Your thoughts please

[martynv8]

I am not talking large and small engines running at different RPMs for the same power output, with different gear ratios, different cars with different aerodynamics and body weights, of course there is a difference. I was talking a 4.0 litre Land rover V8 as opposed to a 4.6 litre Land rover V8. Same car, same weight, same frontal airflow resistance, same gear ratios. At any given RPM the 4.6 will use more fuel than the 4.0 litre. That is where the simples comes in. Land rover seem to agree with me, as the mileage figures for the 4.6 are marginally worse than those claimed for the 4.0 litre engine under all conditions. So your argument is out of the window i am afraid.

Unless i'm mistaken, the point being made (very simply put) is that even though the 4.6 will obviously use more fuel at the same RPM as a 4.0, the 4.6 under many circumstances needs less RPM.
I suspect real world figures would show little between them, but who cares, they are both thirsty petrols. The 4.6 having a gearbox that's actually up to the job & generally better trim levels won it for me.

 

[Saint.V8]

Given the SAME gear ratios in the gearbox and final drive units, the 4.6 and the 4.0 engine crank speed would be the same for any given road speed....the size of engine is irrespective...pure mechanics will dictate that....

The 4.6 will need to be 'on throttle' for less time as its acceleration is quicker (marginally) so in effect getting up to a speed will be quicker and thus you won't be on throttle for as long which is where a saving could be made, but the difference in the real world would be negligible....

BUT as for RPM speed differences at a given road speed....that is incorrect!

 

[Datatek]

Given the SAME gear ratios in the gearbox and final drive units, the 4.6 and the 4.0 engine crank speed would be the same for any given road speed....the size of engine is irrespective...pure mechanics will dictate that....

The 4.6 will need to be 'on throttle' for less time as its acceleration is quicker (marginally) so in effect getting up to a speed will be quicker and thus you won't be on throttle for as long which is where a saving could be made, but the difference in the real world would be negligible....

BUT as for RPM speed differences at a given road speed....that is incorrect!

You forget the slip in the torque convertor.
Fuel consumption is not strictly revs related, low revs wide throttle opening will inject the maximum fuel for that particular engine speed wheras a light throttle opening at the same revs will inject a lot less. If it were not so the engine would never accelerate.
Engine efficiency also varies with revs which may be why there are differences in consumption in the same circumstances.

 

[Saint.V8]

You forget the slip in the torque convertor.
Fuel consumption is not strictly revs related, low revs wide throttle opening will inject the maximum fuel for that particular engine speed wheras a light throttle opening at the same revs will inject a lot less. If it were not so the engine would never accelerate.
Engine efficiency also varies with revs which may be why there are differences in consumption in the same circumstances.

Very true, the variables at play in working these things out are quite substantial...

How about this....??....If you have to ask what MPG a V8 does, you are not in the right head-space to own one!

 

[Datatek]

Very true, the variables at play in working these things out are quite substantial...

How about this....??....If you have to ask what MPG a V8 does, you are not in the right head-space to own one!

Exactly But you have to be wrong in the head to own a Rover V8 in the first place.

 

[wammers]

Unless i'm mistaken, the point being made (very simply put) is that even though the 4.6 will obviously use more fuel at the same RPM as a 4.0, the 4.6 under many circumstances needs less RPM.
I suspect real world figures would show little between them, but who cares, they are both thirsty petrols. The 4.6 having a gearbox that's actually up to the job & generally better trim levels won it for me.

How the hell can an larger engine powering the same type of car with the same gear ratios need less RPM for a given road speed?

 

[TCubed]

How the hell can an larger engine powering the same type of car with the same gear ratios need less RPM for a given road speed?

Are the ratios the same for the different boxes?

 

[wammers]

Are the ratios the same for the different boxes?

Yes they are all the same gear ratios and final drive ratios petrol and diesel models.

 

[martynv8]

Different torque on all models, even more so on Thor.

 

[StuckAgainSteve]

Yes they are all the same gear ratios and final drive ratios petrol and diesel models.

In the real world the 4.0 owner is having to push the throttle harder to get the same amount of go. So shift up points and revs will be higher burning more fuel. Effectively different gearing.

 

[wammers]

In the real world the 4.0 owner is having to push the throttle harder to get the same amount of go. So shift up points and revs will be higher burning more fuel. Effectively different gearing.

Bull****, only consideration is torque slip. The fact that the 4.6 is injecting more fuel per cycle is irrelevant then? The 4.6 uses more fuel than the 4.0 simple as that.

 

[martynv8]

Bull****, only consideration is torque slip. The fact that the 4.6 is injecting more fuel per cycle is irrelevant then. The 4.6 uses more fuel than the 4.0 simple as that.

Bloody hell Wammers, thought your were supposed to be the expert

 

[wammers]

Bloody hell Wammers, thought your were supposed to be the expert

If you want to talk nonsense carry on i don't give a ****.

 

[martynv8]

Always guaranteed plenty of nonsense on here when it comes to the usual petrol / diesel debate.

 

[wammers]

Always guaranteed plenty of nonsense on here when it comes to the usual petrol / diesel debate.

Petrol.

Urban cycle. 4.0 12.9 MPG. 4.6 12.3 MPG.

Extra urban cycle. 4.0 23.4 MPG. 4.6 22.7 MPG.

Combined cycle. 4.0 18.1 MPG. 4.6 17.4 MPG.

Diesel.

Urban cycle. Manual 22.2 MPG. Auto 20.3 MPG.

Extra urban. Manual 31.9 MPG. Auto 30.7 MPG.

Combined Manual. 27.5 MPG. Auto 25.8 MPG.

Those are Land rovers figures.

If the 4.6 somehow managed to beat the 4.0 on fuel consumption i would suggest pigs may soon be used to deliver air mail. At 50 MPH in torque lock both engines would be doing the same RPM but the 4.6 because it has a larger cylinder capacity would be using more fuel. It has to, physics make it so.

 

[martynv8]

Petrol.

Urban cycle. 4.0 12.9 MPG. 4.6 12.3 MPG.

Extra urban cycle. 4.0 23.4 MPG. 4.6 22.7 MPG.

Combined cycle. 4.0 18.1 MPG. 4.6 17.4 MPG.

I never said the 4.6 gave better MPG, i said there's not much in it. The figures above confirm this without the debate on whether the 4.6 uses less throttle & more torque to get about.

 

[wammers]

I never said the 4.6 gave better MPG, i said there's not much in it. The figures above confirm this without the debate on whether the 4.6 uses less throttle & more torque to get about.

Some people seem to think that 5 litre engines use less fuel than 1 litre engines. I have yet to be convinced actually.

 

[martynv8]

Some people seem to think that 5 litre engines use less fuel than 1 litre engines. I have yet to be convinced actually.

Not the same argument really is it.

 

[Datatek]

Some people seem to think that 5 litre engines use less fuel than 1 litre engines. I have yet to be convinced actually.

Having run a fleet of 35 Ford Escorts, I can tell you that the logs showed the 1.6 to be more economical than the 1.1 or 1.3.

 

[martynv8]

Some people seem to think that 5 litre engines use less fuel than 1 litre engines. I have yet to be convinced actually.

PS, the 4.4 TDV8 gives a better combined MPG than your 2.5 TD