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Christian:
For an emergency stop, you're supposed to press the clutch to the floor and hit the brakes as hard as you can (assuming you have ABS).
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Originally posted by Christian:
Originally posted by Big Jim:
Originally posted by Christian:
Originally posted by Big Jim:
On decel, the engine is not driving the wheels. It is adding resistance and as such is actually assisting the brakes. Depending on conditions, this may not be significant, or it can be very significant.
But surely, unless you a braking extremely softly, when applying braking power you are working against power sent from the engine to the transmission, even though that power is dropping it is still there. I can understand it assisting in slow braking but in fast braking, especially emergency braking, clutch in is best as there is no power being sent from the engine.
At least this is the way I see it in my mind, the physics of this makes sense to me.
Once you lift your foot off the gas pedal, the power generated from the engine turns from propelling the car (positive power) to slowing the car (negative power). Surely you can feel that as you lift the throttle. While this is happening, the engine is assisting braking all the way down to engine idle.
Okay, I may be a little slow here on this one 
If the engine is still producing power then even if that is less than the power required to propel the car at the same speed (hence engine braking), would that power still not be put to the wheels (even if it is less than required for propulsion) meaning that when you brake ... you are braking not only the velocity/momentum of the vehicle but also the revolutions of the engine ... whereas if you brake clutch in/in neutral you are braking against purely the velocity/momentum of the vehicle ... my understanding of physics, back from A-levels 15+ years ago, so it is a little shoddy!
I can picture this scenario ....
Say at 50mph you need 10% disc braking to stop in distance x, if you apply engine braking you would need only 5% (making up numbers here) as the engine braking allows for 5% braking power ... which is why we brake with engine engaged.
But I don't see this ...
If I want to perform an emergency stop (slam on brakes as kid jumped in road) ... my aim is to stop as quickly as possible. At this point I imagine that removing engine power from the scenario would reduce braking, as the engine is rotating it is producing power, which goes to the wheels which causes forwards momentum (even if it is less), which means I have to apply more braking force to the wheels ...
Sorry to turn this into a school lesson, just I don't quite get it. 
Cheers,
Christian./
You are almost there. If it is producing less power than required to propel the car it is engine braking and that engine braking is assisting in stopping the car.
Jim Johnson
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When I slow down from say 55mph I almost always brake a little and downshift to slow down. If I'm going slower sometimes I don't even use the brakes until I'm going 15 or less.
I got over 100,000 miles on my brake pads (backs went first for some reason), and I'm still on the original clutch. So I think as long as you're not slamming it into first at 55mph or popping the clutch, you should be fine either way.
I've also gotten pretty good at upshifting from 3->4 and 4->5 without clutching.
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Originally posted by mvr:
well if im not mistaken when you are engine breaking in snow the engine could cause the tires to lock up. i would try to explane further but i would make myself look like an idiot. id rather take my chances with the antilock.
Yes, esp when going down a hill it's not smart to downshift where your RPM goes say 4000+. Under normal conditions, the pavement doesn't allow any slippage , of course. Under slippery conditions the engine braking (normally allowed because there's friction from the pavement) allows the tires to lose traction and thus create what you called "lock up" but more appropriately be lose traction, like you do when you accelerate in snow.
This can happen in automatics too, if they gear down to a high RPM. My Mech teacher told us about him driving in the snow and a Grand Prix decides to gear down on hwy(no brake lights lit) and it loses traction and kareens into the barrier.
Originally posted by Rishodi:
Christian:
For an emergency stop, you're supposed to press the clutch to the floor and hit the brakes as hard as you can (assuming you have ABS).
Say you don't have ABS (<--me), wouldn't braking w/o lockup and stalling the engine (don't engage the clutch) stop you quicker?
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Originally posted by Big Jim:
You are almost there. If it is producing less power than required to propel the car it is engine braking and that engine braking is assisting in stopping the car.
Okay, that I get. However for maximum braking would the power not be cumulative to the momentum as it is still creating some momentum ... and if you are braking hard, once the velocity drops (if dropping quickly) to match the velocity that the engine is trying to produce, further hard braking is going to be working against the engine too, as the engine is not dropping quick enough to match the drop in velocity, so you're (from that 'sweet spot') braking against both the momentum of the car and the engine's power output (even though it is dropping) ?
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Originally posted by Christian:
Originally posted by Big Jim:
You are almost there. If it is producing less power than required to propel the car it is engine braking and that engine braking is assisting in stopping the car.
Okay, that I get. However for maximum braking would the power not be cumulative to the momentum as it is still creating some momentum ... and if you are braking hard, once the velocity drops (if dropping quickly) to match the velocity that the engine is trying to produce, further hard braking is going to be working against the engine too, as the engine is not dropping quick enough to match the drop in velocity, so you're (from that 'sweet spot') braking against both the momentum of the car and the engine's power output (even though it is dropping) ?
