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Wierd Downshift Issue ('09 G37 Sedan)

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Old 05-26-2009 | 01:04 AM
  #16  
SkyMG37x's Avatar
SkyMG37x
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From: Southern NH
Originally Posted by terrycs
Has anyone watched your AT downshift by itself in manual mode? No matter how slowly I decelerate, my transmission goes through all the gears, but always skips 2 and goes from 3 to 1. I'm guessing this is to reduce the "jolt" coming to a stop?
I watched mine today in this mode and it didn't skip 2.
Old 05-26-2009 | 01:22 AM
  #17  
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SkyMG37x
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From: Southern NH
Originally Posted by cao_123
Do you mind explaining the engineering of the powertrain in simple terms (if possible)? Most of us experience the slightly rough downshift during normal driving in D mode when coasting to a stop at around 20 mph.

I would accept this as a normal sporty characteristic in DS mode but would expect the car to remain smooth in D. However, it does seem to be a "normal" characteristic unless a software update is in our future.
The question is what is considered "normal". While the thread on a S/W update implies there may be a fix, it also says that there is no known S/W update being provided for this specific condition.

When I read comments that a downshift resulting in 2K RPM means the car is defective, then I think there are some clarifications required.

Here are the facts as I understand them. The gear ratios the 7AT are such that torque and power are optimally available. The ratios of gears 1 and 2 are much wider than 3-7, meaning that during upshifts you're benefitting from the higher ratios via torque and acceleration, but during downshifts the same gear ratio will mean that the engine needs to rev match a bit higher (although 2K is not high), and engine braking will be more noticeable. While you're feeling this engine braking, the transmission is still in the optimal gear if you need to accelerate again, such as through a curve.

So while I agree that the downshift is noticeable, I don't agree that this is a "defect" although I'll let Infiniti decide if they'll update their S/W to smooth this out, but that will mean either a higher rev match or delaying the downshift at the expense of the subsequent acceleration performance. In a 6 speed manual, downshifting at 20 MPH into 2nd is normal with similar results, although under the driver's control so it's not as much of a surprise. Personally my car hasn't downshifted harshly even though it's noticeable especially when my foot is on the brake.
Old 05-26-2009 | 02:20 AM
  #18  
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Reno-ID-Doc
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From: Reno, NV
SkyMG37x has explained the scenario quite well. Thanks Sky.

I think many of us have been used to before to some form of delay in downshifting in most AT out there. Hence there is a phenomena many have posted of the G37 "lose of momentum due to engine brake (dynamic deceleration)" This downshifting program that resides in our G37's software is tweeked to bring a more instantaneous performance (much decreased delay) but pushes the equation curve to a slightly higher rpm match for the next lower gear, wherein you would notice a more pronounced dynamic decelaration effect from the engine. From the specs of the gear ratio, this would be more highly noticeable for gears 3-2. Somebody in our forum have posted a nice table of gear ratio comparisons (5AT vs. 6MT vs. 7AT).

From an engineering standpoint, the only solution is to wait for a new transmission that would have closer difference for the gear ratios of 3 & 2. But I guess, up until then, we have to contend with this phenomena (which I would not assign as a defect) & get through the learning curve.
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