Hey All,
I have a 91 Z28 with a stage 3 fixed pressure 700R4 built by a local performance transmission shop. My engine puts out 450 HP at the crank and about 330 at the wheels. I have a 3.90 Strange Axle.

I have blown this transmission up twice in 3 or so years. The first time the sun shield warped and a sprag cracked. It was then refreshed by the shop in Sept 2021(the shop has a lifetime $450 refresh warranty). I was driving on the highway recently and attempted to 4-3 down ship using throttle pressure only and instead it went 4-Nuetral...twice. Hit my rev limiter both times(~5800 rpm). Anyway they just finished rebuilding it under warranty(free this time) and told me essentially "don't throttle downshift while in OD, instead manually put it into 3rd to "hot rod it"".

I talked to my local performance shop who is installing the rebuilt trans and they explained that OD downshifts at power are really hard on transmissions of this era and even Ford's have the same issue and to do just what the trans shop above said to do.

Anyone have any other advice/opinions on this?

Personally I feel that if I paid for a built Stage 3 I should be able to expect it perform just like a normal 700 with a stock engine in front, including highway speed throttle downshifts. Even though I say that if it is technically infeasible then that's just how it is.

I am considering in the future a 4l80e upgrade but still noodling on it.

 

Almost any Over-Drive Auto Trans (4L80E or Other) will not last the way you wish to use Over-Drive.
I have THM700-R4 Units Warrantied up to 1,200 HP...
With the stipulation that OD be used for ONLY light throttle cruising.

There is no way to further increase the strength of the Input Sprag, in order to use OD the way you wish.
The 4L80E is the same story.

Have fun in D3, cruise in OD.

 

Hey, thanks for the response!

Glad to hear that I'm not being taken for a row.

I can and will adjust how I use the transmission.

Honestly this is the first I have heard of this and the vendor never mentioned it before now.

 

So wait...at this horsepower level (450 at the crank)....I'm cruising down the road in OD, and a situation evolves that requires me to evade an accident using the throttle. I can't expect the trans to kick down on its own without damaging itself? I have to shift manually to D first?

If so, at roughly what horsepower level does this become an issue?

 

Honestly this depends on multiple variables (how the Transmission has been Built/ Modified)...
There are some small modifications that can be done to the Input Sprag and Overrun Clutch that definitely help with Power Handling.

Someone would realistically have to design a new Input-Drum, that could house a substantially larger Input Sprag...
This would be the only way to use OD the way that you guys would like.

The highest power handling Unit, that current parts allow for (for my Shop, this would be a 1,200 HP THM700-R4/ 4L60E)...
Under approx. 500 HP - Ft/Lbs could be fine...
Engine RPM, Throttle percent, Torque-Converter, and Vehicle weight are all variables that will effect the above Power Rating.

Light Throttle, Low RPM OD/ D4 down-shifts to D3 can be just fine.
High RPM, Full Throttle down-shifts are another story.