Doctor Diesel

Fuel-frugal Fiat in fine fettle

Web02Hello Doc.
First the data: November 2005 Fiat Panda 1.3 MultiJet, and mine since May 2008. I’m covering 12,000 miles a year, and the car has now just turned 100,000 miles. More than half of these miles are in the form of 40 to 100-plus mile journeys on single carriageways. Oddity: The specification says my maximum torque is at 1,500rpm and maximum power is 70bhp, but a car clinic gave the maximum torque at 2,300rpm and maximum power as 76bhp, so I seem to have a non-standard turbo or something. This means I have a wide band below the speed at which any decent grunt starts to appear – at around 1,800rpm. Over the years, I have greatly improved its economy, partly in the usual ways, by less aggressive driving, more anticipation etc. But I have also changed to using a lower gear than I used to before, so that the engine is always running well within its torque band – say around 2,000rpm. This means using third gear rather than fourth at 30mph, and fourth instead of fifth at up to 45mph. This greatly improves the driveability of the car, with more engine braking when it is wanted, and instant acceleration when needed, but I also believe that the engine will be most fuel efficient when it is turning fast enough to get the turbo working. Am I right in thinking this? By the way, I don’t usually reset the trip meter until it resets itself after 4,000km, or 2,485 miles. The dashboard display is over ten per cent optimistic, depending on the weather conditions (warm and dry or cold wet and windy), and the mixture of journeys I have been making, the display will settle to between 75 and 85mpg, which in fact means I am actually achieving between 65 and 75mpg – much better than more modern cars which claim much improved economy.
Duncan Martin

You are indeed right Duncan. Firstly, regarding the quoted torque and power figures and rpm: I think you are saying that you had a dynamometer run performed at a car clinic, but even if you didn’t, the following is relevant. Dyno runs are performed in an appropriate gear, usually one below top gear, on account of the fact that in days past this was often the 1:1 direct drive, with top being an overdrive with a ration of less than 1:1, because the transmission losses are minimal in that gear. Nowadays that’s not often the case, but they still tend to use the gear below top. Anyway, they get the car ticking over on the rollers at about 1,000 to 1,500rpm in a low gear, get some revs on, and then drop it into the chosen gear, drop the clutch quite brutally and boot it at maximum throttle, running the engine right up to maybe 4,500 to 5,000rpm, by which time the power is dropping fast, and the peak torque is long gone. So it’s a dynamic test, not a steady state test, and at the lower engine speeds there is an inevitable degree of turbo lag. So the torque figure recorded at say, 1,800rpm, would increase if the engine was held at that speed, although that’s only a real world situation if the engine is pulling on full load at a steady 1,800rpm, which would only happen on a hard pull up a steep hill. So, in the borderline area where the turbo is not really working, the torque figures produced (by calculation from the power output) are depressed from the potential maximum, which I’m fairly sure is what the manufacturers derive, from bench tests with the engine out and direct power measurement from the crankshaft taken at a series of steady speeds, which generates the higher figures. There are also calculation corrections made for transmission losses, by the way, which are measured and recorded during the engine run-down period, at the end of the test. I am also of the opinion that manufacturers like nice round figures, and like to quote high torque at low engine speeds, so they probably round down the engine speed figure, like from 1,450rpm to 1,400rpm, etc.
Your test maximum power figure of 76bhp is not out of the ordinary, as (in contrast to the torque figures) manufactures often tend to understate power outputs, sometimes I think to get lower insurance groupings. I have seen figures for tuned 1.3 MultiJets up to 90 to 95bhp, but referring back to some data, I see that peak torque both tuned and untuned was at somewhere around 2,700 to 2,800rpm, and increased by conversion from approximately 125lb ft to 150lb ft of torque. So I feel that all that you say is correct, and your philosophy of not flogging the engine at low rpm is correct. In fact, based on the figures above for peak torque engine speed, you might well take around 2,500rpm as being your best economy/max torque figure to work with.
The 1.3 MultiJet is/was a great engine and I well remember being told at the launch (in the Punto) that it was designed for a life of 200,000 kilometres, or nearly 130,000 miles, with the chain driven camshaft (not belt) being one of the significant plus points. The few negatives with the 1.3 MultiJet have come with emissions controls, and the addition of the diesel particulate filter a few years ago naturally generated problems with folk who bought 500s, in particular, for local running around. Sales of 1.3 MultiJet 500s have actually been very low, and I don’t think Fiat really push sales of 500 diesels now, which is sad; but then you would struggle to get the extra cost of the MultiJet back over a low annual mileage.
Hope this all makes you feel good, as it certainly should, and well done for getting those sort of figures for the fuel economy, by the way.
Doc D

P.S. Are you sure that you are running on standard wheels and tyres? It’s unusual for an on-board-computer’s mpg figures to be quite that far out!

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