Hello all, I have a TH350 behind a fairly mild 455 build up. Of course I heard that the preferred trans for a 455 would be a 400, but since my TH350 was rebuilt fairly recently before the engine swap, I wanted to get my money out of it. It is now "slipping" equally in all gears. It feels like a car with low trans fluid feels. Obviously that was my first theory, but the fluid level was full and it looked clean. The car probably only has less than 10,000 miles on since the trans rebuild, with about 7,000 of it with the 455. I put my foot in it every once in a while, but I don't race the car. I wouldn't say I drive it hard. Is it possible the 455 has done in the TH350 already? One friend suggested it sounds like a convertor problem, since the slippage is equal in all gears. The convertor is a OEM type, nothing upgraded. Any thoughts?
Check the vacuum modulator. It has to have a leak free vacuum supply. Make sure the metal line is not crimped. Pull the modulator out and point the vacuum nipple down. If any ATF comes out replace the modulator. You can also check the modulator with a vacuum tester. It should hold vacuum.
Thanks Larry, I will give it a shot. Is the modulater easily accessed by pulling the pan? I have no transmission knowledge/experience so it will be a learning experience for me. And Larry, I also would like to take this opportunity to thank you for all you do on this site. I know every time I have posed a question here, you always give knowledgeable advice. You seem to reply to almost every post on here, and rarely get thanked for it. So, just wanted to let you know some of us notice. Thanks again!
Thanks Devon, that looks easy enough. I'll give it a shot in the next few days. I appreciate all the input. So do these vacuum modulators commonly fail?
I wouldn't say failure is common, but it's an easy item to check compared to tearing into the rest of the transmission's internals. Another good test is to watch line pressure while performing driving tests, which requires having a liquid-safe 0-300 psi gage attached to a length of hydraulic hose, which in turn is attached to different pressure ports on the side of the trans case. If you're not that adventurous, it's an easy job for the shop to perform and can tell a lot about what's happening in the different hydraulic circuits. More detail here: http://books.google.com/books?id=bu...ook_result&ct=result&resnum=1&ved=0CEAQ6AEwAA Devon
You're welcome. Glad I could help. I see Devon has provided a picture:TU: The modulator is easy to remove. Like Devon, said, failure isn't common, but when they do go, they can cause all kinds of symptoms. It's a simple thing to check. Verify the vacuum connection is intact and leak free. Then see if the modulator has any ATF on the vacuum side, and that it will hold vacuum.
Gents, I pulled out the modulator today, and the vacuum side was dry. There did not appear to be anychange in the vacuum line to the modulator. I guess my next step from here will be to pull the pan to check the filter and look for shrapnel. A friend of mine that has some transmission training said since it shifts normal, it didn't sound like a failure of the transmission internals, but more like a convertor failure. Is there any way to tell without removing it?
It is possible that you have a stator malfunction inside the torque converter. There is a one way clutch that keeps the stator stationary as long as the speed of the two halves of the converter are different. As the converter approaches coupling speed, the stator free wheels. There are 2 possible malfunctions, constantly stuck, or constantly free wheeling. A constantly free wheeling stator will make a car accelerate poorly up to about 30-40 MPH, but then it will seem normal at higher speeds. A constantly stuck stator will manifest itself as good acceleration up to a point, but top speed will be limited to 40-65 MPH. Maybe you have a free wheeling stator. The transmission has to come out either way. If you find a lot of material in the pan, could be catastrophic failure inside the converter. Doesn't sound like that though.
Thanks Larry. Your second hypothesis is about right. The car starts from a stop ok, but once I get in to the higher RPM's of each gear it starts to slip. It won't do much over 45 without serious slippage. Well it sounds like I will be pulling the trans. I guess this is a good opportunity to upgrade that stock convertor. The TA website recommends a "small stall convertor." Any idea what they mean by that? Here are the specs: <TABLE border=0 cellSpacing=0 cellPadding=4 width="100%"><TBODY><TR><TD width="100%" colSpan=4>Product ID: TA_290-94H 455H.491"-226'/.502"-235',112' </TD></TR><TR><TD class=detailimage width="100%" colSpan=4 align=middle> </TD></TR><TR><TD width="100%" colSpan=4>Good street/strip cam. Gives 30 HP over stock cam. Stock rockers ok, TA valve springs recommended. Small stall converter recommended. Power Brakes ok. Fair Idle. 9.0:1 or more CR. Average Idle 1000 in Park, 750 in Gear. Idle Quality: Smooth Idle - just like stock Good Idle - mild lump, but doesnt impair driveability Moderate Idle - nice performance sound Fair Idle - heavy lump Rough Idle - very lumpy race type idle VALVE LIFT STOCK 1.55 RATIO: IN: .476 EXH: .486 TA ROLLERS 1.60 RATIO: IN: .491 EXH: .502 DURATION AT .050 IN: 226 EXH: 235 ADVERTISED IN: 290 EXH: 294 LOBE CENTER: 110 POWER RANGE: 1500-5500 I am also running 10:1 slugs, Stage 1 heads, a B4B and full length headers. The car is street driven 98% of its life, and is full street trim. It is currently running 2.56 rear gears, but will be getting 3.55's or 3.64's in the next few months. I want a convertor that will be noticebly faster, but not miserable around town. Does such a unit exist? </TD></TR></TBODY></TABLE>
Can't wait to see the torque converter recommendations as I have a similar cam. Bet(hope) someone suggests one of Jim's small but tight converters, sure would like to try one of those myself. Good luck, Steve
So at this point I am at a crossroads. The 350 is definetly going to need to come out either way. I have a 400 with the long tailshaft. Cost wise, it sounds like I will be spending about the same to repair the 350, or rebuild and use the 400. The 400 came with the yoke from the donor car. I believe I have heard from folks here previously that the 400 needs the short tailshaft to swap in without shortening the driveline. If I do that, will the 400 yoke attach to my existing driveline? Or am I totally wrong about most of this!
I have swapped in many TH400s for a TH350, and never yet had to change the drive shaft. You must use the shortest yoke, and it will be close. A TH400 is the reliable way to go behind a big block, and you even have the option of switch pitch. Changing the 400 to a short tail is trivial in a rebuild. The kickdown cable goes away, the 400 uses an electric kick down. good luck, Bruce Roe
So bruce, are you saying the yoke from the 350 was also compatable with the 400? No yoke change necessary?
The yoke has to be changed for a TH400 output shaft. The shortest version of the yoke will likely allow using the same drive shaft, always worked for me. Do note there are 2 versions of the trans shaft. Some have an O ring ahead of the splines. The idea was a smooth section inside the yoke extended forward past the splines to slide over the O ring, and the yoke had a center vent let air out. A yoke with no vent will work on either type shaft. But a vented yoke on a shaft with no O ring will be leaking oil out the vent. Bruce Roe
