In general a shift kit is supposed to mechanically shorten shift times and reduce slippage during shifts. Without a tune a lot of this will be undone. Yes you can command faster shifts in the tune, but what you end up with is big trims as the PCM tries to get it there. It really can only get you so far, my dually really doesn't shift that snappy compared to my 1500 which has a Superior shift kit. Both are commanding faster than stock shift times, but admittedly I kept the torque reduction intact on the dually and deleted it on the 1500. If you take the time to dial a shift kit in properly the effect would be fast shifts with minimal trims. I really don't think any of the shelf kit is going to be bang on in that respect. Think of any given shift kit, even stock, as being the VE for the transmission, i.e. the base tune before any trims are applied.
Your dually prob weighs a good bit more and so the extra weight absorbs the shift energy before it's felt by the driver. And you're absolutely right about COTS kits not being sufficient to get you there.
For most transmissions, a good tune with some well executed hydraulic and mechanical mods will yield optimal results and in most cases, you can do a better job "fine-tuning" the mechanical shift calibration without spending money on a shift kit.
Here's some general steps builders take when putting together a trans, esp for performance:
- drill clutch (gear) feed orifices in the separator plate per converter stall, vehicle weight, final drive, intended application and desired shift firmness, taking into account the below mods.
- set clutch clearance based on desired firmness (staying within required ranges, the more clearance you have in the pack, the firmer your shift will be into that gear, all other things equal.
Note, firm doesnt necessarily mean fast so clearance and hydraulic mods must be phased in with each other so you arent too fast or too slow in terms of gear shift speed.
- restrict or block off accumulator circuits for the 3rd and 4th* acc circuits rapidly increases speed of apply (reduction in dwell between shift command and applied element fully applied)
*It's not generally recommended to completely delete the 1-2 accumulator circuits in most GM and Ford longitudional transmissions
- install high rate return springs in direct (th350,400,4L60,80) and intermediate clutch (4L80e, th400, th350) to quicken downshifts (and prevent centrifugal apply)
There's other things but those are the main steps.
You pair all of the above with a well-matched tune, the trans will shift like a dream and last a long, long time.
