Starts in 2nd Gear? - Page 3 - PeachParts Mercedes-Benz Forum

kerry · #31 08-10-2006, 03:13 PM

I have no explanation as to the cause, but I know the sense of 'drag' from my motorhome. I have a BlueBird Wanderlodge with a CAT 3208 diesel and an Alison 4 spd auto. At governed rpm, it will probably do about 22mph maximum in 1st gear (which I have done uphill). However, when starting out from a stop on flat ground, at around 15-18mph in 1st gear it 'feels' as though the engine has run out of 'oomph' and is slowing down and when it shifts into 2nd it seems to regain it. It's a strange sensation which I though indicated a problem when I first bought it, but I now thinks it's a normal part of its performance.
I can't say I've ever noticed it in either my 77 or 85 300d's.

fastpakr · #32 08-10-2006, 03:21 PM

Quote:

Originally Posted by kerry edwards

I have no explanation as to the cause, but I know the sense of 'drag' from my motorhome. I have a BlueBird Wanderlodge with a CAT 3208 diesel and an Alison 4 spd auto. At governed rpm, it will probably do about 22mph maximum in 1st gear (which I have done uphill). However, when starting out from a stop on flat ground, at around 15-18mph in 1st gear it 'feels' as though the engine has run out of 'oomph' and is slowing down and when it shifts into 2nd it seems to regain it. It's a strange sensation which I though indicated a problem when I first bought it, but I now thinks it's a normal part of its performance.
I can't say I've ever noticed it in either my 77 or 85 300d's.

This actually makes perfect sense, and applies to any vehicle in any gear. As you accelerate past the torque peak, acceleration will begin to decrease as torque falls off. At some point past the RPM at which peak horsepower occurs, you'll be better off in the next gear up, because the engine will be making enough extra torque at a lower RPM to make up for the higher ratio of the upshift. This is what I was referring to in my earlier post when I said:

Quote:

until the torque curve begins falling off to the point that the torque jump from reducing engine speed would exceed the falloff from using a gear with a lower torque multiplication factor

SD Blue · #33 08-10-2006, 07:46 PM

[QUOTE=fastpakr;1242308]No. Driving through mud bears no similarity to this discussion whatsoever. The reason that a higher gear is used in mud is to spin the tires faster to self-clean the tread. First gear would redline while the wheel speed was too low. Try again.

What in #$%()*U$ do you mean by 'higher rpm's with relation to speed'?QUOTE]

Gee Whiz professor! Try this: go out to the car and try to take off. Try it once in first gear then once in second gear. What do the rpms of the engine do before the car starts to actually move.

didget69 · #34 08-10-2006, 08:39 PM

Quote:

Originally Posted by fastpakr

The only WAG I can come up with is that it's a function of exhaust flow and building manifold pressure via the turbo, but otherwise I'm completely baffled by this dragging that you describe.

The 'granny gear' 1st gear in the Dodge 6-speed, or in other heavy truck 4 or 5 speed transmissions, is designed to get heavy towed/hauled loads rolling from rest more easily, or is also used to slog through muddy fields, pull hills, etc... the dragging feeling described is due to "running out of gear" quickly, e.g., the engine runs through it's rev range in that gear & feels sloggy or out of breath - can't think of a better way to describe it... My buddy has a 1980 ex-Air Force deuce & a half that creats the same sensation in low-range & using 1st gear -

Bryan in NC

fastpakr · #35 08-10-2006, 08:42 PM

Quote:

Originally Posted by didget69

The 'granny gear' 1st gear in the Dodge 6-speed, or in other heavy truck 4 or 5 speed transmissions, is designed to get heavy towed/hauled loads rolling from rest more easily, or is also used to slog through muddy fields, pull hills, etc... the dragging feeling described is due to "running out of gear" quickly, e.g., the engine runs through it's rev range in that gear & feels sloggy or out of breath - can't think of a better way to describe it... My buddy has a 1980 ex-Air Force deuce & a half that creats the same sensation in low-range & using 1st gear -

Bryan in NC

I absolutely agree with that. Just like starting in low range (2.48:1 reduction) on my Exploder, you get unbelievable acceleration for a second or two, then you're out of gear. We're 100% in agreement on everything you posted. The problem here is that SDBlue is trying to make the impossible argument that acceleration is actually slower in first gear.

fastpakr · #36 08-10-2006, 08:43 PM

[QUOTE=SD Blue;1242574]

Quote:

Originally Posted by fastpakr

No. Driving through mud bears no similarity to this discussion whatsoever. The reason that a higher gear is used in mud is to spin the tires faster to self-clean the tread. First gear would redline while the wheel speed was too low. Try again.

What in #$%()*U$ do you mean by 'higher rpm's with relation to speed'?QUOTE]

Gee Whiz professor! Try this: go out to the car and try to take off. Try it once in first gear then once in second gear. What do the rpms of the engine do before the car starts to actually move.

Which has absolutely ZERO to do with this argument. RPM change because of slippage in the torque converter is meaningless for this discussion. Let's try this again - what is your straight forward physics explanation for reduced acceleration when you're generating GREATER torque at the back end of the transmission?

SD Blue · #37 08-10-2006, 10:13 PM

[QUOTE=fastpakr;1242626]

Quote:

Originally Posted by SD Blue

.............. RPM change because of slippage in the torque converter is meaningless for this discussion. Let's try this again - what is your straight forward physics explanation for reduced acceleration when you're generating GREATER torque at the back end of the transmission?

I believe the question was why do they start out in second gear.

Now re-read what you just said.

fastpakr · #38 08-10-2006, 11:25 PM

[QUOTE=SD Blue;1242742]

Quote:

Originally Posted by fastpakr

I believe the question was why do they start out in second gear.

Now re-read what you just said.

I did, and?

greasybenz · #39 08-11-2006, 12:15 AM

wow this topic is going out of hand!!

Lets just agree with didget69's post. And it explains why the truck is slow in 1st.

OK? end of discussion, my question has been answered

SD Blue · #40 08-11-2006, 09:28 AM

[QUOTE=greasybenz;1242904]..........Lets just agree with didget69's post. And it explains why the truck is slow in 1st.................QUOTE]

Exactly, my understanding is the same. We are talking about the range of rpm's, torque, ie... acceleration throughout each gear. This relates to all the way from dead stop to shift point. Nearly every multispeed transmission I have seen spends very little time in first gear due to the fact that rpms climb rapidly. Evidently, MB set the gearing in some of their auto trannys to take advantage of the range of rpm's in second, efficiency probably improves also and this may be due to initially higher rpm's caused by the effect of the torque convertor of an automatic transmission. (note that 1st gear is still maintained when you actually need it for a heavier load, steep incline, etc.) If my ideas on this helps someone to dig furthur and come up with on even more precise answer, great.

I apologize if there was a further question to this viewpoint. However, this turned into what I felt was an unnecessary personal attack for whatever reason. As I stated earlier, the intent was to describe in a breif statement of layman's terms. Others have shed light in areas not mentioned by myself. And I believe that is what forums are about, not some perception of an intelligence battle.

Have a good day.
(I will unsubscribe from this thread.)

fastpakr · #41 08-11-2006, 09:40 AM

Just so we're clear - now you're saying that you would accelerate faster in first gear, but it doesn't normally use it because it doesn't need it? If that's the case, then we're on the same wavelength now.