Remap for fuel economy

[clanless]

I've been looking at these tuning boxes that claim to increase MPG.

With fuel at an all time high price, one of these contraptions is starting to look quite attractive.

The only sticking point for me - how can a tuning box or remap for that matter increase mpg? Diesel has the same energy content - so to travel a given distance, surely you need to use the same amount of fuel?

Or am I missing something obvious?

[stevoxsi]

I had one and they seem to work! More power and more mpg. My theory is the engine has less work to do because of the extra power. But the two are exclusive! Go easy, more mpg. Drive hard, faster but bye bye diesel!

[john_h]

yeah i can vouch for that...if i drive with a heavy foot i get 44mpg and when easy 57mpg

[Bird dog]

The best way to increase mpg on a diesel is to raise the gearing and fit the least powerful engine that still drives the car forward with reasonable aplomb (exactly the opposite of what a tuning box does). This is what most manufacturers are currently doing to their own diesel line-ups, hence why Ford, Skoda, Citroen and others are introducing 1.6 diesel versions of their larger Mondeo, Superb, C5 type cars.

Tuning boxes and remaps also mess up your trip computer read out.

They actually make it look like you are getting more mpg, when what is actually happening is that the ECU can't 'read' the amount of extra fuel that the tuning box/remap is using and only registers the amount of fuel that it actually thinks it's using, which will be less than normal because the tuning box will always be adding fuel to the engine under normal running conditions and that means that the ECU thinks the engine is running on less fuel under the same conditions that it would be without a tuning box/remap.

Ironically, the more fuel that tuning box puts into the engine the less fuel the ECU thinks that the engine is using. So the faster you drive, the more economically the trip computer says you are driving relative to what the normal readout would be!

Remaps can be different in that respect. Some will take account of the additional fuel while others won't. It depends on how honest they are being with their product.

[stevoxsi]

I disagree. I worked my mpg out properly and it was better with the tuning box. Also when the car was standard I was returning better mpg than a friend of mine from my 110bhp compared to his 90 doing roughly the same journey and a similar driving style.I always find an underpowered car needs working too hard to get up to speed to return decent mpg. When up to speed on a flat road, fair enough the smaller capacity engine will perform better but lets face it, how much of your driving takes place and a flat 60mph road? I'm actually getting slightly better mpg from my 2.4 D5 Volvo than I was from the 2.0 HDI 136 307.

[clanless]

Interesting reply, thanks. Just as I suspected, you don't get owt for nowt. The first law of thermodynamics - dammit, is correct.

Following this through - how does the size of the engine have an impact upon fuel economy? The energy content of diesel is a constant - so for a given speed and distance, surely the size of the engine is not relevant.

So, say a 1.6 and 2.0 engine drive a car at 30 mph for 10 miles - the larger engine, although using more fuel per cycle, has more oomph - so doesn't work as hard as the 1.6. Each will use the same amount of fuel, surely.

[Gibbo]

Originally Posted by stevoxsi

I disagree. I worked my mpg out properly and it was better with the tuning box.

Are you sure its not a placebo though? My car always drives better and returns better MPG when I wash, polish and valet it!

Its very difficult to get really accurate readings with everyday driving. I fill up every 10 days or so. Some weeks I get 500 miles out of a tank, most others I get almost 600.

[Bird dog]

Before we got our Peugeot 3008 1.6 HDI 112bhp Sport, we had a Vauxhall Vectra 3.0 SRI V6 diesel. All I can say is that I do not have to change gear any more often or at a higher engine speed in the Peugeot than I did in the Vauxhall (both have six speed manual gearboxes with very, very similar gearing: 50mph @ 1500rpm in 6th Both of similar weight) at normal driving speeds. This tells me that putting a tuning box on the Peugeot isn't going to make a blind bit of difference to fuel economy, simply because the added power and torque that the tuning box provides isn't going to get used under normal circumstances where you would expect to get better mpg. I can drive the Peugeot in just the same way that I drove the Vectra with less power and torque in situations where I would expect to be getting good mpg. Indeed the Peugeot is more economical with less power and torque because the ability to use the extra power is no longer there. In short - The 3008 is just as gutsy as the Vectra where it matters with regards to mpg.

Sure the Vectra would leave the Peugeot behind in a straight fight, but the Vectra would also be using more fuel in doing so.

We also put a tuning box on the Vectra. It used more fuel - can't think why. Probably because the only way you could feel any real benefit of putting a tuning box on it was to drive it at faster than normal (i.e. silly) speeds.

The only people that may really benefit from a tuning box with regards to mpg are people that tow caravans etc quite often, and even then it would only be marginal improvement.

[Bird dog]

Originally Posted by clanless

Following this through - how does the size of the engine have an impact upon fuel economy? The energy content of diesel is a constant - so for a given speed and distance, surely the size of the engine is not relevant.

So, say a 1.6 and 2.0 engine drive a car at 30 mph for 10 miles - the larger engine, although using more fuel per cycle, has more oomph - so doesn't work as hard as the 1.6. Each will use the same amount of fuel, surely.

I bought the 112bhp 3008 diesel with the 1.6 diesel engine. If I had bought the 2.0 diesel I would not expect to get the same mpg as the 1.6 when travelling along at the same speeds, as indeed the official fuel consumption figures do show. Moving those larger pistons up and down must have something of an effect on fuel economy.

Maybe on an Autobahn the 2.0 would have better mpg if it were cruising alongside a 1.6 that had reached its maximum speed?

I should also say at this point that gearing has a lot to do with mpg, and it tends to muddy the picture a bit when trying to compare what might be just seem to be the difference in engine cubic capacity. Higher gearing is best for economy. Lower gearing is best for performance. But putting too low gearing with too much power and vice versa, can be counter productive both in terms of performance and economy.

[stevoxsi]

Definately had improved mpg with the box in. It was a DTUK one. Cheap ones not good but the digital ones I like. But as I say just my experience. Better a remap though!