Ive only had my 09 G37 for a couple of months and mostly drive in D.Well recently i started trying out DS and when I drive at highway speeds and then slow down. in atleast 1 of the downshifts theres a big flare not really jerky as if you downshifted in a manual but i can hear the big rev of the exhaust..
anyone else experience this?

 

That is the car rev matching

 

why is it doing it? is it normal? why doesn't it do it in D

 

The DS mode keeps the car in a lower gear, and allows you to make the most of the higher revs. D mode is mostly for fuel economy (such as it is), and keeps the car in high gear. It also is reluctant to move from a gear, as that uses incrementally more fuel.

DS mode is for spirited driving, and will downshift aggressively to keep your revs higher than they would be in D mode.

 

Its supposed to do that. Thats the sport mode. Dont get scared, your car is fine. Simply just rev matching. Its a racing thing.

 

Whats happening when you down shift is that your transmission shaft is turning at a certain RPM that is relative to the gear you are in. This equates to the RPM of the engine but not precisely so. When you down shift you are switching to a lower gear that rotates faster along with the engine RPM. If your moving at say 60 MPH in 5th gear and your RPM are say 2500 and you downshift to a lower gear, you are forcing the engine via the transmission to rev faster. This is normal and nothing to really worry about. Just don't go wild doing it because it puts extra strain on the engine and transmission. It is good for "getting out of dodge" or passing an obnoxious driver ahead of you. Fuel economy will degrade since the engine RPM is increasing and your giving it extra fuel to speed up the car up quicker.

 

My understanding is the computer controls the throttle of the engine to match its revolution to the speed of the tranny instead of the tranny "forcing" the engine to rev faster. Whichever the case is, engines are meant for revving. Just dont go crazy bouncing around the redline.

 

The whole idea of rev matching is to keep suspension upset to a minimum. Really, you should already be in the proper gear before entering a corner.

If this sounds greek to you, do some research on high speed cornering strategy. There is little to gain by downshifting for no reason in the course of day to day driving. Since you got an automatic, let the computer decide the best gear for you.