Optimum Revs in 1.9TDi ?

[PassatNoMore]

I was lecturing Mrs P the other day for driving our car like a petrol engined motor - cruising along at 30 mph in 4th but the revs are about 1100 - I told her that she's best to keep the revs around 1800 -2000 for best fuel economy and that labouring the engine uses more fuel. I doubt whether it'll make much difference tho'.

Anyway i was watching Top Gear on Dave and Clarkson run an Audi V8 diesel from London to Edinburgh and back on one tank of fuel - he said that the revs were never much above 1200. If he's right then that blows my theory about 2,000 being optimum. Now I'm confused :confused:

[skodakid]

Complex subject, at 1900 rpm the engine is the most efficient at full engine torque (floored throttle, turbo fully wound up). But that has nothing to do with economy. I wont bore you with graphs, lectures about mapping and the theory about gearing, but as a rule of thumb, the slower the engine speed and the higher the gearing, the more economical you are driving. There is a point at which you can drive too slowly and as you point out the engine labours, but this varies with load and is pretty near the 1000 rpm level for the 1.9 tdi.

Edited 25 September, 2009 by skodakid

[Guest]

Mr P, I too saw the Audi economy run with Clarkson at the wheel. Don't forget...the car he was driving had a V8 diesel in it. Although he was keeping to 1200rpm, this V8 was providing the same number of firing strokes as a 4 cylinder unit revving at 2400rpm. My Fabia 3cylinder diesel 1.4tdi 80 PD is running at about 1400rpm at 30mph in 4th gear, although it sounds as if it is doing nearer to 1100rpm due to fewer firing strokes per crankshaft revolution when compared to a 4 cylinder unit. The main criteria is don't work the engine too hard at low revs as it will labour as has been said, using more fuel. Also the oil pressure is relatively low at around 1000rpm or just above and therefore is not a desireable place to hang out at.

[PoloGaz]

I've never had a TDI that sounded right doing 30 in 4th. Anything below 1500rpm in 4th, 5th or 6th unless I'm going down hill, it's time to change down. I always think there needs to be a "3 and a halfth gear" cos 3rd is a bit too short for 30mph and 4th is too long.

[Hauptmann]

I tend to rev the 1.9TDI's to about 3000rpm before changing gear - if nothing else it excecises the turbo vanes. In steady speed driving my 'rule of thumb' is 30 in 3rd, 40 in 4th and 50+ in 5th. The engine is labouring at 30mph in 4th, even in a small car like the Fabia.

[KenONeill]

Define "optimum". And while you're at it say what you want to optimise for.

Seriously, at 1100 on a flat road most of the grumbling noises are probably from the DMF, not the engine labouring.

Best economy will be 1200 to 1500, before the turbo cuts in, and best acceleration will be around peak torque in first and second, rev it to stay above 2000 after the upshift in 3rd, and above 2500 after shifting 4th to 5th.

[PassatNoMore]

Thanks guys - I agree that the rule of thumb (20 in 2nd, 30 in 3rd, 40 in 4th etc) seems to work best. "Optmum" maybe wasn't the clearest title for this thread. By optimum, I mean keeping the revs in a band where you get the best mix of fuel economy and power when you need it.

I think that if you're tootling along at 30 in 4th, the engine is probably labouring even if it doesn't sound like it and if you suddenly need a burst of speed the engine won't respond very well with the revs that low.