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      12-23-2012, 04:39 PM   #1
SomeRandomer123
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Question 8AT - Reverse Gear

Noticed something recently! Was hoping other Automatic or Sport Automatic Transmission owners could chime in too. Mine's a Sport Auto, although I think the standard will also do the same.

The other week, I was travelling down a narrow lane and an oncoming car approached, so I stopped and engaged R to go to a passing place 300-400m back. Being a confident reverser (not to come across big headed!), I got a move on and briskly accelerated to about 10-15mph. As I was accelerating there was a bump as if the car had changed gear in M/S mode (where the changes are fully sharpened up like a Double Clutch Transmission) and the revs dropped, although I wasn't studying the needle.
That got me thinking then, do we have multiple reverse gears in our 8AT's?! But then I was also arguing to myself that the bump was a bit too pronounced for Comfort mode, and obviously, you can't engage R in M/S or DS mode.

So today, I went dropping off Christmas presents to the relatives and again, the road is only wide enough in this section for one car and as Sod's Law dictates, another car approaches. A Renault Scenic on this occasion!
Seeing the panic in the driver's face at the prospect of reversing, I thought I'd just bloody get on with it. The difference this time being, I was reversing up a VERY steep hill, not the flat as before! So much so, I was worried about breaking traction being RWD, but thankfully the 255's had plenty of grip!
So, I went from D to R and stepped on it. This time however, I monitored the revs more closely, and as I began to travel backwards they shot up to 2000rpm and stayed there as I was accelerating until there was a bump and they dived down to approx 1200rpm.
Following this event, I thought again!
Torque converter. With this car being my first automatic, and one of the finest ever to be put into a car, I've gotten used to 1st gear being engaged without any noticeable slip from the torque converter - it's just like fully engaging the clutch as you pull away in a manual transmission.

So, this is where I need your evaluation; my conclusion:
When accelerating in R, the torque converter doesn't fully lock up until about 5-8mph and when it does, it does so sharply, giving the bump sensation I was experiencing. And R is only one gear. Seems like a long gear though if it's doing ~ 1200rpm at over 10mph.

Now I need your thoughts: why is this? Why doesn't it act like 1st gear does, giving you complete connection?

And Merry Christmas to you all!
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      12-23-2012, 05:27 PM   #2
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There are 9 gears in total (8 forward; 1 reverse).

More information here:

http://en.wikipedia.org/wiki/ZF_8HP_transmission
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      12-23-2012, 05:38 PM   #3
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Quote:
Originally Posted by spark58 View Post
There are 9 gears in total (8 forward; 1 reverse).

More information here:

http://en.wikipedia.org/wiki/ZF_8HP_transmission
Yep, got that bit. I'm wondering more why the torque converter behaves like it does in R. Certainly not for smoothness as it locks up.
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      12-23-2012, 10:42 PM   #4
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The gearbox is only designed to handle max 3,500rpm in reverse gear, it may have sth to do with it.
U seem to like to experiment ur car to the limit in ways ppl have never thought of.
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      12-24-2012, 03:13 AM   #5
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And whats wrong with that?
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      12-24-2012, 05:03 AM   #6
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Quote:
Originally Posted by SomeRandomer123 View Post
Noticed something recently! Was hoping other Automatic or Sport Automatic Transmission owners could chime in too. Mine's a Sport Auto, although I think the standard will also do the same.

So, this is where I need your evaluation; my conclusion:
When accelerating in R, the torque converter doesn't fully lock up until about 5-8mph and when it does, it does so sharply, giving the bump sensation I was experiencing. And R is only one gear. Seems like a long gear though if it's doing ~ 1200rpm at over 10mph.

Now I need your thoughts: why is this? Why doesn't it act like 1st gear does, giving you complete connection?
Not sure what is going on, the gear ratio is similar to 1st gear (3.68 1st vs. 3.35 reverse). Not sure how lock up is intended to work in reverse, may be doing so, in a more abrupt manner. Typically I suppose, a driver wouldn't be driving like that in reverse.

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      12-24-2012, 06:43 AM   #7
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Quote:
Originally Posted by clarence View Post
The gearbox is only designed to handle max 3,500rpm in reverse gear, it may have sth to do with it.
U seem to like to experiment ur car to the limit in ways ppl have never thought of.
Ahh is that right? Interesting! People were also saying that the car rev limits itself to 3500rpm in N or P, but I'm not fussed on trying that because it's not a problem I'm ever going to encounter in a real world situation, plus I don't like an unloaded engine to achieve a high engine speed.
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      12-24-2012, 06:48 AM   #8
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Quote:
Originally Posted by HighlandPete View Post
Not sure what is going on, the gear ratio is similar to 1st gear (3.68 1st vs. 3.35 reverse). Not sure how lock up is intended to work in reverse, may be doing so, in a more abrupt manner. Typically I suppose, a driver wouldn't be driving like that in reverse.

HighlandPete
Cheers for that info too. Yeah lock up is quite abrubt and I'm just wondering why this is, especially R being a weaker gear. True, but that said, I wasn't experimenting purposely to see how fast I could go in R, these are real world applications where I needed to get on with it! And still only around 10mph. And on both occasions, less than 1/4 throttle used, even on the enormous incline! It was feathered really for the first instance on the flat.
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      12-24-2012, 02:06 PM   #9
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I used manual for years and 320d is my first automatic too in this time i never released the clutch fully for reversing in manual transmission, always halfclutch. Bimmer is doing that in a way i think, when the limit reaches gearbox fully engages and that is your bump??
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