Hey from down under,

Just been through through Franks Custom Trans tips, tricks and mods thread. Understanding the finer workings like he and some other posters obviously do has to be a gift. Unfortunately I don’t have it but probably like most others on this forum I am quite capable of fitting parts that other more knowledgeable people like him have recommended/designed/supplied.

I am currently trying to finesse a project car conversion running a 2005 GM Holden 6.0L gen 4 V8 with 4L65e. The trans has been professionally reconditioned approximately 2000 kms ago with OEM parts but also some selected pieces including a Sonnax smart shell and super servo etc. Overall the trans runs very well.

The engine and transmission are being controlled with aftermarket ECU’s. The TCU is an older HGM Compushift 2 which I have become quite familiar with. It has all the usual period 4 speed auto control programs onboard, including the GM 4L60e variants. Although not as detailed in parameters as the OEM TCU, it gives me fine tuning of shift rpm points and pressures as well offering extras like tap shift/manumatic mode using column mounted paddles. The actual tunnel mounted shifter is now a 4 position only with PNRD, with D position giving 1-4 forward gears either on auto or tap shift modes.

Because I prefer a firmer shift feel and a degree of engine braking, I have fitted a Sonnax HD valve body kit, their partno: GM054-HD, details at:

https://www.sonnax.com/parts/5581-he...red-valve-body

On the face of it, this piece appears to be a more expensive version of Franks OD VB mod with the additional benefit of several Sonnax VB improvement parts and bringing back OEM internal operating tolerances etc. As advertised, it does give me some form of engine braking on gears 1-4 inclusive.

Using the TCU and VB, I get reasonably fast tap up shifting, 1 through to 4 no problem and the same for downshifts 3-2 and 2-1, both with throttle and on overrun. My problem is that in tap shift mode the 4-3 downshift on over run is generally slow, taking approximately 2-3 seconds from the down tap (shown as commanded on the TCU head unit) on overrun. This only improves if you blip the throttle to an indicated 5% throttle around town or to around 15% throttle at higher speeds on the open road, at or immediately after the downshift tap.

I would like the over run tap downshift to happen as quickly as the 3-2 and 2-1 over run shifts and was wondering if there is anything I should look at or any obvious way to improve that 4-3 shift. I have already played with shift pressures in the 3-4 shift table with no appreciable difference made. I assume playing with shift speeds the same way isn’t going to make any difference since the 4-3 shift is being manually commanded. Any advice would be welcome.

Cheers, Pete. NZ.

 

Assuming you want the OVERUN braking to occur faster. The only way I know would be to enlarge the feed to the overun. But I am thinking that is not the issue . But rather the hydraulic timing. Have you compared using BI DIRECTION controls to force the downshift ? Just to make sure it not just the PCM reacting slowly? Also note that if you do bring the overuns on sooner . You risk the band not having fully released yet (NEVER MIND THE BIDIRECTIONAL STUFF I SEE YOU ARE PADDLE SHIFTING.) but that is a risk since it unlike other gears only bleeds of via the apply circuit back though the valve.
Things you could try - stiffer spring in he 1-2 valve (may however result in a slower 2-1 in command to action on a kickdown. Enlarge feed to the overun (may result in a momentary bind up as band release if happens to soon) Never really had anyone concerned with engine braking happening faster at part/low throttle . But those are about the only solutions I can think of except maybe just maybe a customised solenoid with a larger bleed and hole in it .(I did this once with the 2-3 trying to get a faster 2-3 shift command to action. But had no notable effect which is why I ended up adding the spring to the 2-3 valve which did indeed have a notable effect. ) So based on that if your up to the hassle I would try a stiffer spring in the 1-2 valve as the most likely to accomplish the goal.