Shift firmness - What causes it

Mark20

Original poster
Member
Dec 6, 2011
1,630

BoldAdventure

Member
Jun 28, 2012
1,634
I just rebuilt my transmission and included a transgo shift kit along with the all the other upgraded parts and I know that did a lot for shift firmness. I have been curious myself what PCM's "mod" would do, especially now.
 

dfc739

Member
Jul 29, 2012
170
Des Moines, IA
Mark20 said:
PCM 4 Less offers several shift firmness levels (I have level 2 in my tune). But what causes/defines shift firmness? I ask this becasue there was a mention of line pressure in the thread about Tow Mode without a button. (http://gmtnation.com/f73/tow-mode-without-button-6516/#post119366)

I've always been told that shift firmness has to do with line pressure because the more line pressure, then the more pressure that is put on the clutches. More pressure on the clutches equates to faster/snappier shifts because there is less slipping between the frictions and the steels.
 

ScarabEpic22

Member
Nov 20, 2011
728
A few factors go into determining shift firmness including line pressure. Also shift times, torque management settings, input shaft torque, separator plate setup, valve body springs, etc. With just a tune and not actually opening the transmission up, you can raise line pressures a little (need to make sure you dont raise them too much and overwork the pump), decrease shift times, and remove some/all torque management. If you drop the pan and install a shift kit with servos, different springs, drill the separator plate, etc then you increase the pump/line pressure volume mechanically which firms the shifts up.

For those wondering if just adding a shift kit and leaving the tune alone is ok, it is BUT you'll get a lot better results with a tune as well. My 02 I6 has a shift kit, Vette servo, and my tune and the shifts are perfect IMO. Nice and firm at part throttle, but they dont snap your neck driving around town. When you punch it, then it shifts quickly and much firmer. I probably could make it shift faster, but honestly I have no reason to and Ive been very happy with how it shifts for the past few years so I havent changed it.

Now the SS is a different beast, a tune helps a ton but they definitely need a shift kit to help handle the power. Im probably just going to run my stock trans as long as I can, then get a fully built one.
 

BoldAdventure

Member
Jun 28, 2012
1,634
Sounds like I should probably do the PCM now. We had our Transmission built and she's shifting firm, crisp and quickly. Holding RPMs longer and kicks down better as well.
 

Jkust

Member
Dec 4, 2011
946
On my minivan, the pressure control solenoid quits working when you come off the highway and things warm up in stop and go. Every gear from 1-4 absolutly slams into gear. It was on of the issues with that particular gm tranny. Been doing it since probably 2005 or 6 and you never know when it will or won't do it. Point being line pressure.
 

Mark20

Original poster
Member
Dec 6, 2011
1,630
How do they actually increase the line pressure? I must admit I really don't know the magic (er, mechanics) inside an auto tranny.
 

ScarabEpic22

Member
Nov 20, 2011
728
Mark20 said:
How do they actually increase the line pressure? I must admit I really don't know the magic (er, mechanics) inside an auto tranny.

There are 2 ways in our transmissions because they are electrically controlled (instead of the original 4L60/700R4 that are vacuum controlled). 1 is to simply raise the desired line pressure in the tune. The second is to actually modify the valve body of the transmission to increase line pressure mechanically.
 

Mark20

Original poster
Member
Dec 6, 2011
1,630
Do they speed up a pump, modulate a valve at a different frequency?
 

ScarabEpic22

Member
Nov 20, 2011
728
Mark20 said:
Do they speed up a pump, modulate a valve at a different frequency?

Increase the pump flow is what Ive seen, usually stiffer springs do this (more flow at a lower pump rpm). Tuning changes the solenoid apply time.
 

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