Turbo 400 trouble

[Robbie Welch]

How much clearance does the converter need when installing the trans. Combo is 468 chevy turbo 400 trans. Has anyone heard of to much pump pressure driving the converter into the engine?

[Tom Goldman]

Robbie, the converter needs a minimum of 3/16" clearance between the drive and the pump gear.
You can get by with 1/8" ,but 3/16" buys a little insurance.
Converter pressure will try to push the converter forward from the pump, slightly, but this is normal, and not to be confused with converter "balooning".
You really need to make sure the converter is not bottomed in the crankshaft hub as well, that can lead to crank thrust bearing and pump bearing issues.
Some of the thick plate type flexplates can cause converter clearance issues when used without a midplate.
Line pressures in a race or performance 400 are usually not nearly as high as a high HP Powerglide, usually 175# or less.

[Henry S]

From what i have found, is that pump pressure is not the only problem with t400. Most of the crank endplay problems come from too much cooler line pressure. anything over 50 psi cooler pressure is concern for a crank killer. I have seen 70 to 100psi cooler pressures on some t400's. You can google search and find more solutions. fwiw restricting the convertver feed in the pump is the cure for crank killer T400's.

[Tom Goldman]

Usually the cooler pressure issue is on cars using too small a cooler or undersize lines to the cooler.
Some transbrake valvebodies also up the line pressure fairly hi and aggrivate the problem.

I've only ever had to restrict 1 th400 that had this problem.
Heres a picture from a forum I found a few years ago.
Restricter size preference seems to be between .090 and .150

I would check the cooler pressure and the cooler for restriction before making any mods to the pump.

[Robbie Welch]

Thanks Tom & Henry, this is a customers engine that came to us with a the trans side of the trust bearing burnt and the bearing welded to the crank.The crank was useable and everything else rebuilt and checked out good. The customer checked the converter clearance and drive shaft clearance all said to be good. Well 2 races later and its back again with the same problem converter side of the trust bearing burnt. Since then the converter has been checked by the (manufacturer) and was good and the trans is bring checked. Both times after the run when he lifted the engine shut off when this occurred (I believe it was going on during the run and just locked up after the run). Also he had a less powerful engine in the same car with out a problem. With that engine he had motor mounts and a rear engine plate but I heard he cut the rear plate loose to gain header clearance after the first engine. He also checked engine movement in the car and said there was none. When checking cooler pressure do you just put a gauge in the line or do you use a port on the trans. Thanks Robbie

[Tom Goldman]

Robbie ,the side port is the line pressure port......Line pressure depends on many factors ,but should be between 165 -200#
Tee into the cooler line to read the pressure on the bottom cooler out line from the trans.
Does the customer do his own transmissions ? ...He may know if the pressure regulator spring was replaced or if any horse shoe shims were added.
If the cooler pressure is high , remove the cooler from the circuit , and measure the pressure on the loop to see if there is a restriction in the cooler raising the pressure.

[Alan Roehrich]

Did he take the plate loose from the frame, or did he take the plate out completely? If the mid plate was removed completely, that is the problem, unless the converter was also altered. The converter should be made to compensate for the thickness of the mid plate. When you then remove the mid plate, the converter is "too long", and you have a thrust problem.

Also, it is possible for the pump itself to develop a leak internally, and have line pressure leak into the converter charge/cooler circuit, that will create excessive pressure and burn the thrust bearing.

I've also seen people block off the cooler passages and create this problem. I just don't like blocking the cooler, if I'm not going to run a cooler, I make a small loop from steel tubing to run between the two cooler fittings, I find that to cause no problems at all, where I've seen various methods of blocking off the cooler cause all sorts of problems.

[Robbie Welch]

No, he doesn't build his own trans. The plate is still between the engine & trans I believe just cut off. Tom I'll get with him and find out about cooler & line size. Can ATI check pump pressure (they didn't build the trans). He'd like to find the problem before putting the engine back in the car.Thanks again Tom & Alan

[Tom Goldman]

I'm sure ATI could dyno the trans and moniter pressure.

[Alan Roehrich]

Tom is correct, ATI can dyno the transmission. I can't say what the cost would be, probably not inexpensive, considering they did not build or sell it.