What ultimately governs fuel consumption.

[Dom996]

Is it merely engine capacity and weight of the car, or can we figure state of tune, power, and at what revs the power band is?

 

[WhyAyeMan]

Many, many things.

The size of the engine, whether it has a blower on it or not, whether that blower is a turbo or a supercharger, whether the engine is a diesel or a petrol, how well the engine is mapped, how light the vehicle is, how the vehicle is geared, how aerodynamic the vehicle is, how heavy footed the driver is, etc etc etc etc........................

 

[Dom996]

Well I'll tell you why I ask.

I am about to change from an Audi 90 Quattro 2.3l 20v to an Audi 100 Quattro 2.8l V6. On paper the MPG is about the same. Now the BHP of both cars is similar. The 100 weighs a bit more, but gives it's best torque around 2k revs lower. It's also a 'lazy engine' there as the 90 is a bit tuned.

See where I'm coming from?

 

[lovegroova]

The 100 will be worse as it's heavier and has a larger capacity engine. The peak torque at lower revs means you'll be using more fuel at lower revs as you'll be making a higher percentage of peak power at lower revs.

How much fuel you use is related to how far and for how long you keep the throttle open.

A look at the official figures will give you an idea of the relative difference between the two, if not the actual figures you are likely to get.

 

[Kebabhead]

Tyre pressures as well as maintenance of a car can affect fuel consumption

 

[WhyAyeMan]

Well I'll tell you why I ask.

I am about to change from an Audi 90 Quattro 2.3l 20v to an Audi 100 Quattro 2.8l V6. On paper the MPG is about the same. Now the BHP of both cars is similar. The 100 weighs a bit more, but gives it's best torque around 2k revs lower. It's also a 'lazy engine' there as the 90 is a bit tuned.

See where I'm coming from?

Yes, and I agree with lovegroova on principle, but its not as simple as that. The more powerful torquey engine will not have to work as hard, so may well be just as easy on fuel, see what I mean?

So many things affect MPG, its very difficult to call. Generally speaking though, a small, naturally aspirated engine in a light aerodynamic car will be very good.

 

[SeanT]

A small turbocharged engine in a light aerodynamic car might be even better though....

 

[WhyAyeMan]

A small turbocharged engine in a light aerodynamic car might be even better though....

Explain?

(not saying your wrong btw, just curious).

 

[lovegroova]

Yes, and I agree with lovegroova on principle, but its not as simple as that. The more powerful torquey engine will not have to work as hard, so may well be just as easy on fuel, see what I mean?

So many things affect MPG, its very difficult to call. Generally speaking though, a small, naturally aspirated engine in a light aerodynamic car will be very good.

Take a look at the fuel consumption figures of any range on the same car 3-series, Mondeo, whatever and you'll see that the bigger the engine, the higher the fuel consumption. Always. (There may be an exception somewhere but you get my drift.)

 

[lovegroova]

Explain?

(not saying your wrong btw, just curious).

Assuming everything else is equal, he car with the most efficient engine will have lower fuel economy. If the turbo can be used to make the engine more efficient, then there you go.

I've no idea whether this is do-able in practice though.

 

[WhyAyeMan]

Take a look at the fuel consumption figures of any range on the same car 3-series, Mondeo, whatever and you'll see that the bigger the engine, the higher the fuel consumption. Always. (There may be an exception somewhere but you get my drift.)

I kind of agree, but sometimes the difference can be smaller than you might imagine due to not having to work the car as hard. If you have to rev the tits off it just to make it move, you'll still end up using as much fuel as in a bigger engine worked at a much gentler rate (except that with the bigger engine, you actually have some power in reserve - never a bad thing). Example, when i had my micras (shudder) my economy never matched the claimed figures because they were so gutless so i had to work them that much harder, yet in my more powerful cars, they were'nt much worse because i never had to work them as hard... if that makes sense

 

[SeanT]

As above, a turbocharger can make an engine more efficient, the reason that turbocharged cars have historically been inefficient is (a) they tend to get clogged anyway and (b) they used to be intentionally overfuelled to prevent chocolate piston / subaru syndrome.
If you fit a closed loop lambda system to a 400+ BHP cossie you can see 30 to the gallon on the motorway (but about 8 around town!)