Q: I have a Turbo 400 transmission with a 4,000-rpm stall converter and a transmission cooler. My trouble starts when the transmission temperature gets to about 210˚ F—the transmission starts blowing fluid out of the top vent tube. Even this hot, the trans doesn’t slip and shifts just like it does when it’s cool, whether I’m cruising or cranking the engine up to 7,800 rpm. As far as I know, the pump is set up for stock line pressure. The transmission also has a flat filter and a deep pan.
A: Your converter will slip any time engine rpm is under 4,000—and a slipping converter generates heat. As the trans fluid heats up, it expands and fills the pan. A stock transmission pan holds approximately 4½ quarts of fluid, but your deep pan holds more. That means the fluid level when the transmission is cool is about the same as the level of hot fluid in the stock pan.
What all that means is when that extra fluid gets hot, it has to go somewhere—and that somewhere is into the transmission itself. All those moving parts whip that fluid into foam. Since foamy fluid takes up a lot of space, it is forced out of the transmission via your top vent.
The solution is one of two things—lower the fluid temperature (maybe by going to a larger cooler), or switch back to a stock capacity transmission pan.
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If the fluid level reads over full on the dipstick when at 210 degrees then lower the fluid level to the full hot mark while at 210 degress . That way it will still have room to expand if it gets hotter and should still be on the proper level when cold. I would also get a deep sump filter to draw cooler, none arrated fluid.
A couple of problems here ! petrolium based atf cokes at 225f (starts to form little carbon particles) so you are at the outer limits 2 didnt mention vehicle size/weight but coolers are rated by BTU expelled and vehicle size if you have too small a cooler you need to address this 3 Im certain if you checked with your convertor manufacturer that they dont recomend a 4000 rpm stall speed for your application 4 When hydra Matic design the deep pan option it was for heavy duty (towing RV industrial) It was adopted by hiperf because it works It is deeper to provide more fluid so it can disapate more heat and preserve the fluid 5 The deep sump option is more than the pan there is also a specific (longer) pickup tube and a specific dip stick if you are missng parts you will have problems Point being that deep pan th400 have gone billions of miles Check your cooler how its plumbed where its positioned,Where are you getting your temp reading should be about 25f difference between in and out ,size,Get a convertor thats more realistic,Make sure that you have all the correct parts for the deep pan And finally God Bless the engineers at HYdraMatic for designing a trans that even when abused it still works and bless GMC for letting them build it Treat your th400 with respect its one of a kind Do the engineering and enjoy the the tire shredding 2/3 shifts at wot
Just written a few hours ago: “no, no, no, no, this is now, 1956, was then, PLEASE, GOD, will you use RED LINE PRODUCTS and don’t endanger yourself or anyone around you anymore????????”
racers for christ says the same.
A lot of cars and engines can’t use as you say a more realistic converter. I run a 5000 stall on the street and my trans temp never goes over 190 deg. I use a big cooler and a heat sink. A 4000 stall is not that high.
What size cooler do we consider a big cooler to be?