RE: PH Blog: freewheeling
Discussion
filski666 said:
so what do you do on a steep descent - ie when you see this sign?
Your decoupling system disconnects you from your engine braking - you set fire to your brakes and you crash into a pile of children and kittens?
I'd like to think there's a tilt sensor somewhere that will prevent de-coupling above a certain threshold. Or maybe this is just a ploy to sell more brakes at the exorbitant OPC prices! Your decoupling system disconnects you from your engine braking - you set fire to your brakes and you crash into a pile of children and kittens?
p.s. anyone with a Sport button on a Porsche that doesn't use it all the time is missing the point. When I sold my Cayman S for an M3 I kept hitting the similarly located START/STOP off button as I was that used to hitting Sports Plus as soon as I got in the car.
Edited by CJP80 on Thursday 26th July 13:17
dom180 said:
Chris Harris said:
dom180 said:
Good review but why do 99% of reviews fail to mention the chassis setup/options fitted when reviewing latest pork.... - let me guess 20 inch alloys with PASM but no diff/PTV or chrono/mounts but only as GT Porsche tested that car!
What the hell has the chassis set-up got to do with a specific piece about the de-coupling system?Since Porsche are so configurable, if you're raving about the latest model but failing to tell us the spec, we can only guess on options/what works and, since no-ones done a what options work best on a Boxster yet or a PASM and/or PVT and/or/chrono v non, be nice to get as much detail as possible.
Edited by dom180 on Thursday 26th July 13:10
from the Porsche website:
Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
MIP1983 said:
I imagine it's not going to coast if you get on the brakes. Simple answer though, get a manual.
I have driven the new Boxster (2.7) with the manual. Whilst the gear change is fantastic, there's one massive failing that was enough to put me off considering a manual 2.7: the gear ratios - they're ridiculous! Country lane blats use only 2nd and 3rd. It completely ruined the experience for me. Shame as it was brilliant otherwise. 
loomx said:
What the hell?
Effectivly putting it nuetral will only save fuel in really old cars, like carb'ed cars because they fuel all the time!
In any half modern fuel injection engine, you lift off, the injectors turn off and the inertia of the car/drivetrain turns the engine over, so effectively in that coast you are getting infinite MPG.
You may get infinite MPG but for quite a short time/distance as your rate of decelaration is quite high due to friction/engine braking whereas coasting means that you can go much further without using any fuel. I can't see many real world situation where this will actually work out though ! And you also end up braking more often, creating phantom traffic jams on motorways.Effectivly putting it nuetral will only save fuel in really old cars, like carb'ed cars because they fuel all the time!
In any half modern fuel injection engine, you lift off, the injectors turn off and the inertia of the car/drivetrain turns the engine over, so effectively in that coast you are getting infinite MPG.
filski666 said:
from the Porsche website:
Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
It's called resistance Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
be it aerodynamic or rolling..Hmm, I just don't get this, the only way I could see it working is if you were going so fast that there was a massive ammount of drag from the engine being at really high revs and you want to keep going at a constant speed.
I could see how it would completely ruin the car, I currently drive a 328i 8 Speed Auto, and I always downshift with the paddels to make use of engine braking.
Surely though for this system to actually work well, it should decouple the engine, and turn it off completely, that at a certain speed or detection of first brake pressure, or low vaccume in the servo, restart the engine.
I could see how it would completely ruin the car, I currently drive a 328i 8 Speed Auto, and I always downshift with the paddels to make use of engine braking.
Surely though for this system to actually work well, it should decouple the engine, and turn it off completely, that at a certain speed or detection of first brake pressure, or low vaccume in the servo, restart the engine.
loomx said:
Surely though for this system to actually work well, it should decouple the engine, and turn it off completely, that at a certain speed or detection of first brake pressure, or low vaccume in the servo, restart the engine.
Or have electric motors that can be clutched in to power the brake servo. Driving the air con is harder though, that uses a fair amount of power. I once owned a 1970 Saab 99 and as has been mentioned, that came with a freewheel feature. I don't recall that you could turn it off but if you were feeling lazy it permitted clutchless gearchanges. It might well have been a hangover from Saab's 2 stroke days but the argument for keeping it was said at the time to be to prevent front wheel skids when lifting off the throttle abruptly in icy conditions. As such, it was a part of Saab's character as a producer of cars fit for a Scandinavian winter and which did things its own way, rather than following the herd (cf fitting a gearchange lock rather than a steering lock, because who wants the steering in their car potentially to lock up at the wrong moment!).
filski666 said:
so what do you do on a steep descent - ie when you see this sign?
Your decoupling system disconnects you from your engine braking - you set fire to your brakes and you crash into a pile of children and kittens?
