Coasting in neutral or with clutch down?

[alohaspirit]

Originally Posted by PaleMelanesian

I concur, with two exceptions:
1. You are forced to brake for an unexpected stop. You might as well DFCO and save fuel and brakes.
2. DFCO to control speed on a long downhill.

I was thinking about alternating the two due to a long hill


Neutral Coast until you get to around 60

then DFCO until around 45

[lusth]

Originally Posted by alohaspirit

DFCO < Neutral Coasting < EOC

Going down a slight hill, my instantaneous MPG maxed out at 99 using DFCO. Slipping into neutral lowered the MPG to 45.

At least in a new car (2009 Pontiac G5), DFCO > Neutral Coasting (for this one experiment).

[Christ]

Originally Posted by lusth

Going down a slight hill, my instantaneous MPG maxed out at 99 using DFCO. Slipping into neutral lowered the MPG to 45.

At least in a new car (2009 Pontiac G5), DFCO > Neutral Coasting (for this one experiment).

Instantaneous MPG AFFECTS overall average, but does not INDICATE overall average.

DFCO is still < Neutral coasting, when you consider ALL the inputs necessary to make the comparison.

I leave you to ponder that.

[chipsndukes]

I haven't read the whole thread thoroughly, but wanted to throw in my $0.02 on the original question, I hope I'm not interrupting...

I found recently on the expressway while pulsing and gliding between 60-75 mph that my 5th gear synchronizer started to give out.
It didn't like being shoved into gear at 60 mph from a transmission stand-still (out of gear).

So I began to leave it in 5th gear and just press the clutch, while my synchronizer still isn't totally gone.

At slower speeds (40 mph), it doesn't seem to matter, so I take it out of gear when coasting.

Just my $0.02.
Chipsndukes

[Ptero]

Why didn't you double-clutch or float to synch? I wouldn't even consider wearing my synchros. Your synchros, which are brass, will last forever if you match your gears.

Double clutch
From Wikipedia, the free encyclopedia

A double clutch (also called a double declutch) is a driving procedure primarily used for vehicles with an unsynchronized manual transmission.

In a gearbox with neutral between each gear, a typical shift actually involves two gear changes, once into neutral, and again into the next gear. During any shift, disconnecting drive components via a clutch properly unloads the engine and transmission of undue pressure applied by the opposing components. Fully utilizing the clutch for each shift out of, and then into each gear is double clutching. Due to the absence of a neutral spacing, double clutching is ill-advised for sequential gear changes, as in a fully sequential gearbox such as a typical motorcycle.

Keeping the clutch pedal depressed while in neutral, as is performed during a typical shift, gives more economy of driver motion and effort compared to double clutching. Taken to extreme, sequential gearbox shifts and non-clutched shifts are also very quick and effortless. However, significant wear can take place on the separated clutch plates any time the engine and transmission have varying drive loads. In simple terms, wear occurs the more the clutch has to "slip" to match revolutions between the engine and transmission. Double clutching can minimize this clutch plate wear by encouraging matching of engine and transmission RPMs before the clutch plates are engaged.

History and Theory
Before the introduction of transmission synchronizers (in the 1920s) and helical cut gears, double clutching was a technique required to prevent damage to an automobile's gear system. Due to the difficulty and most often unnecessary redundancy involved in learning the technique, coupled with the advent of synchronized gearing systems, it has largely fallen into disuse. However, drivers of large trucks often use the double clutching technique when unable to keep the transmission unloaded during shifting, as large vehicles are usually equipped with older, simpler and more durable unsynchronized gearboxes.

The purpose of the double-clutch technique is to aid in matching the rotational speed of the input shaft being driven by the engine to the rotational speed of the gear you wish to select (directly connected to rotating wheels). When the speeds are matched, the gear will engage smoothly and no clutch is required. If the speeds are not matched, the dog teeth on the collar will "crash" or grate as they attempt to fit into the holes on the desired gear. A modern synchromesh gearbox accomplishes this synchronization more efficiently. However, when the engine speed is significantly different than the transmission speed, the desired gear is often unengageable even in a fully synchronized gearbox. An example is trying to shift into a gear while traveling outside the gear's speed or directional range, such as reverse while moving forward.

Double clutching, although time consuming, eases gear selection when an extended delay or variance exists between engine and transmission speeds. When shifting up on a non-synchroniser equipped vehicle, the clutch pedal is pressed, the throttle is released, and the gearbox is shifted into neutral. The clutch pedal is then released. As the engine idles with no load, the RPM will decrease until they are at a level suitable for shifting into the next gear. The driver then depresses the clutch again and shifts into the next gear. The whole manoeuvre can, with practice, take no more than a fraction of a second, and the result is a very smooth gear change.

