing, the cars might be in different gears at the same speeds in the drive cycle. The whole test is only a matter of minutes long too.

All in all, I don't think it's especially representative of real-life. I expect that if I had access to 500PS, I'd probably use most of it at times - but I wouldn't be expecting 23MPG if I did!

This whole testing regime gives rise to all sorts of anomalies like the Peugeot 307 - whose 1.6 (90BHP) diesel has exactly the same figures as its 1.6 (110) horse diesel. So who in their right mind would buy the 90bhp version? I think the answer is that the lower powered car probably IS more economical in real life - it's just that neither car used 90bhp (or anywhere near it) to follow the EC drive cycle. The differences between the two would only become apparent when the conditions of use lay within the range of differences in the engine managment computer's mapping.
 
#8 ·
Did you get those figures from the "official" fuel consumption database?
Yes, I did and I take your point. I'm sure it has a bearing on the figures. However, comparisons have to be made using the same tests or they are not comparisons. 'Real World' comparisons are very difficult to do accurately. Of course they would appear different if someone drove the 500PS car really hard but that's not a fair comparison because it'll be achieving performance which is not possible in the other vehicle! The official figures are at least directly comparable.

Also, if it's true that the tests are tough enough to mean that the 3 litre car is struggling (relatively) in comparison to the 5 litre, the fuel consumption of these little 1 litre cars would be awful. But they're not. They are much more efficient than the bigger engines, so that argument doesn't hold water. Dougie's comment that the V8 is just ticking over would suggest the opposite is true. So, 8 litre V12s all round, then :cheese:

Despite my dislike of the German cars, I'm inclined to think it has to be that the BMW engines are better engineered and thereby more efficient. It will be interesting to see if the new 169 is also more efficient, despite having increased power.

I understand the comments about style from Alfa Black but the one doesn't preclude the other! There's no reason why you can't have both.
 
#7 ·
Had a Volvo S80 T6 twin turbo once with 100 more horspower than my 2.5 Sportroninc and that would flirt with 40mpg on a motorway run (low revs)

I'm averaging mid 20s mpg but console myself that the equivalent BMW or anothet T6 would have cost me about £5,000 more to buy - thats a lot of fuel!

It all evens out - but the Italians do style not economics, and leave running costs to others to worry about - I think it's always been this way!
 
#9 ·
I agree you need to make the comparisons using a standardised procedure but that doen't mean much unless you then drive the cars in a "standardised" way! Pretty much all petrol engines these days will have (I guess) very similar efficiencies and the bulk of the difference (again, guesses!) will be down to the rest of the car - weight, gearing, rolling resistance, aerodynamics etc. What you really need before making comparisons of the efficiency of the German and Italian engines is a "Specific fuel consumption" figure - usually expressed in kg/kwh so it's a measure of how much fuel it gets through while producing 1 kilowatt for 1 hour. That takes all the other factors in the car out of the equation. Suppose, for example, that the gearing on one car was such that it was always 1 gear higher than the other car during the EC drive cycle. That would make it seem much better on paper but wouldn't necessarily mean anything at all in day-to-day running. The "highway" component of the drive cycle lasts about 7 minutes and of that 7 minutes only about 10 seconds is spent at a steady 75MPH (the rest being much closer to 50MPH). Quite how that relates to cruising at 80MPH for an hour on a motorway is beyond me!

Also, on the urban cycle, you're allowed to start off in 2nd gear if the manufacturer has designated first as not being a "normal road use" gear. Maybe the German cars have used this dodge? I guess we need to find a forum for the other makes of car and ask the owners what they typically get but I certainly wouldn't make a buying decision based on the "official" figures alone!
 
#13 ·
I certainly wouldn't make a buying decision based on the "official" figures alone!
No, I agree entirely. Nor would I ever buy on economy alone! It is an interesting subject, though, and it's becoming increasingly important as the fuel prices continue to rise. I've never seen the 'specific fuel consumption' figure quoted anywhere...
 
#10 ·
My dad reckons his 540 (4.4 litre) beemer is better on fuel than his 3 litre GTV was and it is an Auto. It is just ticking over most of the time at normal UK road speeds.
 
#11 ·
Alfa engines are traditionally more thirsty than the norm, even the diesels for some reason. I was surprised to read that even the new 3.2 doesn't offer much better consumption than the old Busso V6. Kind makes you think why they bothered given the new one is pot ugly in comparison. Must be the emmissions, can't be owt else.
 
#14 ·
No, it's extremely difficult to get hold of - which is a pity because it's the only fair way of just comparing the engines by themselves. Certainly, I've never seen one quoted for any Alfa engine. I'm surprised that so many people see Alfas as trditionally poor in this respect though. The fuel consumption of my old 12 valve V6 164 was one of the things that impressed me most about it!
 
#17 ·
New owner picked it up 10 minutes ago...... a very sad day for me but all good things come to an end, now the hunt begins for something new. The guy who bought it currently has a GTV so he is obviously a petrol-head so I am happy, says he has been to national Alfa day and he does frequent the forums so perhaps he is known to us already.

Take care

Mike
 
#20 ·
IIRC the V6 was first seen in 2.5 form with carbs in the Alfa 6, which appeared maybe late 60s early 70s so the design compared to a modern BMW is almost pre-historic, but the heads and fuelling have clearly had many updates to keep it fairly up to date.

That said, we are driving one of the best looking cars in the world so why are we talking fuel consumption?
 
#21 ·
we are driving one of the best looking cars in the world so why are we talking fuel consumption?
Why does the way a car look excuse other such factors?! This seems to be a recurring theme here but it doesn't have to be exclusively one or the other! I brought up the subject because other cars, with more power, appear to be more efficient as well. I don't know about you but I'd prefer a 5 litre V8 with 500PS and better fuel consumption in my Alfa :lol:
 
#23 ·
as King Jeremy put it, your new girlfriend might be a great cook, great in bed, marvelous with children, but if she looks ugly, you will not want to show her to your mates.
But you're missing the point I'm making, which is that you can have the power/economy AND the looks!

I suppose there is the diesel option, out of the UK anyway.
Yeah, but it's not that powerful and the sound...
 
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