I've always slotted my car of the moment into top (or fifth gear) just as soon as the engine's torque allows. I've contented myself that the lower revs will aid economy, but I may possibly get a bit of plug sooting (I tend to drive old cars). I guess I compensate for this as I'm not afraid to use revs to overtake / get clear of traffic / etc.
On balance it seems to work, but I'm now driving a newer car with electronic-shutter controlled intake length, antiknock sensors etc etc. It's a 2.0 litre V6, and while the power is a bit peaky, I find I can keep the car in fifth and drive almost like an auto - it'll easy (though not quickly) pull away from 1000 revs without complaint.
Am I kidding myself? Is the antiknock gubbins just retarding the ignition so much that it's really inefficient at these times? Am I likely to gain nothing in mpg simply because of that?
Just curious about others' thoughts on this. I'm not bringing "appropriate gear for the circumstances here" as it's hardly likely that a driving instructor would approve of me loafing around in fifth everywhere.
With an engine such as this- i.e. one with not alot of torque until quite high in the rev band, you might well find that getting staying in 3rd / 4th until you have more revs on the clock will be better for mpg.
Our 2 litre 20v Volvo certainly was better on fuel to stop in 4th until about 55-60mph.
Using low revs and a high gear for acceleration is bad for the crank and main bearings. Fuel economy is "mainly" governed by less throttle pedal and not by lower engine revs. Towing a caravan with the Chrysler very rarely sees fifth gear, even on the motorway. I did use 5th on my first outing with the 'van and the fuel consumption was scary, much better if I keep the revs in the power band. Anyway, if you can't be bothered changing gear get an auto :))
I've never driven a car anything like your one, so I can't really comment on the engine. However I suspect that pulling away in 5th isn't going to do the clutch much good.
Depends. It may be better - low revs, low torque / power + I'm certainly not flooring the throttle *because* I'm in fifth = less stress for the clutch to handle?
The top gear / floor the throttle test is for a suspected failing clutch - this scenario encourages clutch breakaway by slapping more power through than it can handle. It's a completely different scenario if you're feathering the throttle.
Ever tried driving a car with a failing clutch? The higher the gear the better if you want to make progress...
Every IC engine I have seen so far had it's best full power efficiency at or just below max torque. Efficiency also increases as you open the throttle. (Do not mistake efficiency with mileage consumption here). Given that, you want to operate your motor below max torque rpm with high loads, i.e. open throttle. In other words, use high gears (there's your fifth) and low rpm for best mpg. This has been my experience for all the different engines I have driven so far, no matter how old or recent the design. Please excuse my bad english, I am not a native speaker.
Ignoring the "appropriate gear for the circumstances" bit in terms of driving purity, I have always read that it's mechanically unsympathetic to push a higher gear with greater throttle opening - and I'd suggest that from
1k revs fits that bill.
Not knowing a great deal more on the subject (from an engines internal perspective) than what I've read, I understand it's bad for big end bearings, which are likely to be expensive to maintain, if not quite difficult.
Plus, using wide throttle openings, low revs and a big gear isn't entirely the best approach for fuel economy - although seems to be the taxi driver's approach.
Wouldn't you be more suited to an auto? And as a general rule (I appreciate that they can get wrong-footed) observing how and when a modern automatic changes gear is a reasonable take on which gears are appropriate. With the caveat that they tend to do things that manual drivers wouldn't ie change up when lifting off the throttle, in all other respects they can reveal what is best, overall, in terms of gear selection.
You're right there! They would normally tell you to come out of fifth below 50mph. But you won't get good fuel economy by letting the engine labour in too high a gear. Also you won't be able to control the car's speed as easily using the throttle. It's important to stay in the useful part of the torque curve. If you've got a wide torque curve - e.g. a big lazy engine - then you can stay in a higher gear longer.
Diesels are happier to slog at low revs than petrol engines, which is why taxi drivers do it.
BMC actually tried to design a gearless Mini by tuning the engine so that it had a very wide torque band, but it was unsuccessful. The prototype is at Gaydon (or at least it was).
I didn't actually say I used _wide open_ throttle with a high gear - did I give that impression?
If I'm on the level ground or even a very slight incline, the car will accelerate very gently with a **barely open** throttle from 1000rpm in fifth gear - it's certainly isn't quick and I don't tend to do it if there are impatient people behind.
I'm experienced enough not to labour an engine unduly - my main point was the contention that anti-knock technology, while preserving the engine from pre-detonation, effectively masks the area where the "old-fashioned" driver will be detecting low down engine strain.
Nope, I fear I've given the wrong impression of my driving style. I do work the stick but am mindful of leaving my car in a gear that the engine can cope with.
This is a strange engine, I maintain that power doesn't really come on stream until around 4000rpm, and the implication is that the torque curve is not very flat, but in part throttle scenarios it does seem as flexible as the multitude of big lazy V8s I've driven. Given that it displaces only 2.0 litres I assume that the intake length tuning system is doing a great job.
You're not complicating matters Peter - you're spot on! If there any semipro cyclists on the group perhaps you'll chime in here?
OT: cycling stuff...
