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ian_uk1975
925 posts
71 months
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waremark said: Can you explain this point? If you are changing down you need higher revs; but as soon as you declutch with your foot on the brake the revs reduce. How can the engine speed match the road speed without use of the throttle, unless you have kept your foot on the clutch long enough that your speed is now extremely low? In a car with any kind of flywheel effect (ie. the vast majority), the revs won't drop a huge amount unless changing down particularly slowly. I think a bigger point, which I should've thought of when I posted that reply, is that if you're changing down under braking and trying to match gear selection to road speed, engine braking will be minimal (assuming you're not trying to use engine braking to supplement the foot brake). The lack of engine braking when the clutch is re-engaged will mean there is little resistance against the clutch, so the change will be much smoother negating the need to rev-match. If, on the other hand, you're changing down to overtake, there will be significant engine braking in the lower gear, so rev-matching becomes far more important to effect a smooth change and not unsettle your passengers or the car. Same would apply if changing down under braking to use engine braking as a brake-assist.
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vsonix
1,065 posts
32 months
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Somewhatfoolish said: I wonder what happens if you just put the car in top gear, let it idle, and slowwwwwwwlly engage the clutch. In most cars that would have you doing about 25 to 30 once the clutch was fully engaged. It'd most likely just not work unless you had quite beastly amounts of torque. But I do (traffic permitting) occasionally set off from home and work through the gears up to fifth using only the clutch and no gas pedal. Then depending on which route I take there is a long downhill stretch which means on a clear day I can be half a mile or more along my route before I eventually need to press it.
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ian_uk1975
925 posts
71 months
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vsonix said: Somewhatfoolish said: I wonder what happens if you just put the car in top gear, let it idle, and slowwwwwwwlly engage the clutch. In most cars that would have you doing about 25 to 30 once the clutch was fully engaged. It'd most likely just not work unless you had quite beastly amounts of torque. But I do (traffic permitting) occasionally set off from home and work through the gears up to fifth using only the clutch and no gas pedal. Then depending on which route I take there is a long downhill stretch which means on a clear day I can be half a mile or more along my route before I eventually need to press it. Engine torque barely factors into this, actually... since the engine would be idling (which infers you're not touching the throttle), the amount of torque actually being developed will be negligible since the throttle is almost completely closed. Starting-off in a high gear is a more realistic thing to attempt, but this has a lot to do with gearing as well as engine torque. Most engines with vast amounts of low-down torque are geared appropriately (for example, my 600bhp+ 7-litre V8 Corvette Z06 only turns around 1,500rpm at 80mph... which it's happy to do because it's so torquey). If I attempted to set-off in 6th, I'd have to be travelling at at least 40mph, or so, before the road speed was matching my idle rpm (ie. not trying to stall the engine by dragging the engine speed below idle).
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R300will
3,598 posts
20 months
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ian_uk1975 said: vsonix said: Somewhatfoolish said: I wonder what happens if you just put the car in top gear, let it idle, and slowwwwwwwlly engage the clutch. In most cars that would have you doing about 25 to 30 once the clutch was fully engaged. It'd most likely just not work unless you had quite beastly amounts of torque. But I do (traffic permitting) occasionally set off from home and work through the gears up to fifth using only the clutch and no gas pedal. Then depending on which route I take there is a long downhill stretch which means on a clear day I can be half a mile or more along my route before I eventually need to press it. Engine torque barely factors into this, actually... since the engine would be idling (which infers you're not touching the throttle), the amount of torque actually being developed will be negligible since the throttle is almost completely closed. Starting-off in a high gear is a more realistic thing to attempt, but this has a lot to do with gearing as well as engine torque. Most engines with vast amounts of low-down torque are geared appropriately (for example, my 600bhp+ 7-litre V8 Corvette Z06 only turns around 1,500rpm at 80mph... which it's happy to do because it's so torquey). If I attempted to set-off in 6th, I'd have to be travelling at at least 40mph, or so, before the road speed was matching my idle rpm (ie. not trying to stall the engine by dragging the engine speed below idle). But if you want to move from a standstill in high gear you need shed loads of torque? bhp isn't going to do it because that's high rpm stuff.
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doogz
18,667 posts
56 months
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What do you think is going to happen if you try and pull away at idle in 5th gear?
How would it "not work"?
My 1.1 106 I had years ago could pull off in 5th, with no right foot. By the time you got the clutch all the way out, you'd be doing about 25mph in that little puddle jumper.
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