Although double clutching is a testing requirement when obtaining a commercial driver license, many experienced truckers will float gears or slip shift, thus eliminating the clutch except for starting and stopping.

Conversely, in order to downshift, engine RPM must be increased while the gearbox is in neutral and the clutch is either engaged or disengaged. This requires the driver to shift into neutral, apply throttle to bring the RPM up to a suitable speed, and finally shift into gear. This operation can be very difficult to master, as it requires the driver to gauge the speed of the vehicle and throttle accurately. Double clutching occurs if the clutch pedal is released while matching engine speeds in neutral and again engaged prior to shifting into the next gear.


[edit] Technique
A related downshifting technique is called heel-and-toe, in which the brake and accelerator pedals are pressed simultaneously. Classically, the brake is pressed with the ball of the right foot and the accelerator pedal is controlled by the right heel, while the clutch pedal is pressed by the left foot. However, many variants are possible, with the brake and accelator pressed by sides of the right foot.

Proper Heel and Toe technique aids both slowing the vehicle while at the same time accelerating the engine for a matched downshift. Note that Heel-and-Toe may be used with any type of gearbox when simultaneous braking and downshifting is necessary to save component wear. Though difficult, mastering the Heel-and-Toe technique in conjunction with necessary clutching is essential for high performance driving (e.g., rally racing) to stay in the optimal gear regardless of the simultaneous braking, accelerating, and clutching required for shifts. This allows the engine to stay in the RPM "powerband" and allows one to drive as fast as possible. Left foot braking while accelerating the engine with the right foot to accommodate downshifting in a clutchless situation accomplishes the same feat.

The purpose of the heel-toe-double-clutch is to downshift into the correct gear, and thus optimal engine RPM, for exiting the corner while placing the least wear and tear on the entire drivetrain. Note that racers will often skip gears during downshifts depending on the vehicle speed; there is no need to shift through every gear when significant velocity has been lost.

[chipsndukes]

Originally Posted by Ptero

Why didn't you double-clutch or float to synch?

Thanks Ptero.

Could you describe the process a little better?

The engine is idling and the transmission is in neutral, clutch out.
The vehicle is rolling at ~65 mph, when I clutch and try to select 5th gear, it grinds.

What should I do differently according to the text you posted?

Thanks,
Chipsndukes

[Ptero]

For pulse and glide, you'll want to master the slip. Always be gentle. Ham-handed people should avoid learning the slip in their own cars. You can break off gear teeth if you get clumsy.

SLIP

Get a tach if you don't have one.
Go 65.
Make note of the exact rpm. e.g. 3070
Go 55.
Make note of the exact rpm. e.g, 2750

Now, at 65 mph, touch the clutch and shift into neutral.
Take your foot off the clutch and off the gas.
Your idle will drop to around 500 rpm.
Coast down to 57.
Now bring your rpm from 500 to 2750. Be steady and precise, no hunting up and down.
As you hold the rpm steady at 2750 and your speedometer drops from 56 to 55, quickly jab your clutch (just a little - NOT to the floor) and slip the stick into high. A good slip shift will not make any noise except for a little "snick".

This technique works for all speeds and all gears (upshifts require doubleclutching) but you have to memorize the match points. To learn match points in big trucks that I'm not familiar with, I cheat by pulling the shift lever lightly against the spinning gears. A light accelerator foot, with an eye on the tach, will find the match point. Little transmissions with synchros can make this hard to do.

Once you are good at this, you will find the clutch is unnecessary, as well as the tach. It's like playing a musical instrument.

[PaleMelanesian]

That's fine if the engine is running. What chipsndukes is talking about is with a stopped engine, moving into 5th gear to do a bump-start. I've not had trouble with the synchros, but I can see how it might be a problem in some cases.

[Ptero]

I have started a new thread to address this and similar issues. See:
Hypermiling techniques that are invalid on the Hypermiling thread.

[chipsndukes]

Originally Posted by PaleMelanesian

That's fine if the engine is running. What chipsndukes is talking about is with a stopped engine, moving into 5th gear to do a bump-start.

Yes PaleMelanesian, that is correct.

I don't know how I made the mistake, my post above indicates the engine is idling, in fact it is off.

Thanks!
Chipsndukes