My Mazda 6 has a 2.0 naturally aspirated petrol, but being a 'SkyActiv' engine (14:1 compression and long gears) it generally drives below 2k revs at lower speed limits, and at low speeds the drop between gears can be as little as 250rpm. By that I mean around town I'll generally drive at 30mph in 4th on long straights with little by way of hazards, and drop to 3rd for roads with banks of ATS, more pedestrians etc to give better retardation and more flexibility for acceleration sense.
Please bear in mind I live in Liverpool, and as such the 30mph roads I'm talking about are generally all miles-long straight dual carriageways with two lanes each side, not little single carriageway B roads. Obviously as you get closer to the city centre the roads become more crowded, bendier and often smaller (and demand different approaches), but for the purpose of my question I'm talking about the suburbs and their long D/Cs. Here's an example:
With such long straights it's pointless screaming along in 2nd, and 3rd isn't much better than 4th at these speeds (about 1,800rpm instead of 1,400rpm). The gearing does seem to present me with a problem when it comes to avoiding BGOL. If I've understood von's post on ADUK correctly, using the clutch to prevent stalling/shuddering is only fine if the resulting gear is 1st. Anything else is 'overlap, and not on the grounds of safety'.
So how do I approach this from an AD perspective and avoiding BGOL? Consider this:
- I'm in 4th (or even 3rd) at 30mph and the ATS is red ahead, with a car waiting at the stop line.
- I ease off the gas (I'm a few hundred yards away) and/or apply the brakes to shave off some speed.
- I'm planning to stop but looking to go. If the light stays red, I stop. If it goes green, I select an appropriate lower gear and accelerate again (in theory).
The problem is, as I'm already cruising below 2k rpm (often closer to 1,500rpm around town) I've not much 'play' left in the rev band for slowing down. By the time I'm down to 20mph the car is already shuddering and threatening to stall as I've 'run out of revs', so my options seem to be:
1) Clutch in just before the 'shudder' and continue losing speed. If I end up braking to a stop (light stays red) then all's well as I then select first. This isn't considered BGOL. If the light goes green before I stop then I may well be looking to move down to 2nd (20mph plus). That would be BGOL by Von's definition as I had the clutch in before selecting a non-first gear.
2) Adopt a lower gear between 'spates' of braking (i.e. down through the gears). This obviously isn't System.
3) Drive along shuddering loudly and hope I don't stall before I can select a lower gear and accelerate (lol).
One might suggest I am in too high a gear around town in this case. As detailed above though, short of driving around town in 2nd gear - which is loud, not especially smooth, and uneconomical in the extreme - what to do? I know my friend has a similar issue in his diesel engine, in that it wants to be around 2k revs but that leaves him only 1,000rpm for slowing due to hazards and he then hits the same 'wall' as I do.
On higher limit roads it's not really an issue. The car will happily travel in the same 3rd or 4th gear right the way up to (and beyond) NSL and remain quiet, smooth and pliable with bags of 'space' up or down the rev range. This gives closer to 2,500 / 3,000rpm at a steady road speed, with flexibility to slow and adopt the next gear down where necessary without impacting the drive (and allowing full BG separation), as well as an extra 3,500rpm for acceleration before approaching the red line. Generally then, I'll also be using 3rd and 4th on roads up to and including NSL, dropping to 5th on longer/emptier bits to rest the engine and aid economy, and back to 3rd/4th where required (approaching hazards or busier areas). Sixth gear doesn't really get a look in unless I'm on a long, straight, empty motorway stretch and expecting to cruise for many miles. More often even on the motorway I'm in 4th or 5th gear, such is the gearing.
As such you can see how adopting the same two gears at 30mph is practically already only ticking over, and as such presents me with difficulty when trying to apply system to slowing and down changing without using BGOL (or else driving everywhere in the relatively short 1st/2nd!). So with such leggy gears and the choice between driving everywhere around town in 2nd or seemingly unavoidable BGOL, what am I missing?
Apologies for the long post but it's something I've been struggling to work through alone this week. Does anyone have any ideas or advice? Have I managed to completely misunderstand BGOL? Cheers.