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TDI 101 Got a simple/basic TDI question? Are you a newbie (new to the forums). Feel free to post your question here.

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Old July 18th, 2011, 03:51   #1
deucelee
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Default TDI MT different from other MT's? (how to drive question)

Guys, a couple of QQ's about the MK6 TDI's...

1.) Say you are on the highway, your exit is 1/4 mile away, there's no cars behind you. Can you just put your car in neutral and let your car glide to the exit? (sorry, might be a dumb question but i do this all the time with MT cars and was wondering if the TDI is different)

2.) What gear do you guys typically put your 6 speed MT on when driving in the city doing 30-35mph? Or should I just go with your rule of thumb of "ok to cruise at 1800rmp"?

3.) Summary: Anything I should avoid doing with a TDI and a MT that is a big no-no compared to other gas MT's out there?
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Old July 18th, 2011, 04:51   #2
MayorDJQ
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Quote:
Originally Posted by deucelee View Post
Guys, a couple of QQ's about the MK6 TDI's...

1.) Say you are on the highway, your exit is 1/4 mile away, there's no cars behind you. Can you just put your car in neutral and let your car glide to the exit? (sorry, might be a dumb question but i do this all the time with MT cars and was wondering if the TDI is different)

2.) What gear do you guys typically put your 6 speed MT on when driving in the city doing 30-35mph? Or should I just go with your rule of thumb of "ok to cruise at 1800rmp"?

3.) Summary: Anything I should avoid doing with a TDI and a MT that is a big no-no compared to other gas MT's out there?
1. There's nothing physically unique about a TDI transmission. If you want to coast, go ahead. But, to idle in nuetral requires fuel. Coasting in gear requires no fuel.

2. 3rd gear. Don't lug your TDI. Keep the RPMs 1600+ when cruising.

3. Don't lug the engine: use the RPMs. Once warmed up a little, I shift 1 to 2 and 2 to 3 at 3k. Depending on the road, I might shift to 4th a little sooner.
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Old July 18th, 2011, 06:56   #3
deucelee
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Originally Posted by MayorDJQ View Post
1. There's nothing physically unique about a TDI transmission. If you want to coast, go ahead. But, to idle in nuetral requires fuel. Coasting in gear requires no fuel.

2. 3rd gear. Don't lug your TDI. Keep the RPMs 1600+ when cruising.

3. Don't lug the engine: use the RPMs. Once warmed up a little, I shift 1 to 2 and 2 to 3 at 3k. Depending on the road, I might shift to 4th a little sooner.
Thank you Mayor...I appreciate it...thanks for the gearing and shifting info...

wow, this is good education for me. I've always thought that if it's at idle, then the rpm is at 900 or so, vs. the 2000 rpm you'd be if you left it in gear and just didn't press the gas.

I did some research and others do confirm what you said. They do make a point that leaving it in gear slows the car down, in some cases by quite a bit, then you have to use fuel to accelerate. (i.e. you can put to neutral a lot sooner and let it coast, vs. pressing on the gas for a longer time before letting go of the gas).

More questions:
1. So what you guys are saying is that as long as you don't press the gas, even if the tranny is in gear and the rpm is higher, then no gas will get to the combustion chamber (or in our case diesel)?

2. Assuming the gas used being on idle while coasting is the same as the additional gas you're using due to tranny assist braking, it better to put more pressure on the brakes in the long term (i.e. coasting on idle) OR could you possibly harm your tranny in the long term by using tranny assist braking?

I'm not a tranny expert by any means. And if I'm wrong, it's because I absolutely don't know. I'm just thinking there could be some benefits for NOT using the tranny to help with braking in the long term of it's life. If anything, that my brakes are cheaper to replace then a tranny. Is my reasoning not applicable in real life?
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Old July 18th, 2011, 07:13   #4
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For #1, a good rule of thumb: If the engine RPMs are above 1000 and your foot is not on the accelerator, no fuel is injecting into the motor.

You can verify this with a scangauge (reads >9999 MPG), as well as with your head out the window driving along a fence. The diesel clatter disappears during the above conditions.

