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[Mr Skrunts]

In that case then it would be interetsting to see the Top Gear test rerun with the V8 Audi to Scotland and back from London on a hot sunny day starting with an empty tank at 2pm and see what the difference is.

Dont forget the air density on a hot day is less and will effect the running of the motor, much like driving at high alitudes.

All above my head Jon, but I find some of the theory very interesting allthough not allways the case in reality.

[Magsmews]

Hey everyone has an opinion, thats what makes us all unique  
I am prepared to  try anything that might help the squeeze on me poor old pocket at the mo!!

[Entwood]

Its dead simple mate .... to burn petrol efficiently you need 14.7 times as much air as there is petrol, by mass,  (known as the stoichiometric ratio)... when the air pressure is low the engine takes in less air, so the ECU gives less fuel, so you have less power ... most folks don't know this and compensate by flooring it a bit more ... so using more fuel as well as making the ratio "wrong" by overfuelling to get more "power" ... inefficient operation.

On a cold dry day when the air pressure is high its little like having a tiny turbo on your car ... more air = more fuel = more power, so you can back the throttle off a tad ... more efficient operation.

HTH  

[feeutfo]

my old ZX9(edited;sorry thats a motor bike, just in case you wondering) used to run noticably smoother and crisper in cold air temps.esp after a warm day. throttle response much improved. went like slippy snot until carb icing stopped play. they never did sort that prob. :-?

[Mr Skrunts]

Its dead simple mate .... to burn petrol efficiently you need 14.7 times as much air as there is petrol, by mass,  (known as the stoichiometric ratio)... when the air pressure is low the engine takes in less air, so the ECU gives less fuel, so you have less power ... most folks don't know this and compensate by flooring it a bit more ... so using more fuel as well as making the ratio "wrong" by overfuelling to get more "power" ... inefficient operation.

On a cold dry day when the air pressure is high its little like having a tiny turbo on your car ... more air = more fuel = more power, so you can back the throttle off a tad ... more efficient operation.

HTH  

That confirms a few things for me.

I allways felt my my car went a lot better at 6 am on the wy to work rather than the trip home at 4 pm.  I put it down to tighter tolerances on the engine being cold and "Tight" rather than being hot and "losr"

[ians]

However !  I've yet to come across a motorist who doesn't think that the first half of a full tank goes down slower than the bottom half.

I've driven a variety of vehicles over these past 27  years or so and in every case, the first half lasts longer in terms of milege than the last half. Why is that prey tell ?

Could be a number of explanations such as the guage is not linear.

[psychnurse]

However !  I've yet to come across a motorist who doesn't think that the first half of a full tank goes down slower than the bottom half.

I've driven a variety of vehicles over these past 27  years or so and in every case, the first half lasts longer in terms of milege than the last half. Why is that prey tell ?

I must say mate I experience quite the oppoisite. have always wondered why though.

[davlad22]

My diesel astra goes noticably better when colder. Especially in the evenings.

[Omegatoy]

However !  I've yet to come across a motorist who doesn't think that the first half of a full tank goes down slower than the bottom half.

I've driven a variety of vehicles over these past 27  years or so and in every case, the first half lasts longer in terms of milege than the last half. Why is that prey tell ?

quite simple really, the bottom half of the tank is smaller!!! top half has almost sqaure edges bottom half has sloping edges to push the fuel into the centre for the pick up pipe, thats a simple explanation but it gives you the gist of i :yt!!

[Kevin Wood]

Its dead simple mate .... to burn petrol efficiently you need 14.7 times as much air as there is petrol, by mass,  (known as the stoichiometric ratio)... when the air pressure is low the engine takes in less air, so the ECU gives less fuel, so you have less power ... most folks don't know this and compensate by flooring it a bit more ... so using more fuel as well as making the ratio "wrong" by overfuelling to get more "power" ... inefficient operation.

On a cold dry day when the air pressure is high its little like having a tiny turbo on your car ... more air = more fuel = more power, so you can back the throttle off a tad ... more efficient operation.

HTH  

You might actually find the opposite is true. You'll have more power with colder air but, when cruising on part load, thinner, hotter air will require less throttling of the engine, which actually improves its' efficiency.

Any difference is probably lost in the noise though.

Kevin

[willyboy]

Very interesting read bud nice one will think on in future..

[duggs]

Omega toy...How can that be possible..Despite the shape of the tank...the fuel gauge shows how much was and how much is left.
The top 35 litres takes longer to go than the bottom 35 litres...?

[feeutfo]

how much does the filler neck hold??? got more than 70 litres in myn acouple of times, i am sure.
brim it rather than stop at the first click off. first half is then more than 70 ltrs??? :-?

[Lazydocker]

Omega toy...How can that be possible..Despite the shape of the tank...the fuel gauge shows how much was and how much is left.
The top 35 litres takes longer to go than the bottom 35 litres...?

The shaped tank explaination is correct, although I would have said the shaping was more to do with making the tank fit the available space!

As for the guage... Remember it isn't calibrated, it's just a guide! It's fed from a potentiometer which just gives a linear resistance output as the float arm drops. Fuel guages are notoriously inaccurate at the best of times!

I hope this helps you with the mystery!

[duggs]

Good point.....