Quote:
Originally Posted by euromodder
Downshifting is used to help engine braking, appears to use less fuel as the fuel flow is cut off.
Yet it's essentially a braking technique, and braking wastes fuel unless you're powered by gravity and need to check your speed.
Coasting in idle beats engine braking.
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Reiterating, downshifting is used to increase engine, or "compression", braking, as opposed to using a car's brakes to slow or stop.
Until many auto manufacturers cottoned on, generally in the '70s', to the importance of improved braking/or the costs for better brake systems fell/or consumers demanded it [front disc brakes were an option on the '69 327 Camero], downshifting for slowing or stopping was very important. I worked with a guy who used to race a NEW C-Type Jag in the early 50's [a car that today would be worth $5M, if he hadn't totaled it at Sebring, and ended his racing career]; he said the large drum brakes just weren't up to repeatedly slowing the car. He made that statement to comments I made about my '55 TR2, in the '90's, but still with drum brakes. Plus it can also be important to balance the car while braking hard, and without a more modern brake bias proportioning system, racing corvettes in the 1970's found that "compression braking" was something that drivers of Porsche 911's had to be more careful with, and helped at least one Corvette driver (another guy I knew) "keep up" with the drivers of those rear-engined cars, at some corners. And anyone who's driven cars from the 1920's or 30's knows the value of the 3-2 shift, when braking, and also the dangers (broken gears/skids/spins/lots of grinding/jerky ride) with trying a 2-1 shift. I asked a friend why the 5.0L Mustangs, in showroom stock, way back in the late '80's weren't so much faster than his Nissan 2.0L NX; he said the that, because of all their power, the Mustangs only had one good "stop" per lap!...meaning, of course, that they needed to rely on their downshifting, for braking, setting the car up for the turns, and for accelerating.
Downshifting when braking makes sense if you can anticipate a "near stop", when you might have the chance to not have to stop, but so that you are then in the right gear to accelerate again, when traffic/lights change...with the added bonus that you might be able to help some of the traffic behind you to flow better.
And somewhere what gear you car is in should be tempered with the idea that it should be in the right gear for the generally anticipated conditions - being able to get out of the way of a truck is a better idea than getting killed while trying to save 2 cents worth of fuel. And then there's the benefit of using a lower gear when driving in hilly/curvy country, and the obvious used of compression braking when driving down a mountain. And I might as well though in, or when your car is heavily loaded with people and/or stuff...it makes sense to err on the side of caution.
We suspect that more people manually shift up when accelerating or when they've reach their desired cruising speed, or when they want to reduce the car's torque at the road, like when driving on snow.
The other appropriate use for downshifting is when braking for a corner, but not just for manual transmission cars. But everybody knows how to heel & toe, right? You might want to slow for a corner, and might want a lower gear to accelerate faster out of the corner, or use the higher torque at the road afforded by a lower gear to help you steer the car through the corner, when at the limit of adhesion (either race "racing speeds" or when driving on snow or gravel). Modern cars' brakes are generally pretty strong, and can mask a botched heel & toe effort. Without some braking, however, a botched down shift - especially in the rain, snow, gravel, or in the middle of a wet corner, when the driver has totally mis-matched the revs during the shift - can really unsettle the back-end of a rear wheel drive car; rear tires chirping, a skid, or spin shouldn't be unexpected.
Another great use for engine braking is in areas where you see a lot of children in, or beside, the streets. Running along in 5th or 6th gear, I'll downshift a gear or two, in part to slow the car, but also to create some noise; I want those kids - often engrossed in a ball game - to know that there is a car coming.
If more people thought about, and "practiced" slowing and stopping, traffic would flow better, and we'd all use less fuel and time, when driving around. Oh, and the roads would be safer, too. I recall what JP Montoya told Jeff Gordon, during their "Trading Paint" exhibition: "If you improved the brakes, the car would go faster!"