For #2, tranny-assisted braking is actually good for the engine / transmission as it puts a "negative"' load against the bearings and keeps things seated. Some folks seem to debate this though. The brakes (on the mkIV anyway) are capable of well over 100K before pad replacement so don't worry about those.
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Old July 18th, 2011, 07:37   #5
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The TDI seems to have less pronounced engine braking than many gassers, so that lends itself towards more coasting in gear, which as said above, helps fuel economy.

I'd keep the car at 1800, so if 6th gear puts you under that, you are likely in the wrong gear.
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Old July 18th, 2011, 12:19   #6
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1) Nothing bad to coast in idle if you don't want to loose speed. Coasting in gear you'll gradually loose speed.
2) 3rd.
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Old July 18th, 2011, 16:33   #7
deucelee
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thanks for all your replies guys...it's gonna b hard to try to break my habit of coasting on idle...it doesn't seem like there's terrible consequences if you do it, so maybe i'll try to do half and half.
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Old July 19th, 2011, 16:06   #8
Curious Chris
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My Dad trained us to never coast in neutral as you have no control at that time. And the clutch doesn't care as it is engaged when you come off the go pedal, whereas the brakes will have to be applied to slow the car down.

The when you have it in neutral it is tricky to know what gear the engine really wants to be put in when you let the clutch out. With the clutch in and you see the rpm in the 800-1000 range, you know the engine wants you to downshift to accelerate.
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Old July 20th, 2011, 03:52   #9
Henrick
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If you're at least a bit mroe experienced driver you know which gear engine wants to be (if necessary) in across whole speedometer scale. It just takes a moment of sight and you know it.
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Old July 20th, 2011, 06:40   #10
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Quote:
Originally Posted by Curious Chris View Post
My Dad trained us to never coast in neutral as you have no control at that time. And the clutch doesn't care as it is engaged when you come off the go pedal, whereas the brakes will have to be applied to slow the car down.

