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Probably a silly question but why are most powerfull German cars electronically limited to 155mph? I noticed Jags are but cant figure out why.
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One possibility is that this is 250kph, which seems like a nice round figure to limit them to?
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I think it was a response to the green/morality lobby. A gents agreement was forged between certain car makers to stop them leapfrogging each others top speeds, and thereby being deemed irresponsible thus risking having some draconian limit forced on them. 250k for autobahns seemed like something they could get away with. Seems to have worked so far.
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I recollect it was also a response to the tremendous carnage on German roads. Not to mention road closures and inconvenience from blocked roads and diversions...
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Slightly off thread but my wife's Xantia HDi does 30 mph per 1k revs in top gear (90 at indicated 3000 revs). The red line starts at 4750 revs which theoretically gives a speed of 140 mph (ish)(and 150 at 5000 revs) yet the top speed is quoted as about 110 mph. Now, naturally I haven't tested this on the motorway but is the top speed limited by
a) Aerodynamics
b) The engine won't pull 4750 in top because of the gear ratio
c) some kind of limiter on top gear?
d) something simple and extremely obvious I haven't thought of!!
interested in your thoughts
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a. Yes but because of 'd'.
b. Yes but because of 'd'.
c. Yes 'd'
d. It aint got a big enough engine.
To be more precise you cant get enough air and diesel into it fast enough to produce enough power, without spending a fortune, and if you could it would blow up unless you spent another fortune so you might as well buy a 'bahn'stormer in the first place, and be able to stop without the aid of the scenery when you did wind it up to 140mph.
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a) and b), I think. More specifically, b) because of a), air resistance being by far the biggest obstacle at that sort of speed, and one that increases by the square of it (twice as fast gives four times the resistance). Mind you, the Xantia looks fairly slippery, so I'd be surprised if it couldn't do a bit more than that, unless the gearing really is too tall.
A simple example is an unfaired motorbike, which cannot do much more than 120mph, however powerful, because the rider would have to be tied on! With a fairing, 200mph is possible, although pretty scary, I imagine...
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Thanks JB and JBJ
JB Note I did NOT say I WANTED to do 140 mph, it was a hypothetical Q, though my wife seems to think that 100 is a comfortable cruising speed !(It's so quiet, I didn't realise) Also, if it won't pull 4750 revs in fifth because oof the lack of air and fuel, how come it will in 2nd and 3rd gear?? (My wife calls that "Lets show that idiot in the baseball cap driving an F reg BMW 316 what a middle aged woman driving a diesel can do" - don't read more into that than you need to!!)
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Because it'll reach max revs in second long before the aerodynamics come into play. So you dont need to overcome so much force. It has a surplus of power at this stage and is only prevented from applying it by the rev limited (a fuel strangler)or the pistons parting company with the con rods if it didn't have one.
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So it is mainly a) and not d)????? not enough air and fuel??
Thanks for answers - I put a) first because I suspected that was the main culprit. I know it, like all diesels, has a limiter, probably more sophisticated than that old truck I drove 30 years ago which just had a bolt under the throttle pedal which you could tighten down so it would do a max of 50mph instead of 40!!I will not be going out to test the theory!! - though my wife might - if the engine doesn't get too noisy!!!
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Sorry I didn't quite answer what you asked namely 'why will it rev past 4750 in other gears but not fifth'. The answer is IT WILL rev to max in fifth ------- if you jack the wheels up. In other words to do a certain speed, a certain amount of work is required. At 4750 in 5th i.e. 140mph or so, more work would be required than the engine can do (because of all the drag etc.) so it peaks out at a speed where work required to do that speed matches the work available. If you really like to try it, run downwind in a force eight gale. It'll do 140 then because you've reduced the aerodynamic drag.
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You may get it above 110 if you can find a long enough downhill on a motorway. In my more reckless days I remember a friend in a 4 speed fiesta (i think) screaming the engine and then shifting into neutral when the car sounded like it was about to blow - and on a steep downhill it did do more then. Come to think about it, shifting to neutral at perhaps 90mph+ sounds rather stupid.
So I would assume you could get 140mph on an ideal (steep) downhill in gear and more in neutral
Just don't tell the policeman I told you to do it!
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