Discussion
I really like small cars. I don't even mind modestly-powered ones because it is fun to maintain momentum.
But I hate trying to get the modern 1-litre ones (whether 1.0 n/a or 1.0T) moving from a standstill in a hurry, as is frequently required to join the busy, fast-flowing A-roads in my area.
The problem is the tendency of engine revs to 'hang' during upshifts (which I believe is to reduce emissions) but requiring a very-slow-motion 1st-2nd gearchange to allow the revs time to drop, but that costs valuable seconds, loses forward momentum and hugely increases time exposed to danger on poorly-sighted NSL junctions.
If not a slow-motion gear change, it requires a very early upshift from 1st-2nd, causing the car to bog down and not be any quicker to get up to speed than making a slow-motion gearchange as above.
The final option is to just dump the clutch with no mechanical sympathy and accept a jolt to the transmission, and get a new clutch every couple of years.
So do you have a modern, 1-litre car?
What do you do to work around the rev hang?
If your revs don't 'hang', please share the make and model.
I would be interested in peoples thoughts.
But I hate trying to get the modern 1-litre ones (whether 1.0 n/a or 1.0T) moving from a standstill in a hurry, as is frequently required to join the busy, fast-flowing A-roads in my area.
The problem is the tendency of engine revs to 'hang' during upshifts (which I believe is to reduce emissions) but requiring a very-slow-motion 1st-2nd gearchange to allow the revs time to drop, but that costs valuable seconds, loses forward momentum and hugely increases time exposed to danger on poorly-sighted NSL junctions.
If not a slow-motion gear change, it requires a very early upshift from 1st-2nd, causing the car to bog down and not be any quicker to get up to speed than making a slow-motion gearchange as above.
The final option is to just dump the clutch with no mechanical sympathy and accept a jolt to the transmission, and get a new clutch every couple of years.
So do you have a modern, 1-litre car?
What do you do to work around the rev hang?
If your revs don't 'hang', please share the make and model.
I would be interested in peoples thoughts.
It's very common with DBW throttles but it shouldn't be of the order of seconds. The worst I've ever experienced is maybe half a second (which feels like forever); most engines are considerably less than that. I've never noticed a correlation between this behaviour and the size/power of the engine though. In fact some of the worst culprits I've driven have been relatively powerful models.
If it's really costing you seconds on your gear change, I'd imagine you've got a sticky throttle position sensor.
If it's really costing you seconds on your gear change, I'd imagine you've got a sticky throttle position sensor.
Edited by kambites on Monday 7th May 13:57
kambites said:
It's very common with DBW throttles but it shouldn't be of the order of seconds. The worst I've ever experienced is maybe half a second (which feels like forever); most engines are considerably less than that. I've never noticed a correlation between this behaviour and the size/power of the engine though. In fact some of the worst culprits I've driven have been relatively powerful models.
Maybe it's more noticeable with 1-litre cars than larger engines because with the larger engines it's easier to make an early 1-2 change to allow a smooth shift and still have decent acceleration available from low rpms in 2nd.My wife's 2009 Vauxhall Agila 1.2 used to do this a lot. It was most noticeable when cold and the effect was pretty much noticeable any time you came off the throttle quickly, the revs would hang for a millisecond and then fall off slowly. It was irritating but hardly a deal-breaker in an already woeful car like that one.
It's caused by the ECU delaying the closing of the throttle to keep combustion going when the driver has lifted off sharply, to minimise emissions of unburned hydrocarbons from the exhaust. I didn't like it because it gives the car the slightly mushy pedal-response that you used to get from older diesels with sluggish turbos. I've read of people mapping it out where the really want rid of it.
It's caused by the ECU delaying the closing of the throttle to keep combustion going when the driver has lifted off sharply, to minimise emissions of unburned hydrocarbons from the exhaust. I didn't like it because it gives the car the slightly mushy pedal-response that you used to get from older diesels with sluggish turbos. I've read of people mapping it out where the really want rid of it.
Nanook said:
Do modern 3 pots have heavier flywheels than 4 pots of similar size?
I would have thought so.A noticeable thing is that my wife's 1L/3-cyl Viva manoeuvres very easily with just the clutch but my 1.6L/4-cyl Swift is easy to stall if trying to manoeuvre with just the clutch. Also downshifts are easier/smoother with the 4-cyl.
That suggests the 1L/3cyl has a hefty momentum-retaining flywheel whereas the 1.6L/4-cyl has a light flywheel.
The Swift's rev hang is much less - it prefers fast upshifts otherwise its revs drop too much; drive it with a 'relaxed' style and the gear changes will jolt because the revs dropped too much during the gear change.
Note that I'm not complaining about my car, the Swift; I chose it specifically because it can make quick smooth getaways and copes well with fast upshifts. But I would have preferred a 1L tiny town car, if only I could have found one that didn't rev-hang and which I could fit in comfortably.
Loyly said:
It's caused by the ECU delaying the closing of the throttle to keep combustion going when the driver has lifted off sharply, to minimise emissions of unburned hydrocarbons from the exhaust.
Almost the opposite, as I understand it. Modern injection is very accurate - the system could easily slam the throttle shut and cut all fuel injection as soon as the pedal is lifted but doing so would result in the small amounts of unburned fuel remaining in the system mixing with the far greater quantities of air still in the intake manifold. The resultant very lean burn would generate of lots of nasty nitrous oxides, which aren't allowed. To get around this, the ECU shuts down both air supply and fuel supply in a much more controlled fashion, so it can maintain the correct stoichiometric mix throughout the process.