AFAIK serious cyclists maintain the highest gear that their legs will power, electing not to change gear *at all* unless absolutely necessary for extreme inclines and the like.
I have a few cycling-crazed friends - easily spotted by their extra lean legs and bulbous knees ;-) They maintain that the best style to adopt is to find a gear that you're most comfortable with and stay in it 90% of the time.
Apologies, you didn't say that - although I was trying to say you did, it's just that the scenario you describe, lends itself to the situation where you'd try and push a high gear from relatively low revs.
Personally, I find that most cars I've driven don't seem that content in gears when the revs are below 1500 or so. That's just really a gut feeling thing. More so, the bigger the gear (ie less of an issue when in 1st or
2nd).
Whether there's any mechanical issues, or not, with driving like that, I guess it's hard to say, it just seems wrong - especially for 5th.
Accepting that, though, why would you push such a big gear, from low revs?
Well as I said, driving an auto can give you an idea of where the engine revs and gear selection should be ideally - I'm not saying that an autobox always gets the gear selection correct for every scenario - of course they can be wrong footed - but as a generalism, you tend to find that they have the engine revs and the gears about right.
As I said, I just find that most cars I've driving don't really like being below 1500 revs under load, that's not to say they won't do it, or that it will truly cause harm, just seems to be how the engine is happy to perform. And I tend to follow that with the gear selection I use - especially with the bigger gears (3rd, 4th and 5th). There may be some slight variation in this, and clearly, bigger engine displacements, or more cylinders, and more torque can kind of mask this.
I'm guessing it's a 2.0 6 cylinder - so either a Rover 75 or a Lexus IS200?
Personally I'd still recommend not going so low and expecting throttle response and acceleration - especially in the bigger gears.
As to the "cab driver" analogy - I'm guessing many drive so because of an over-simplistic rationale that biggest gear possible, plus lowest revs is best for fuel economy But kind of ignoring any consideration of where in the rev range and gear selection possibilities that the engine may be most economical for the road speed - either that or they're trying to save the clutch life by hardly ever using it.
Who knows has anybody ever asked a cab driver why they tend to drive like that?
Ah - see there I was, figuring I'd been clever, and you go and give then answer and reveal I'm not ;-)
I guess the answer is - and I think it will be a little subjective, if you car / engine feels comfortable driving how you are driving, then I'd suggest there's little to worry about mechanically. In cars I've driven, I kind of get a feeling for being at the sweet spot in terms of revs and gear selection, and I'm working on the basis (which may be flawed) that if you're doing something mechanically unsympathetic, you tend to get some feedback ;-)
It's that last aspect, "without complaint," that means everything... but then again if it's quiet you might not be able to tell! Plugging my OBD-II Scanner into the Ka shows how hard the engine is working relative to how hard it _can_ work under those circumstances and it's interesting. It's easy to plot Throttle along the X axis and Calculated Load along the Y-axis, and you get a broadly linear plot from 16.9% Throttle (idle, no pressure) to around 50% (Calculated Load is typically 100% by this point). From 50% to
95% the ECU does some funky stuff that I can't measure concurrently with the rest of the stuff (like enriches the mixture). Anyway I reason that an unusually high Calculated Load with a low Throttle means it's labouring...
I've done some work on this with our Ka and my OBD-II Scanner... and my results are inconclusive simply because it's difficult to measure fuel consumption with just choice of gearing as the only variable...
My work has been specifically about efficiency rather than fuel consumption, but for a given amount of power required, the fuel to be burnt _should_ be the same. Any differences will be because of friction or an inefficient burn.
Higher engine speeds means higher friction, although at extreme low engine speeds a colleague argues that low oil pressure may result in higher friction because the oil isn't able to get to all the places where it's needed.
Most of my work has involved driving along at 30 mph in third, fourth or fifth gear. It's complicated by using either 30 mph indicated by the speedo or by the ECU (the ECU is near as dammit what the GPS kits I've used report).
Using 30 indicated, 27 ECU, the donk is churning over at 2,350, 1,700 and
1,350 rpm respectively. There's little in it when driving along on a flat road without accelerating but _as soon as we try to accelerate_, the Calculated Load figure for fifth gear spikes up with relatively little acceleration. As in, barely increasing pressure on the Throttle by one or two percent (as reported by the OBD-II Scanner). It's around 65% as rapid to climb in fourth but in third, the engine is far far happier to accelerate (peak torque is produced at 2,300 rpm).
At an ECU speed of 30 (33 indicated), we have 2,600, 1,880 and 1,490 rpm respectively. Fifth exhibits the same behaviour, four is much, much better, and in third, it's on a song. :)
Plotting various stuff on a chart (not all of it on the website) shows that a labour / non-labour engine speed for acceleration that doesn't hold up busses :) is from around 1,850 rpm in the Ka (with this final drive) for third, fourth and fifth gear.
This means using third gear for an indicated 30 mph if I think I'll be changing speed... like in the city.
Have we noticed any difference in fuel consumption? Inconclusive - no difference. It hasn't hurt to drive around at 30 in third... and we have much better acceleration for the 30 to 50 dashes. :)
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