The transmission (TH400) in my 69 Firebird has been restored before I bought the car. The transmission is not original to the car, it decodes to originate from a 67 GTO, but that would be similar to a 69 Firebird 400, in my opinion. It works like a charm, but the shifts are harsher than I'm used to from automatic transmissions (I have only owned low performance cars before). The car is an A/C car, so it has 2.78 gears in the axle.

The first to second shift always bark the tires, when I accelerate fully (it shifts at around 5000 rpm). When it shifts to third gear (still wide open throttle acceleration) the speed is so high (around 80 mph), so the tires don't lose their grip. Instead, there is a VERY hard "thump" that shakes the whole car, it almost feels like something is about to break.
Of course, in normal driving it shifts much smoother, but you still feel the shifts. It is not as smooth as the average automatic trans.

Does this sound normal for a standard TH400 trans (together with a standard 400 engine), or could a shift kit have been installed when they restored the trans?

Ronnie Stensson
Sweden

Sounds like a shift kit was installed by a previous owner.

Stall speed should take care of that.

vots

in my TH350 I took out a spring when I installed my B&M shift kit....I removed the "granny spring", which makes it shift harsher , crisper....yes it gives it a "jerk" when shifting....cannot remember the name of the spring, but its a small one.....you may have a similar situation....Anyone buying my car at some point will have that same question maybe....unless I remember to tell him/her (If Im` still alive to tell, when my car gets sold!)

Sounds like mine.

I had a tran pak kit put in my tranny two rebuilds ago. Just because a tranny is rebuilt doesn't change the parts that make it shift hard.

Mine also shifts way to hard under WOT. I'm going to break my rear if I don't get it changed out soon. I was quoted $200 for a new shift improver kit. I want a firm shift not a harsh shift. Must be my old age.

This is how mine was also. I did the calibration govener kit by B&M with some improvement. Did not get the shift that I was looking for Untill I changed the converter.

Need to redo the v/b in my 400.
2nd bangs to hard in cruise mod.
Dont feel it during WOT though.

as mentioned earlier...a higher stall convertor will take care of the harsh shifts

I'm now one step closer to solving the problem with too harsh shifts. I just changed the oil and the filter in the transmission, and it seems to shift much smoother now. I have only driven the car around the block where I live, there is too much salt on the main roads to make me want to drive any longer distances. Therefore, I haven't been able to test how it shifts at full throttle, but it sure feels promising.

I strongly suspect that the trans had ATF type F in it. I drained as much as possible and refilled it with Dexron fluid.
Now I'm waiting for the spring to come, so I can take out my Firebird for some longer trips.