Doctor Diesel

Irrational ratios

Irrational RatiosIt’s me again, bellyaching about gearing yet again! I am presently using a Toyota Aygo hire car in Spain (petrol, I’m afraid), in which you have to rev the engine like blazes to get started in first without stalling, even on the level. Crazy! Anyway, a month ago we bought a three-year-old SEAT Ibiza 1.2 TDI CR Ecomotive with only 21,000 miles on the clock. Its elegance, economy and general comfort impressed us, but we were made aware of a potential irritant. On our way to a suitable place to try the car, the young sales assistant spied a good stretch of road and evidently thought: “Ah! Dual carriageway means fifth gear!” At 35mph, and at all of 1,050rpm, the boom and vibration were indescribable and little better when we reached 50mph and 1,500rpm. After 21,000 miles the engine might be used to that kind of treatment, but is labouring like that no longer seen as being harmful? Whilst second gear takes you from standstill to 30mph in a trice, the step up to third is an incredible 75 per cent – about the same as second to fourth in most cars. It would be manageable if the engine didn’t labour below 1,800rpm though, with that booming resonance, even when a dashboard signal actually recommends engaging each gear from below 1,500rpm. On a modest, but lengthy, urban gradient, with traffic conditions allowing a steady 28 to 30mph, third gear is unthinkable and, for a diesel, even a small one, this is plain silly. The jump from third to fourth is some 41 per cent, and whilst a wider gap would have resulted in a sensible third gear, fourth still labours at 43mph. Surely SEAT boobed here? Fifth (top) gear is wonderful, with a mere 2,120rpm showing at 70mph, though many would resent having to drop down to fourth at 60mph, when labouring sets in. 

I wonder if a six-speed box could be fitted, but still the 1,400 to 1,800rpm range would be unusable. Tunit, who seem to win golden opinions and have been most helpful by phone and e-mail, assure me that they could achieve higher torque at lower revs per minute, so am somewhat inclined to take this route – unless you know of any simpler and more obvious remedy, or unless you consider that a minimum driveable figure of 1,800rpm could indicate a fault. I reckon the Tunit treatment could pay for itself in two to three years. One of my saucier friends points out that I’ll be a historic old codger by then. Well, I’m already a historic fan of Diesel Car and aspire to be a contented old codger in due course. Best wishes to you and your team, 

John R Turner, Glasgow

My word, my brain hurts a touch after digesting all that John, but I certainly take aboard your various points. I recall feeling something of the same frustration when I drove a similarly geared Polo 1.2 TDI 75, but with regard to its gearing and performance more than its lack of refinement. Your quoted road speeds are speedometer figures which, by my calculations, are around 10 per cent high. According to SEAT gear ratio figures, for instance, your 70mph at 2,120rpm is actually only 65mph, give or take a few tenths. So that makes some other figures somewhat less extreme, although I can certainly understand you (apparently) habitually taking off in second gear, and the gearing is certainly far from ideal, although possibly aimed at producing the best EC test fuel economy figures!

Volkswagen Group TDI engines often have quoted maximum torque speeds (in this case 133lb ft at 1,500rpm) that are well on the optimistic side, and I’ve seen independent dynamometer plots that support this, showing peak torque speeds 400 to 500rpm higher than the quoted figures. But let’s get this in perspective. Much of your irritation is generated by the apparent lack of torque, although this would not be so irritating if the significant boom and vibration were not apparent. I do think the latter is worth checking out – specifically in the areas of engine mountings and the exhaust system, to body “hangers”. The Ibiza’s three-cylinder engine is not the smoothest of beasts, but it is very robust though, with its short and sturdy crankshaft, and my memories of the 90bhp 1.4 TDI variant in a (lower geared) Audi A2 are not of a particularly boomy or inflexible engine. There is a somewhat psychological effect with a three that makes it sound as if it’s labouring, because there are 25 per cent fewer firing pulses than with a four. But you should be able to run this engine down to 1,200 to 1,400rpm (although maybe not under heavy load) without causing any damage. But at 21,000 miles, and possibly with a mechanically unsympathetic previous owner, this engine may still be pretty tight, and if you keep trying to run it at low engine speeds it may stay that way. I suggest that you start giving it a bit more stick in the lower gears, and ask the supplying dealer (or a helpful independent garage) to check that the exhaust system is hung properly, and that the bushes in the engine mountings are not perished or damaged. You can even get someone to turn the engine off while you have the bonnet open and observe to what degree it shudders and shakes on shutdown.

It’s also even possible that the injection system is a bit gummed up and that the low-engine speed spray pattern is suffering, so I would source some additive such as the excellent Exocet Diesel Power Restorer and give that a whirl. As regards tuning, possibly by Tunit, who are certainly very well spoken of, this may well produce more low-down torque, but I’m not utterly convinced that it will necessarily eliminate the boom and vibration, and not entirely convinced that you’ll see your money back within three years. I hope some of the above is helpful. But it’s easy to get hung up on such a problem when, with a bit of thought and flexibility of attitude, you could often drive around it. Best regards,

Doc D

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