| MDM, wrote |
it feels like it's doing an average as I step on the pedal.. doesn't respond quickly.
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There is no perceptible lag due to throttle motor reaction time in either my wife's or my DBW cars. I can take a picture of the vacuum/boost gauge in any one or more of my cars, in an effort to show that the difference between the mechanical-linkage variations which I own behave just as quickly as the DBW ones...
Which BMW is it? IIRC BMW were using these about 20 years ago on their dual-intake-plenum V12 motors, (balancing the vacuums struck me as a challenge)
| MDM, wrote |
if the motor is under full power, would I be able to push it out of drive and into neutral then?
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Yes. There is NO connection between linkage operation and motor load in most automatics. Dual-clutch transmissions especially... although in those both clutches are computer-controlled, and operated by pump-driven hydraulic actuators... even more potential points for a catastrophic failure.
In a traditional manual transmission engine drive and shifter position is directly linked, so motive force will come into direct 'contact' (and you can 'feel' this in your wrist when performing a clutchless-shift).
| ScotcH wrote |
Basically you're dealing with unboosted brakes at that point like many old cars, and plenty of broken cars ...
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I've driven, and played with many un-assisted cars. -Take the old VW beetle. The brake arrangement in them is NOT SUPPOSED to be boosted, and the 'gearing' of the brake pedal (. in specific terms the mechanical advantage, the 'leverage' of the pedal/actuator relationship) is MUCH greater than in boosted cars.
As a result, if you disable the servo in a modern boosted vehicle, or if it fails (such as a MkIV platform VW GTI... an example which occurred to a colleague only last week, which is why it is fresh in my mind) then you'll have discovered the world's fastest-acting laxative.
| ScotcH wrote |
my wife's Fiat spider has no vac assist on the brakes (I diconnected it cause it was leaking), and it stops just fine ... I can lock up the whels no sweat.
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I've never worked on a spider, so I honestly can't say with any certainty, but I suspect that it may not have the same assistance ratio that many modern cars do... but that's conjecture. -Certainly, in last week's VW brake servo failure incident, the driver (who is bigger than me, -I'm just over 6' tall- and stronger too) could NOT lock the wheels. In addition, he had to drop a gear -FAST- throw BOTH feet on the pedal, and pull the handbrake, and STILL couldn't stop in the prescribed distance... locking up the wheels was utterly out of the question. -I know, because I tried the car before I fixed it for him.
| ScotcH wrote |
While the effort is certainly MUCH greater, it is still perfectly effective to use the brakes while unboosted. Yes, the pedal will feel rock hard, but there is leverage at work, and you're still appliying like pressure (like 1000s of PSI) to the brakes when you stand on the pedal. The clamping force is enough I would think ... but then car makers have obviously made other idiotic enginieering design changes, so perhaps they boosted the brakes to the point that they are no longer fnctional without the vac assist.
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See above. -Did the Spider also come in un-boosted versions? -Either lower-spec or earlier/older models?
If so, it's quite probable that the
amount servo-assist was only very small, just to 'assist' in the typically-understood sense of the word, rather than being a significant and essential part of the required braking force.
Certainly, I assure you that you could NEVER stop a Lexus with no vacuum in the chamber and a stuck throttle, and sadly neither could any of the people who died in California last August.
| KB_S1 wrote |
Try manually activating the throttle yourself whilst downshifting or applying a bit of left foot braking whilst accelerating.
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I'm heading to work now, so I'll give it a go, -at least I'll TRY. Mine is a DSG, so things are a little tricky, but I'll watch the vac/boost gauge. It's a very reliable indicator of throttle-plate position, so you can actually see the throttle response.
Hmmmm, in a manual, how would you do a 'heel-and-toe' rev-match cog-drop on a corner entrance if the throttle is inoperative... Let me see what it does.
Keith
