TCU - Transmission Control Unit, interchangability??? 4EAT

[Boostedballs]

So, I used to drive a 93 N/A legacy and now I drive a 91 SS.

The N/A had a really hard upshift into second and sometimes 3rd. I know it was probably some kind of fault with the transmission or TCU. I liked it.

The SS shifts like my grandmother and it's costing me about a second on the drag strip. The transmission seems great when I'm not bashing on it so I think there is nothing wrong with it.

Question- Does the TCU in the N/A control timing differently than the Turbo TCU does?

I have a spare turbo trans and I have a spare N/A TCU. It would tickle me silly if I could get those hard shifts in the turbo!

[93forestpearl]

Question- Does the TCU in the N/A control timing differently than the Turbo TCU does?

No.

The old TCU does not control the firmness of your shifts like a late model GM does.



Some tranmission techs will install a mild shift kit on all the Subaru autos they rebuild.




The reprogramming kit from tans-go is what you are looking for, but it takes a full teardown to install it.

[Boostedballs]

I'm familiar with the trans-go kit but they want an arm and leg for it and I don't want to dump any more cash into the 4eat when I am sourcing a 5mt.

I was hoping someone knew something about the failure that my N/A model was having with hard upshifts. I would like to duplicate the "failure" on my SS transmission. I know it will be short lived, but I have a spare turbo 4eat and I would like to see what it'll do in the 1/4 mile.

I read somewhere that a bunch of timing is pulled from the engine during shifts to make the transmission last longer, I just want to see if I can work around it somehow and keep timing advanced through the shift. Also maybe control line pressure externally somehow?

Does anyone know how much timing is pulled from the turbo engine vs the N/A engine and where this is controlled?

[Legacy777]

The ECU and TCU are separate computers. The TCU does read off of some of the engine sensors like TPS and speed sensor. The only direct connection between the ECU & TCU is the rpm signal. So the TCU does not have any affect on timing.

What you probably are experiencing is just different shift maps. Not really anything you can do about it. Try putting in the 93 n/a TCU and see what happens. Pretty sure the I/O is the same. It'll either work, or it won't.

[Grayguy]

If your that worried about 1/4 mile times you should be building the auto anyway. Ratchet shifter in a subie is awkwardly awesome.

[Boostedballs]

If your that worried about 1/4 mile times you should be building the auto anyway. Ratchet shifter in a subie is awkwardly awesome.

Well, the 4EAT will soon be replaced with a 5mt; I just was hoping to find a way to make it slam gears like my 93 n/a did. I have a spare turbo 4EAT sitting in the garage if I need to swap it out quick anyway.

[Boostedballs]

I installed the NA TCU in place of the Turbo unit and the upshifts seem to be firmer. There is a moment during the shift where the AFR's go into the 16's though. I'm not sure if the TCU has anything to do with it. I'm leaving the N/A unit in place.

[Boostedballs]

Update:
It turns out that I was hitting the rev limiter on the ECM with the N/A TCU. The injector duty cycle went to 3% over 6400 rpms, that's not good.

I put the turbo TCU back in and my AFR's are stable but the shifts are LAZY!

Too bad, I wish I could up the rev limiter on the ECM so I can put the N/A TCU back in...

I won't even bother with the 2.5 DOHC heads until I can finger it out...