Why would you set fire to your brakes? As soon as you touch the brake or throttle. PDK re-engages the gear in a few milliseconds and you have drive and braking. I put a link in a previous post to Porsche's description of how coasting works. Your decoupling system disconnects you from your engine braking - you set fire to your brakes and you crash into a pile of children and kittens?
jjones said:
how certain are we of this no fuel when no throttle down hill? takes a lot of force to spin an engine over with no explosions to help it on it's way? guess someone could test he theory by switching off going down a hill and see how far they coast and then retry with ignition (best performed on private land of course because of PAS and servo brakes (and be mindful of the steering lock!))
When the engine is turning the PAS pump (assuming it's old style hydraulic pas) will still be turning, so you still have PAS.Also with the throttle shut and the engine turning you will have vacuum for brake assist.
anniesdad said:
filski666 said:
so what do you do on a steep descent - ie when you see this sign?
Your decoupling system disconnects you from your engine braking - you set fire to your brakes and you crash into a pile of children and kittens?
Why would you set fire to your brakes? As soon as you touch the brake or throttle. PDK re-engages the gear in a few milliseconds and you have drive and braking. I put a link in a previous post to Porsche's description of how coasting works. Your decoupling system disconnects you from your engine braking - you set fire to your brakes and you crash into a pile of children and kittens?
I know - my parents did it when was little - they had to stop and let the brakes cool down before carrying on with the journey.
and your point about the system re-engaging when you press the brake - yes, fine, and as soon as you come off the brakes again it disconnects again - so you shoot forward as you have no engine retardation.....so you have to go back on the brakes again.......so (as I said) you will be on the brakes the whole way down the hill - which WILL cause then to overheat on suitably long and steep gradients.
Edited by filski666 on Thursday 26th July 13:45
McSam said:
filski666 said:
from the Porsche website:
Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
It's called resistance Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
be it aerodynamic or rolling..
kambites said:
loomx said:
Surely though for this system to actually work well, it should decouple the engine, and turn it off completely, that at a certain speed or detection of first brake pressure, or low vaccume in the servo, restart the engine.
Or have electric motors that can be clutched in to power the brake servo. Driving the air con is harder though, that uses a fair amount of power. Just do what all the BMW's do with the autostart stop, only activate it if a set criteria are met, i.e. if the climate control is set to 16 degrees and its 20 degrees in the car, the engine will not turn off. If its okay it will turn off and restart if it warms up.
Porsche seem to have basically put a half arsed system in a car, which in the real world actually just frustrate you. And if it starts coasting when you want to use engine braking you will end up using more fuel! which in my opinion would be most of the time, when I lift off, its because I want to slow down.
What I dont get, is in Porsches blurb, it says it has brake engery regeneration... which wont work if the engine is decoupled... So I guess this now only works when the engine is running and you are breaking
Captain Muppet said:
When the engine is turning the PAS pump (assuming it's old style hydraulic pas) will still be turning, so you still have PAS.
Also with the throttle shut and the engine turning you will have vacuum for brake assist.
Depends on the engine, a lot of modern direct injection engines, have a vacumme pump instead because they dont use a throttle as such, the air intake is controlled by the valves lift and timing.Also with the throttle shut and the engine turning you will have vacuum for brake assist.
loudlashadjuster said:
I know what Mr Harris means though, my C-Class sheds speed at a very slow rate compared to any other car I've driven. It might be eco-gubbins, it might just be the 7-speed 'box, but whatever the cause it is disconcerting and I'm still adjusting after 8 weeks in it.
I think that may just be a characteristic of some autos. I used to occasionally drive an automatic Grand Cherokee, and on anything but an uphill gradient, it would maintain its speed for a good few hundred yards before noticeably slowing, which felt weird.filski666 said:
McSam said:
filski666 said:
from the Porsche website:
Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
It's called resistance Another way to reduce fuel consumption is to utilise the coasting function on downhill gradients that are gentle enough for you to maintain a constant speed
now excuse me if my understanding of physics is wrong, but if you are going down a slope, your vehicle is trying to [u]accelerate[/u] due to the force of gravity - normally this can be offset using engine braking to maintain a constant speed - how the flippin' flip are you supposed to maintain a constant speed on a downhill gradient if they have taken away the engine braking....apart from lightly resting your foot on your brakes and seriously pissing off whoever is following you?
(and potentially causing an accident should you suddenly increase that brake pressure which gives no external signal due to the constant brake lights)
be it aerodynamic or rolling..No, my point is that engine braking is far from the sole or even the main contributor to your car not accelerating continously down a hill. What Porsche are trying to say is that the coasting function takes the effort out of those situations where you're cruising down a hill but losing a bit of speed due to engine braking - with the engine disengages, some resistance is taken away and you maintain a constant (or slightly rising or dropping-more-slowly) speed. You could always have done this manually be declutching, though.
McSam said:
My main point, though, is that under no circumstance should you have the engine decoupled and be braking, because any resistance benefit you might have gained from decoupling is irrelevant anyway so why waste the idling fuel?
It's a very good argument for the manual version imoGassing Station | General Gassing | Top of Page | What's New | My Stuff