Yes. The engine is creating resistance to the car's motion above idle speed, since the air in the cylinders is being compressed. As demon stated, there is no fuel coming out of the injectors when coasting. No fuel means no power. None. You are compressing air in the cylinders, and letting it out the exhaust. This take a lot of energy from the kinetic energy of the car, slowing you down. Even in a car with the injectors spitting fuel at idle speed, the force it takes to compress the non-firing cylinder that revolution is less that the force created by the firing cylinder. Thus, the engine produces DRAG on the trans. You can feel this when the transaxle clunks upon letting off the gas. This is the gears locking in the opposite direction: the wheels are now pushing the engine. Once you get down to idle speed, the amount of energy created by the firing cylinder is exactly equal to the energy expended compressing the non-firing cylinder, so it's a wash. Coast lower than idle, and you're going to be hurting your deceleration, as well as probably stall the engine, unless you hit the clutch soon!
Hope that helps. The key is: the engine takes power too, it doesn't just make it. Whether the TOTAL amount of power coming from the engine is positive or negative depends on whether it is above or below the speed it would natually spin with a given amount of throttle.
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so, what i'm getting from all this is that it is better to coast in gear for various reasons. the one that catches my attention is that by doing that the injectors cut off the fuel going to the engine, so the cylinders aren't firing. When coasting in neutral the engine goes down to idle which means the injector is sending fuel to the cylinders and some gas is actually being used.
therefore, at least in theory according to the posts in this topic, it is cheaper on gas to coast in gear instead of coasting in neutral. i had no idea the engine could actually run without there being gas being ignited in the cylinders.
this brings up a question. theoretically, can you turn the ignition to the off position and coast in gear? wouldn't this make sense since there's no gas flowing into the cylinders? i know that it would affect other parts of the engine like power steering and braking, but consider just the motor.
now, back to the argument about engine momentum. i still agree that if you coast in gear and have to slam on the brakes that you would be braking against the velocity and the engine rotation because engine can decel only at a certain rate. if you brake at a greater rate than the engine can decel than you are actually also braking against the engine momentum. which would make sense to put the clutch in for slam braking, right? please clarify.
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Is it better to clutch for a panic stop? Maybe, but not for that reason. The engine has some inertia, but I doubt the car can stop faster than the engine. The car takes a very long time to stop, the engine natuarally drops in revs pretty quickly. I always hit the clutch at some point during a panic stop because I don't want to stall the car. Also, the extra braking power from the engine (I think it probably does provide resistance, even on fast stops) will bias the brakes even more to the front. They are reasonably well-biased from the factory, but err on the side of too much front. If you have a lowered, more stiffly sprung, or more rear-biased car, more front braking power isn't needed. Besides, once you peg the brakes, you have all the power you need to stop. Those discs can lock up very sticky tires without too much of a problem. But that's all academic. Really:
It's a non-issue. Leaving the clutch engaged until you are about to fully stop (or are in a spin) is the best idea, just to keep vacuum pressure up. Downshifting through the gears isn't a bad idea, partly to keep the revs (and vacuum, and engine braking power) up, and also to be able to floor it if some idiot comes barreling in behind you. It won't take long for a good driver to get it into gear and go, but any time at all can be too much in an accident situation.
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Originally posted by Christian:
Originally posted by Big Jim:
You are almost there. If it is producing less power than required to propel the car it is engine braking and that engine braking is assisting in stopping the car.
Okay, that I get. However for maximum braking would the power not be cumulative to the momentum as it is still creating some momentum ... and if you are braking hard, once the velocity drops (if dropping quickly) to match the velocity that the engine is trying to produce, further hard braking is going to be working against the engine too, as the engine is not dropping quick enough to match the drop in velocity, so you're (from that 'sweet spot') braking against both the momentum of the car and the engine's power output (even though it is dropping) ?
Sometimes theory can only go so far in explaining physics. Now it is time to go gather some empirical data to see what really happens. In other words, go experiment on stopping using both engine braking and brakes in various combinations. I think that may be the only way for you to fully see just waht is happening.
The Wright brothers had incorrect theory when trying to make their first flying machine. They had to determine the correct theory on their own to make it actually fly. You can do the same. See for yourself.
Jim Johnson
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Thanks for the lesson  Now I get it, would have understood much quicker if I had re-read the original posts for ... Originally posted by DemonSVT:
When you are coasting in gear the PCM shuts off the injectors in series. You are basically getting free mileage for no real fuel spent. The exhaust will be flat out lean. Engine braking (extreme vacuum) is by far the most efficient state an engine can be run in.
Then I wouldn't have run in circles until 95sleeper pointed it out again and it sunk in!
Cheers all !
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