Hi, Does anyone know what is the optimal line pressure, for a TH400? Do you also know what the max line pressure is, before you might experience bearing issues? Thanks for any help. Regards, John.
Stock is usually about 160 psi in drive with full modulator pressure, higher in reverse. 160 psi is enough for over 700HP, and some claim up to 1000 HP. 200 PSI is safe as far as internally in the transmission. Anytime you bring line up, you want to at a minimum install the HD snap ring in the case for the intermediate clutch pressure plate and if it is a full manual setup without a intermediate band, install a brace on the case where the band lug is at. If you get over 220 psi you can start blowing the case lugs out. It is much more beneficial to dual feed the directs internally and keep the line pressure reasonable. If you are speaking of engine thrust bearing issues with elevated line pressure, this can be an issue on some engines with the stock line pressure. Some transmissions run higher converter/lube pressure than others. You have to check it with a pressure gauge T'd into the cooler outlet. If it is too high you have to modify the pump by restricting the passage from line to converter feed.
Yes i found out the hard, way if you have over cold max 50 psi in the tranny lines your a potential crank killer!
Jakeshoe, Thank you for responding. A friend of mine has taken some pressure measurements, not sure if they are hot, or cold. I will try to post them in a way that that you can understand what they represent. Yes, I am speaking of thrust bearing issues, and I am affraid I have some at the moment. Regards, John.
Norbs, My pressures are higher than that, but I do not know if they were read hot, or cold. I also have some restriction, as in a large pressure drop, when the readings are taken before and after my trans coolers. I am running a remote, in series with my radiator trans cooler. We have heard that somewhere around 40 psi is ideal. I am not sure if that is a hot or cold reading. I will have to ask my friend. When it gets hot, is a higher pressure accepable? Jakeshoe was talking about much higher numbers? Was that a hot reading? Regards, John.
Line pressure is what the trans operates on internally to apply the clutches. Converter feed and cooler pressure is different and will be much lower. GM tech manuals usually show it at ~70 psi as acceptable. Usually you want it less than this. You need some pressure to keep the converter filled but not much. I've been told as low as 5-10 psi is OK, but 30-45 psi is a good pressure to aim for and not excessive. You have to check this pressure on the cooler outlet line, tee the gauge into the line and check pressures.
Thanks Jakeshoe, I talked to my friend, and we are measuring and looking for converter feed pressure. He seems to think we are going to have to reorifice some passage in the trans, to lower the pressures. We are shooting for 40 to 45 psi. We are pulling the motor, and are hoping to have no crank damage.ray: If that is the case we will clean it up as best we can, and throw in some new bearings. If the crank is chewed up, I gotta box it up, send it back and play the waiting game. It will then most likely be a winter project. Thanks for all the help.:TU: Regards, John.
All i can say is that i get 50 psi cold which translated to 38 psi hot. I would say 50 psi hot or cold is the absolute limit. If the converter has a bushing you can pull the front bushing out of the i stator support. to help and you may not have to restrict the pump. Im still doing research on that. You have to make sure the converter uses a bushing inside or not.
Hi Norbs, Thanks for the info and the support. We should know something this weekend, about the crank. The way my luck is going, I'm quite confident that I will be screwed, but ya never know. I only wish I had done as much research on my new trans and converter, as I did on my new engine. Would have saved me a lot of humiliation, frustration, and cash. But who was to know? My bad. By the way, if you've got any pull with the "Man Upstairs", I could use a little help with the condition of my crank and bearings. Regards, John.
The engine is out. The thrust bearing is shot. Crank has some ridges in it. I might be able to get away with throwing new thrust bearings in it, but I want this right. I am going to box it up and send it back. Think I need a new crank. On the trans front, I took my trans in to get it reorificed. Talked to my friend yesterday and it appears that my trans shop did not orifice it, like we asked, but did some other type of mods, after talking to Precision converters. We were not all that pleased at first, but will trust him and give it a try. When I find out just exactly what he did, I will post for some feedback. I don't really care how we skin this cat, I just want some positive progress. Thanks for all the support. Regards, John.:ball:
sorry to hear about your crank same thing happened to me a week or 2 ago. i was going to pull the engine to reseal the oil pan and when i was turning the fly wheel the crank went forward about 1/8" so once i broke everything down on the engine stand yealp the thrust bearing was ate up and probaly just like yours mine put a deep grove into the crank but not the caps. called ta and dave told me that it would be safer just to replace the crank instead of trying to get it fixed. think about the trust in maching a crank you got to have in a machine shop ! with mine i think either the convertor ballooned or the tranny had a stoped up lined either externally or internally and just overheated. i got a 400 and a 350 and im just thinking about getting a tci 400 or a jegs 400. have not had my old trannys rebuilt but i kind of rather get a brand new or pro built unit than haven to depend on the possible crap tranny shops around here. the engine was runny good and wa tuned good and this has definately set me back so i don't want to do this again if you get what i mean lol.
Yes Norb, That is what we wanted him to do. He talked to Precision converters, and came up with something different. We were not very pleased to say the least.:af: When I pick it up, I will post what mods he did, for opinions. He did NOT restrict the converter feed, he did some other things. I hope it all works out. I will not know for sure, untill I get my engine back, from my builder. Regards, John.o No:
Picked up my trans today. I was quite shocked to hear that he decided to orifice the trans after all. Not only did he reduce the line pressure to stock, but he also orificed it, like we asked him to before. He said he also did something to firm up the 2nd and 3rd shifts. He did the mods he wanted and the mod we asked for, after he said he did not.uzzled: I sure hope this works out. He seems to think it will live, we are not so sure, with the reduced line pressure. I hope everything works out. we will have to double check everything, when we get the engine back, just to make sure we have what we should have. I have no idea what to expect. If everything falls into place, and the trans dies, I will bring it back, tell him to fix it, and add some line pressure back. I think I am going to need some Divine intervention here. This is getting beyond mere mortal comprehention. I'll let ya know in a couple of months. (I hope its that soon.) Regards, John.:TU:
Do you self a favor and buy a transmission pressure gauge that goes to at least 200 psi. Make sure you test the pressures before and after the tranny cooler to see if its restrictive. Line psi the fitting is on the left side of the trans near the shifter. I hope you have at least 150 line though?
Stock blue TH400 PR spring will yield 160 psi and this is adequate for approx 1000 HP if the rest of the trans is correct. You don't want to get over 220 psi on the Th400 anyway if you can avoid it.
That's hot at full pressure. You should see 160 psi with the modulator unhooked and 1000 rpm on a stock TH400. Reverse will be higher. If you installed a brake, the pressures would be the same just full all the time since there is no modulator function.
Thanks Norbs, My friend, who is helping me, has two guages. We have been using them to get some pressures. We will be checking everything again, when I get my engine back, sometime in the future. Regards, John.
