C4 Transmission issue

[HawkBrosMav]

We’ve got a pretty good rpm flash during the 2-3 shift. I’ll have to look at the logs, but shift around 5900 on the tach and you can see the tach flash up to about 6400. The 1-2 if there is a flash is much less noticeable on the tach itself.

This has been something we’ve noticed much more since we took the stock 1st gear out and put in a 2.92 ratio. I personally don’t know why that would give us a flash unless when reassembling we got something adjusted incorrectly. Just swapped to a brand new ATI converter and a different cam recently and the car is as fast as it has ever been but the flash seems to be worse.. any suggestions on making it go away and/or if we are losing ET because of it?

[ss3011]

How often do you adjust the intermediate band ? If it lets go too soon on the 2-3 shift it will flare , and you will see a higher rpm . This is a typical problem with most transmissions that have a timing type shift , there needs to be a little overlap between the band coming off and the high gear clutch coming on , or you get a flare . On a C4 there is a band adjustment .

[Bill Harris]

There are a few things that affect the flare including which valve body you have (who modified it to full-manual) and which intermediate band servo you are using. There a quite a few different servos that have various apply and release piston areas and accumulator volumes which affect the timing of the band release. The adjustment on the band has a little effect, but it is a fine tuning compared to the hydraulic circuit variables. You should never have a flare on the 1-2. Second gear is the intermediate band applying to stop the direct clutch drum. You might get a slow or sloppy second gear shift, but unless the forward clutch is slipping it shouldn't ever flare.

[HawkBrosMav]

Originally Posted by Bill Harris

There are a few things that affect the flare including which valve body you have (who modified it to full-manual) and which intermediate band servo you are using. There a quite a few different servos that have various apply and release piston areas and accumulator volumes which affect the timing of the band release. The adjustment on the band has a little effect, but it is a fine tuning compared to the hydraulic circuit variables. You should never have a flare on the 1-2. Second gear is the intermediate band applying to stop the direct clutch drum. You might get a slow or sloppy second gear shift, but unless the forward clutch is slipping it shouldn't ever flare.

We installed the kit to make it a manual valve body. Never had a flare until the trans was disassembled to swap 1st gear ratio. Valve body was not touched during this process.

What should we be looking at in terms of Servo/Band combo or adjustment to correct said issue. The shift is by no mean sloppy or mushy.. driving through the pits it a nice firm solid shift, but on track it's simply a flare.. no burning of fluid or anything all we're noticing is the rpm flash. not sure we are even losing ET as fast as it happens just seems like something we should be able to eliminate.

[HawkBrosMav]

Originally Posted by ss3011

How often do you adjust the intermediate band ? If it lets go too soon on the 2-3 shift it will flare , and you will see a higher rpm . This is a typical problem with most transmissions that have a timing type shift , there needs to be a little overlap between the band coming off and the high gear clutch coming on , or you get a flare . On a C4 there is a band adjustment .

Where do you recommend we sent the adjustment on this band now that it's a used band and not fresh out of the box? I'm honestly not sure where it is set now, we set it when we built it and haven't touched it since... probably a good reason we are starting to see it now...

[Mark Ugrich]

Originally Posted by HawkBrosMav

Where do you recommend we sent the adjustment on this band now that it's a used band and not fresh out of the box? I'm honestly not sure where it is set now, we set it when we built it and haven't touched it since... probably a good reason we are starting to see it now...

I’d start with the adjuster tight , then back off 3/4 turn.

[ss3011]

Take a look at a Ford shop manual and torque the adjuster pin to spec , then back off the pin by the recommended setting then tighten the jamb nut . This setting might have to be changed if the band runs against an aluminum drum . Typically if you back off the pin less, the more overlap or tie up will occur (less flare) . This assumes the band and the drum surface is in good condition . Too tight a setting might cause the band to drag and the car slowdown in 1st gear . The high gear circuit needs to be leakfree . With the VB removed you can aircheck these circuits . Check a see what servo you have , If you have a flare problem , an "H" servo might be a good one to use , there are billet versions of that servo available . Also typically a C4 only has a 4 plate high gear clutch , which may not have enough capacity , depending on torque and rpm . A 5 plate clutch pack helps that . A flare on the 2-3 shift could be telling you the high gear clutch is burnt .

[Bill Harris]

It's a little hard to correlate how changing the gear ratios would affect the 2-3 shift action, but perhaps it has to do with the torque and rotational speeds of the drums involved. A 2-3 flare is actually the transmission state going 2-1-3, with the brief first gear state causing the engine RPM flare. 2-3 shift is two actions on the direct drum. In 2nd gear, the drum is held stationary by the band, and to shift to 3rd the band has to release and the direct clutch has to apply which causes that drum to go from stopped to engine RPM in a very short period of time. If the band is released before the clutch is applied, that is actually first gear and the drum starts to rotate in the reverse direction (!!) before the clutch application catches it and gets it rotating in the forward direction. Maybe the higher first gear is applying more torque and a higher reverse RPM to the drum before the direct clutch can catch it. The lower gear ratio would also increase the load on the direct clutch, making it harder to stop the drum ... thus the increased flair. Like SS3011 says, if the direct clutch is slipping, or doesn't have enough capacity to stop that drum which now has more inertia, that would show up as a flair. If that is the case, it's going to burn it up eventually. You may have to play with different band servos to clear it up completely. Maybe call Joel's on Joy and see if they can recommend a servo to go with that 2.92 ratio. That is a LOT steeper ratio than the OEM which is around 2.46 (if I remember right).

[Mark Ugrich]

It’s been a long time since I’ve played with C-4 stuff, but I would think the servo or the release circuit would affect this.

[jmantle]

Originally Posted by HawkBrosMav

We’ve got a pretty good rpm flash during the 2-3 shift. I’ll have to look at the logs, but shift around 5900 on the tach and you can see the tach flash up to about 6400. The 1-2 if there is a flash is much less noticeable on the tach itself.

This has been something we’ve noticed much more since we took the stock 1st gear out and put in a 2.92 ratio. I personally don’t know why that would give us a flash unless when reassembling we got something adjusted incorrectly. Just swapped to a brand new ATI converter and a different cam recently and the car is as fast as it has ever been but the flash seems to be worse.. any suggestions on making it go away and/or if we are losing ET because of it?

You didn't mention what you had the front band set at. Stock setting with a course thread adjusting screw is 1 1/2 turns, 3 turns with a fine thread (C5) setup. Setting the band tighter than stock will reduce a 2-3 flare but I wouldn't go tighter than 1 turn on the course thread or 2 turns with the fine thread setup. A good test after setting the band tighter than stock is make sure the driveshaft will turn by hand in both directions after you set the band.
As mentioned by others, you may have direct clutch issues that a band adjustment won't fix.

Jim Mantle V/SA 6632