Guys, what is the weak point in these transmissions? What, exactly, breaks?

The trans is based on a Borg Warner Super T-10 with an auto shifting planetary over-drive glued on the back.

Now, I have never had one of the stronger "SUPER" T-10s, but I have had a bunch of the standard T-10 four speeds, and never managed to break one, even with 11.50 wrinkle wall slicks and some fairly decent (N/A) power. They took, and survived, a ton of abuse for me.

Many years ago, some of us used what was called a HONE-O-DRIVE on our street/strip cars. These were manually shifted planetary overdrive units that were either divorce mounted behind the trans, or they had one that bolted right to the snout of a Ford 9" rear end. Now those you could break. But only if you abused them in overdrive. In direct drive, they were quite strong.

I am guessing that is the way of things with the Corvette 4+3 as well. Is that correct?? What goes bad in them, both in wearing out, and breakage due to stress in high performance use??

Thanks!

Vette-robert

 

The 4-speed portion is fine as long as it doesn't get run without oil.

The Overdrive (OD) portion is not fine. One weak link is a thrust washer that wears and breaks its tangs off, sometimes destroying the case. (Doesn't matter if it does, you can no longer source a replacement thrust washer from anywhere.) Another weak link is the carrier bearing. When they fail they can take out the front cover, piston and other parts. Some peeps say the bearing is available. I haven't seen that it is in recent history. Peeps also say that "maintainece" is the key. Changing the fluid regularly is important, but hard part breakage generally isn't related to lube unless there isn't any.

I've held the position for quite some time that owning a 4+3 is a liability. If the OD breaks, it may not be possible to fix it because of the dearth of replacement parts, and now you have a transmission-less car. There are alternative replacement transmissions, but they can cost more than the car is worth. There are several peeps here that have gone to alternative transmissions, but they are committed to long-term ownership.

 

I need to stop reading these kinds of threads.

I'm not sure what I will do when/if my 4+3 goes out. I rebuilt the OD once. I don't know if I would do it again or replace.

 

Yeah, it's a worrisome thing for sure, given the "parts" situation. I am committed to long term ownership of mine. Of course, the problem lies in how that torque link to the rear end bolts right to the trans. Being a fabricator, possibly I could make a way around that to fit a different trans; I don't know...

I am going to put a big block in mine, and the very first thing everyone says is "You're going to immediately shred that 4+3 trans." I have never understood this because as I said, I have had good luck with the T-10 trannies. I don't know. I have wanted a big block, standard trans Vette for 50 years, and here we are finally getting close.

Thanks for the answer, but do the weaknesses mentioned affect the strrength of the setup when NOT in overdrive? Is there any hope for what I want to do?

As I side note, I have taken control of the overdrive away from the computer on my car. I don't care about overdrive in 2nd and 3rd gears, and just use my switch for overdrive for fourth gear highway cruising. I even clutch when I shift it, to save that "fragile" overdrive. (??)

Vetterobert

 

Keep this in mind: The 1-3 gears in the transmission multiply the torque going into the input of the OD. In forward, this torque is held by a sprag which is exactly the same sprag as used in the 700R4. The forward sprag in the 700 is before the gear reduction, so it benefits from the gear reduction that it is driving into. (I.E., the sprag has a 3.07 to 1 advantage when the trans is in first, for example.) In the OD, the sprag is seeing the increase in torque being put into it unless the OD is in 4th gear (1:1 trans ratio.) because the gear reduction is before the input to the OD.

The sprag in the DNE OD is seldom mentioned as a failure point. But. How long does the sprag in a 700R4 last when coupled to a big block where the sprag is benefitting from it's location before the gear reduction? Quick numbers:
> For the OD: 500ft lbs of big block, 2.86 1st gear ratio, that's 1430 ft lbs into the sprag.
> For a 700R4: 500ft lbs, driving a 3.07 ratio 1st gear is 162 ft lbs through the sprag. Even in 4th gear, the full 500 ft lbs is going into the sprag.
I may be wrong, I've never thought about this sprag location deal, but even in direct I think you'll fairly quickly bust the sprag (or something) putting a big block's torque multiplied by the transmission into the input of the overdrive even when the OD is in Direct.

There is also a clutch pack for Direct. This clutch pack is spring loaded so it can be applied with no hydraulic pressure present. Like when backing up. The direct clutch augments the sprag, and also allows engine braking where without the clutch, the sprag would over-run, and there would be no engine braking nor would it back up.

The carrier bearing is in play anytime the unit is in Direct. All elements of the bearing rotate when the unit is in OD. This is why a failing carrier bearing makes noise in Direct, but the noise goes away in Overdrive.

 

the parts are unobtanium, just use it until it breaks

once it does the best swap is probably a TKX

 

I bought a car years ago from in here that was a 4+3 converted to a zf6.

having driven both, the zf6 is miles better to drive. It gives you that feeling that you’re looking for.

anyone on the of to swap or not to swap, do it. It’s worth it.

 

Im in the same boat with you brother. I have a spare 4+3 in the shed in the event I need another one. Lord knows if it’s as functional as the seller implied.

I’m too young, dumb, and sentimental to let my 84 go if it had a failing transmission. It works fine now, but man these threads simply remind me that the wear and years will catch up someday.

 

Dave McLellan flat out says the only reason the 4+3 exists is to make the required MPG goals with a stick car, they didn't have access to a manual trans with an OD at the time and it was required to make it viable.
It's a kludge and creative and interesting but not a super innovative workaround as it had been done before.
In all my research about these cars to get me back up to speed before buying one I somehow managed to miss the fact that there were parts you couldn't get and would need to rebuild the thing.
My TKX swap kit is on a fedex truck as I type.
The ZF, while I haven't driven one in 20 years, is no doubt a worlds better experience but it's not
without some significant hurdles if/when rebuild time comes too and the situation isn't likely to improve with time.
I'm probably going to ebay my 4+3 as it still functions fine though it's obviously tired.
I will not be sad to see it go, my wallet is though.