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Does Increasing Horse Power On A Diesel 2.0 146Hp Dicrease Consumption?

1K views 6 replies 5 participants last post by  dirocyn 
#1 ·
hello! i have a mini paceman 1300 kg with a 2.0 diesel 146horse power. in the city i get a consumption of about 10L/100km.
if i add say another 40hp is there a chance my consumption will decrease?
i m asking because i feel that the engine strugles with the weight of the car. is it too small for such a car?
 
#2 ·
It may do, normally say a 1300cc car is less economical than say the next size up, a 1600cc and thats because the engine doesnt have to work as hard to keep the car going at the same speed, it may well go up a hill in top gear where the lower capacity car you need to drop a gear or two to get the same result, hence the engine works harder uses more fuel, that works up to a point then of course as you get even more power it will use more fuel then the lower capacities, so its a case of getting the power just right for the body size.

A 2 litre diesel with 146bhp should perform quite well in a MINI I would have thought though?
Town driving is never good for fuel consumption as its all stop start.
 
#4 ·
It may do, normally say a 1300cc car is less economical than say the next size up, a 1600cc and thats because the engine doesnt have to work as hard to keep the car going at the same speed
The truth, for petrol cars, is the exact opposite of this - smaller-engined cars are more economical because their engines are working harder, nearer to the point of maximum efficiency (peak torque), more of the time. The guys who achieve 1000mpg from specialist economy vehicles do so by using the 'burn and coast' technique, where the tiny engine is only run at maximum throttle for a while and then switched off to coast - and that's not because they don't know how to drive 'economically'!

The belief that a light throttle opening is efficient is actually fundamentally wrong, since no petrol engine operates efficiently when throttled back - and even the Valvetronic engine isn't achieving peak efficiency when the valves aren't opening much.

You can see this clearly demonstrated in official fuel consumption tests which use the same speed/acceleration for all engine sizes. The same car with a bigger petrol engine does not achieve better fuel economy (unless there is some other factor at work).

And the bit that most drivers cannot understand is that gentle acceleration is actually less economical - because the same kinetic energy is being obtained at less efficient engine conditions.

With diesel engines, which have good part-load efficiency, the effect is much less. Indeed that is why for most people diesels are more economical, since they can drive an engine well below its maximum power output without harming efficiency much. If you compare the official fuel economy tests for diesel cars with different size engines, there isn't anything like such a big range, showing that the larger engine is not much less efficient than the smaller one.
 
#3 ·
I can only speak from the experiences we've had with my wife's 2005 Seat Ibiza FR TDI. We got that remapped nearly six years ago by Revo which took it from 130hp to 172hp (and 318ft/lb).

Driven normally, we do get better fuel economy from it; tooting along a decent road at 55mph we can get over 70mpg. Drive it hard and it's very easy to see the economy is worse than it was. I suppose one cancels out the other and she still gets nearly 60mpg average.

In the much heavier Paceman and Countryman, I'd say a good quality remap would pay dividends when driven normally. I'm still not convinced by the tuning boxes, especially after trying one from a reputable supplier on a VW T5 recently.
 
#5 ·
The belief in light throttle efficiency is a holdover from carburetor days. With a carb, sometimes you really do get better efficiency with a mostly closed throttle. The reason being, greater manifold vacuum creates more effective atomization of the fuel droplets, hence more complete combustion.

Another factor to think about is the total weight of the car. If adding power means adding weight (a bigger engine), that weight will tend to decrease efficiency, especially in start-and-stop city driving. In addition, having more power available will likely change the way you drive. A 0-60 acceleration in 6 seconds requires more energy than a 0-60 acceleration in 12 seconds. These factors combined, most people who swap a more powerful engine will see their fuel economy worsen.
 
#6 ·
A 0-60 acceleration in 6 seconds requires more energy than a 0-60 acceleration in 12 seconds.
Nope, that's the bit that 'everyone knows' but is actually untrue. It takes the same energy to reach 60mph (kinetic energy is just mass and speed) so you want to 'buy' that kinetic energy with the minimum amount of fuel. Doing it in 6 seconds will require more power (energy per second), but for only half as long.

The most efficient way to accelerate any engine, petrol or diesel, is at maximum torque revs with the accelerator wide open, since that produces the highest amount of energy per unit of fuel. With something like a F56 engine with their long flat torque plateau this can be at quite low revs, but accelerating with minimum throttle opening in the belief that it is fuel efficient is an almost-universal error.

Appropriately enough, it was BMW who demonstrated this back in the 1970s.

This does not mean that fast acceleration is always good, since it may get you to a speed where you need to brake for traffic - and braking is always the most fuel inefficient thing that can be done, even if the OBC says that it gives a high instantaneous mpg figure!
 
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