RPM Matching during a 3-2 Downshift on a 700R4
 

With a TH400 and a loose OEM converter, I used to be able blip the
throttle during a downshift and accomplish a lurch-free transition from
3rd to 2nd.

Not so with the 700R4. I haven't practiced much but my recollection is
that simultaneously blipping and downshifting results in a moment of
acceleration, followed by decelleration (post-blip) and then a rough shift
perhaps a second or two later.

Is this a result of the locked torque converter, the valve body
operation or just ham-fistedness on the part of the operator?

It would really be useful to be able to pull off easy, consistent and
smooth down-shifts that didn't unsettle the car much. Any suggestions
about useful mods to the hardware, the ECM programming or my
technique will be welcomed.

Thanks,
Ken R.

well, if it's indeed a 700r and not a 4l60 then it should be as simple as putting in a switch to iterrupt power to the TCC. Simply the hot lead going into the 4 prong plug. leave it unlocked for autocross.

The trans go 2-3 shift kit will allow the trans to downshift to any gear chosen immediately.

Thanks for the suggestions, guys.

Pete K. I'll take a look at TransGo's kit. But, for the moment,
my efforts are aimed more at smoothing the shift event. I'd like
to left-foot brake while I blip and shift in a way that doesn't
bring the rear around.

I like the idea about the manual lockup switch. What about taking
this to another level - revising the TCC parameters in the .Bin for
the ECM? Here are the parameters for two OEM bins for an '89 A4.

http://www.slalom4me.com/images/jpg/tcc_parameters.jpg

I have a bank switch with a remote and a chip I can load multiple
programs on. What about having an 'Autocross' version with different
values? Would modifying these

• 'TCC Unlock Time'
• 'TCC Coast Release'
• 'Low Speed Limit for Low Gear Transition'
• 'Low Speed Limit for High Gear Transition'

move me closer to smoother downshifts?

.

Yes, it is a 700R4 in an '89.

I know by introducing the ECM that I'm partly crossing over into territory
more suited for discussion in Scan & Tune. But can we continue here
from the tech perspective of what is actually needed to change to get
me back to the place I used to enjoy with the old-school TH400.

For instance, does a looser-than-stock converter need to be added
into the equation so I can get the RPM flash needed to match RPM?

.

For the purposes of this discussion, the biggest difference between the TH400 and your 700R4, is the lock up converter. That is hardly fodder for Scan and Tune. If you care to experiment, a simple toggle switch can be installed to temporally, manually, defeat the TCC. I don't have trouble synchronizing the RPMs for the 3-2 down shift.

RACE ON!!!

Can anyone confirm or refute this statement
.

I haven't opened the FSM to confirm, but I gather that the brake switch is
a convenient place to insert a manual TCC control.

The 700R4 went through some changes leading up to '88 or thereabouts.
I wonder whether your earlier version is happier with the down-shift
manuever or if like I proposed earlier, I'm just ham-fisted.

The latency between a change at the shifter and the transmission's
response seems to tie in with the value I see listed as the TCC Unlock
Time.

.

I heard this and tried an experiment. With the car idling, I cracked the feed line to the rad with a wrench while the car was running. I received a fluid bath. I would not be suprised if more fluid was sent in lock up, but some goes to the cooler when the converter is unlocked.

At this time I have no clue as to the pedigree of my transmission. The car came with an early 27 spline input, but that trans was deep sixed when I had a torque holding monster built up in anticipation of a new engine. It is a later year (???) with a Vigilanti 2600 rpm stall converter, and the stock (for TCC lock up) chip.

RACE ON!!!

The 2600 RPM stall would make a difference, assuming the TCC
has released.

I have come across references to OEM values being used for other
applications that are several MPH higher than those cited in my table
above, like 40 MPH. I think they were for TBI vehicles - would TBI be
synonymous with Crossfire? If so, do you happen to recall what your
OEM chip code is?

If my reasoning is correct, perhaps the higher stall and higher TCC
disengagement you have make for smooth down-shifts?

.

Yes, I believe the TCC is disengaged. But I would think that the slop of a looser converter would make synchronization, even more difficult.


Crossfire is TBI, but there are more single, 2 bbl, TBI, V8s than, dual, 1 bbl, TBI, V8s. The part number for my chip should be 1226466, with codes, either "3235CCC" or "3940CDR", according to my parts book


The chip is stock, so the TCC lock up points should remain unchanged. Pete K. is the automatic transmission authority on here, as far as I'm concerned. See what he says. Maybe internal changes to the trans can influence lock up speeds.

RACE ON!!!

Thank you for the '84 p/n and codes. Info about that year appears
to be scarce, compared to '85 and up. I'll look for a bit longer then
check out values from the new cars.

With the TH400 and the 2.xx rear, the 'looseness' provided by the
higher stall allowed the revs to climb with a blip - worked great.
Several people who heard the car first and then looked inside were
perplexed by the column shift.

I agree about Pete K. He has helped me several times directly and
indirectly. I always learn from his posts.

.

Revving the engine with a blip IS the problem, not the cure. You are revving the engine to get the trans input shaft to the speed it needs to be in the next lower gear. With a loose converter, the engine is more free to rev, but less likely to take the transmission with it, due to the slippage.

RACE ON!!!

And I think we have nailed on why a hydraulic automatic transmission is not a good box for road racing.

What if you looked at it from the other end of the telescope?

A 'loose' converter decouples the crank from the input shaft
at the interface between the Pump, Turbine and Stator elements
of the TC.

http://www.familycar.com/classroom/I...eConverter.gif

Downshifting causes the output shaft and the attached planetary
gears to speed up the ring gear and hence the input shaft. The
input shaft is attached to the Turbine vanes within the converter.
Since the converter isn't tying the input shaft to the crank, when the
input speeds up the Turbine, it is free(er) to overrun the Pump
so no (or less) lurch occurs than there would be with a 'tight'
converter that didn't let the Turbine overrun the Pump.

http://www.familycar.com/classroom/I...Planetary2.gif

Blipping the throttle brings the Pump vanes of the TC up into
sync with the now faster turning Turbine vanes attached the input
shaft.

Yes? - No? - Maybe So?

.

With the occasional exceptions

http://www.turbosracingphotos.com/CANAM/66acar6t.jpg

.

A follow up to say that modifying the .Bin values for TCC parameters,
stacking these on a chip capable of housing multiple bins and installing
it with a remote switcher enables the driver to change the TCC
characteristics on the fly. It does not appear to be necessary to
stop and turn off the ignition.

I haven't found further information about whether fluid only moves to
the cooler when the TCC is locked - still 50:50 with weight given to
Pete_K for his test. Since my events are short, I'm not going to
pursue the matter for now - I will assume that the temps do not rise
enough, long enough to be a concern.

.

"Blipping the throttle brings the Pump vanes of the TC up into sync with the now faster turning Turbine vanes attached the input
shaft.

Yes? - No? - Maybe So?"

FYI.
I have a built T700 with shift kit (stage below full manual opp) and 2K convertor with clutch removed.
I can manually downshift at any speed without a lurch
as such.Suspect would be good setup for autoX