The net result of this is that there is effectively a limit to the rate at which you can come off (or onto, in fact) the throttle. The best way to drive around it is to make sure you never make sudden changes to throttle position; if you keep your pedal movements smooth, the relationship between throttle position and pedal position will be much more consistent. Of course in the situation the OP describes, that will slow you down a bit. Or buy an automatic which will be programmed to engage drive at a rate which matches what the throttle is doing.
Edited by kambites on Monday 7th May 14:23
I had an ecoboost 125 for a time and whilst it's fashionable for some to sneer at ubiquitous engines like this one , I thought it was extraordinarily good . It was smooth , near silent(and the noise it did make was nice) but God above , what I now know to call rev hang was absurd .
It took an age for the thing to work out nobody was giving it gas any more so it felt like Isambard Kingdom Brunel had designed the flywheel - making the average diesel feel like a Cosworth DFV in comparison . And yes , it really did screw gearchanging up if you wanted to change quicker than on you might on a pre war Humber with no synchro . Total PITA
It took an age for the thing to work out nobody was giving it gas any more so it felt like Isambard Kingdom Brunel had designed the flywheel - making the average diesel feel like a Cosworth DFV in comparison . And yes , it really did screw gearchanging up if you wanted to change quicker than on you might on a pre war Humber with no synchro . Total PITA
Strudul said:
Ron99 said:
With a big engine, you can shift up early and still have more than enough acceleration.
With a small engine, shifting up early will greatly impair acceleration.
Why not shift later then? High RPM shifts are smoother and the revs will drop faster.With a small engine, shifting up early will greatly impair acceleration.
On some occasions I think I've had some success in reducing the rev hang by flooring it in 1st until it hits the limiter, then making the upshift. I'm guessing the limiter cutting fuel is what helps the revs drop a little faster.
Zad said:
Don't they keep the revs high to maintain the turbo boost, hence resulting in less lag?
It's not just boosted engines; I haven't yet come across any 1.0 n/a or 1.0T that didn't suffer significant rev hang although a fair proportion of the small cars out there are too small for me.Ron99 said:
It's not just boosted engines; I haven't yet come across any 1.0 n/a or 1.0T that didn't suffer significant rev hang although a fair proportion of the small cars out there are too small for me.
We're running a petrol 1.2 3-pot turbo'd 110bhp Cactus as the family hack at the moment. The same engine is fitted to the current C3. I haven't found it to exhibit the traits you describe (at least, not in any way that I find material to day to day use), so might be worth a look. The benefit of the Cactus is that it's actually quite roomy inside while also being very light.
Edited by Gruber on Monday 7th May 20:32
Gruber said:
We're running a petrol 1.2 3-pot turbo'd 110bhp Cactus as the family hack at the moment. The same engine is fitted to the current C3. I haven't found it to exhibit the traits you describe, so might be worth a look.
The benefit of the Cactus is that it's actually quite roomy inside while also being very light.
That's interesting - thanks. The benefit of the Cactus is that it's actually quite roomy inside while also being very light.
I have an Insignia for family duties so wouldn't want my second car to be larger than B-segment and preferably A-segment. However I think I've seen a few Peugeot 208 with 1.2T which might be worth me taking a look.
Have PSA dealt with the mucky valve issues that plagued their earlier DI engines? I hear that Ford's latest 1.5T Ecoboost (as fitted to new Fiesta ST) uses both PI and DI. Unfortunately, and back on-topic of rev hang, at least one magazine review grumbles about the new Fiesta ST.
Quote from a recent Fiesta ST review:
'.....I’m not totally sold on the time it takes the crankshaft to slow down from high revs, I have to say – a function of the counterbalance measures that three-pots need to run smoothly at low engine speeds.....'
My Mum's had an R50 Mini Cooper for years now - the rev hang on that is appalling. Of all the cars I've driven its the most difficult to heel n' toe. Driving slowly, the slow decay of revs isn't that noticeable. Similarly, if you absolutely nail it the engine seems to respond more appropriately. Everything in-between, however, is awful. All gearshifts become a slow, protracted negotiation with the ECU.
Weirdly, the engine responds quite quickly, but almost in a random manner. A small blip of throttle may give you a 250rpm blip, but an extra millimetre of pressure on the throttle would balloon the revs by 1500rpm... and then you have to wait an age, clutch, gear in, waiting, waiting, waiting for the revs to decay enough for you to de-clutch.
Really spoils an otherwise good car. I wonder what it would've been like the with cable-throttle'd K-series engine that the development team wanted to use, instead of the mediocre Chrysler unit it ended up with? Probably would have been terrific (between HGFs, anyway...).
I prefer cable throttles. You always know where you are with a cable throttle.
Weirdly, the engine responds quite quickly, but almost in a random manner. A small blip of throttle may give you a 250rpm blip, but an extra millimetre of pressure on the throttle would balloon the revs by 1500rpm... and then you have to wait an age, clutch, gear in, waiting, waiting, waiting for the revs to decay enough for you to de-clutch.
Really spoils an otherwise good car. I wonder what it would've been like the with cable-throttle'd K-series engine that the development team wanted to use, instead of the mediocre Chrysler unit it ended up with? Probably would have been terrific (between HGFs, anyway...).
I prefer cable throttles. You always know where you are with a cable throttle.
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