The when you have it in neutral it is tricky to know what gear the engine really wants to be put in when you let the clutch out. With the clutch in and you see the rpm in the 800-1000 range, you know the engine wants you to downshift to accelerate.
Once you get to know the car, knowing which gear to use after coasting or otherwise losing a lot of speed should come pretty easily.
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Old July 20th, 2011, 08:53   #11
deucelee
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Originally Posted by Henrick View Post
If you're at least a bit mroe experienced driver you know which gear engine wants to be (if necessary) in across whole speedometer scale. It just takes a moment of sight and you know it.
also, maybe it's just me but honestly i don't feel i loose any control of the car when i coast on idle. When i'm ready for my turn or exit, like Henrick mentioned, i just see what speed i'm currently going and put it in gear to match the speed so i'm at the correct rpm.
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Old July 20th, 2011, 10:20   #12
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My 0.02-
This first part isn’t aligned exactly with the OP questions... Also might appear to contradict responses I made to similar questions in the past, but after some more thinking... Although the question is completely sincere, “At what RPM should I shift up / down?” is too simplistic, and defies a concrete answer. Other things must be factored in, such as: (ignoring weather & safety conditions...)
  • Is gravity presently also helping to accelerate or trying to decelerate the car ? (downhill / uphill)
  • Is the car lightly loaded or heavily loaded?
  • Is slow acceleration acceptable, or is near-max performance needed?
  • Further, during the time when the engine has not yet reached its full operating temperature, many people believe it is beneficial to limit the max operating RPM to some extent, and also to limit the amount of torque requested from the engine well below the max it could produce.
=
Those factors can combine (sometimes in different ways !) to change the answer to the shift point question. At times, I have accelerated my car in 4th gear, pulling from 1200 RPM. But those times would have been going downhill, and where gentle acceleration was acceptable, and I did not have to lug the engine (ie. operate at low RPM with heavily pushed accelerator pedal and high boost etc.)
=
In general, it is recommended to keep the engine RPM above about 1900 when significant torque is being requested. Given the gear spacing of the existing manual trans, then for most driving the engine ends up operating between 2000 and 3000 RPM. But if going up a steep hill, and / or if heavily loaded, and / or if faster acceleration is desired, then it’s probably a good idea to raise the operating speed and keep the engine in the 2500-3500 range, maybe higher. Note I’m not talking about drag racing here- for the absolute maximum performance, we’d get into mapping the horsepower curve and choosing the shift points such that they keep the engine in the range where integrated horsepower under the curve is maximum. That could result in different shift points for different gears. And someone else may chime in and say that it’s not max power, but max torque to govern the shifts (which would be wrong), but I digress...
=
Again in general, it is desirable to let the engine spin at least 1900 RPM if it’s being asked for any significant amount of torque. Quite often the engine will be quite “happy” to maintain steady speed, on level ground, when running significantly slower than 1900. The car I commonly drive is 5spd, not 6spd, but I will offer here- I typically go through village 30MPH zones in 3rd gear at around 2000 RPM (just above 30MPH). The engine will happily carry the car in 4th gear at 33MPH at around 1600, but the problem I have there is- the torque of the diesel is so plentiful, it’s easy to creep the car up over 40MPH in 4th, thus eligible for speeding tickets. So I just let it sing along in 3rd, and if the RPMs get too high, it’s a reminder for me to back off a bit.
=
Probably rude to close this without touching on the OP questions!...
I agree with above responses, there’s nothing specifically detrimental about coasting in neutral. Due to the computer control, the feature is built in such that the fuel supply is shut off if engine speed above idle is detected along with accelerator not depressed. So in that condition, zero fuel is consumed. But the OP counterpoint is valid; in that case, the car is slowing down faster due to the engine braking effect, so what is actually cheaper- to coast farther and easier in neutral, but the engine is consuming fuel, or to coast with engine braking, but consuming no fuel? I can’t answer that, only to quote another forum regular (whose name I can’t recall): Drive more, worry less. I also agree there is no detriment to the manual trans for coasting in gear. Personally I am among the group that typically does not coast in neutral, generally always keeps the engine coupled to the drivetrain while the car is moving. It’s what I was taught, and I observe vast majority of skilled performance drivers do same thing- rarely coast in neutral. I’m sure we’ll find a counter-point somewhere...

I will insert one other thing, though- I believe it's good to be in the habit of NOT sitting with the clutch pedal depressed for long periods of time. I will generally idle at a stop light in neutral with foot OFF the clutch. Watch the opposing light for when it turns yellow, and use the clutch to engage first gear only a few seconds before getting under way. That saves wear on the throwout bearing and pressure plate (not to mention your clutch foot).

Enjoy your TDI !!
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Last edited by Corsair; July 20th, 2011 at 10:26.
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Old July 20th, 2011, 11:51   #13
deucelee
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Thanks a lot Corsair...very good points

I concur with you on the stop light. i always go to neutral to not have to hold the clutch down the whole time.

If anything, I've def got the message from this thread that it does NOT hurt the car by having the car in gear when coasting to a stop. It just slows the car down quicker but no harm. thanks everyone.
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Old July 20th, 2011, 11:56   #14
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"so what is actually cheaper- to coast farther and easier in neutral, but the engine is consuming fuel, or to coast with engine braking, but consuming no fuel?"

This has been debated ad naseum on this list (take a look at the Fuel Economy section sometime). As for me, I do coast in neutral whenever I need to slow down ever so slowley, or when going down hills when I don't, within reason. I am of the camp that I think this uses less fuel than coasting down in high gear (and progressively greater savings the lower the gear you would have been in when you coast in neutral).

Also, if you have been driving a while, if you ever need to put the transaxle back in gear for acceleration, or quicker stopping, you will know exactly which gear and may also give the accelerater a slight blip to bring the RPMs up a hair for a rev-matched smoth gear engagement.

I consistently get 50 MPGs (or better) in my '04 and that is with a large portion of my driving in rush hour in the Washington, DC area.

--Nate
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