My engine came rebuilt in my car.. Took her to my engine builder and he guessed her at 400-425 without opening her up so she is still a mystery to a point. How much HP can the SP 400 handle. I know I can build a th400 big time. Kind of like to keep the SP. I found a 66 olds SP400 out of a delta long shaft. with only 23,000 miles. I'll swap housings and up date. Plus I got a rebuilt SP torque converter never used... so I got hard parts.... Olds was wasted years ago. Interior is way to unworn to be 123,000. Throw a GV behind I'll have the best of both worlds.. I already own the GV...
If you follow the same principals of a stout Th400.. I'd have to say it'd handle the same HP? I've never heard of anyone breaking one..
An ST400 is a TH400. The "ST" stands for "Super Turbine", Buick's name for the TuboHydromatic 300 and 400 series transmissions. The "Switch Pitch" or "Variable Vane" or "Vari Vane" (as it can be called several names) is a specific model of the ST 300/400 transmissions. "ST" does not have anything to do with it being variable vane or fixed vane. The main difference is the torque converter's variable stator vanes and the front pump as well as an internal switch and passages for the varible vane function. (there are a few differences between the transmissions over the years, and some are a bit stronger that others). Switch Pitch transmissions can (and often were) made static pitch when rebuilt by shops that either did not understand them, or believed the feature was a "problem". I took mine to a local guy who has 50 years in business doing transmissions and "specializing" in GM transmissions. He started right off about what a piece of junk the ST400 was, and I started to explain what I was putting it behind and what I was looking to have done, and he cut me off and tried to feed me a boatload of crap about the unreliability, how weak the transmission was, how busy he was and might not be able to get around to it for months, but I could drop it off, and asked me if I even knew how the Switch Pitch worked. So, I stared at him for about 45 seconds, thought about making a case, but thanked him for his time, made an about face and walked out the door. (I know a little bit about transmissions...) If I had a press and a few other things, I would do it myself, it is just easier for me to have someone do it with access to press and tools. So, I sent it to Jim Weise to love on for a bit while I do other things. I am putting this behind a 430 .060, Stage 2 heads, TA413, and 10:1 with a 3.42 posi. Pretty sure it will put the Switch Pitch to task and I have faith and belief that it will be just fine. You should have no issues.
The THM 400 will take all the HP you can throw at it, as long as it is built accordingly. The SP converter will need to be built with that in mind as well. Bet JW can provide one.
ALSO the splines for a switch pitch in the converter & front pump are shorter because of the added feature. This is where the splines get torn out with excessive HP. Now this is years ago & maybe today there are better parts available. I'm sure Jim knows.
Thank you Guys. This has been very help full. I believe I'll be fine Im not a race track guy, so my HP is just there. The 66 trans has not been beat on so the converter & front pump should be in good shape.
SP400s, like any TH400 based trans, are strong but not indestructible. Back when I still had a SP400 with my small valve, iron head motor of maybe 400HP, I managed to break mine not once but twice on street tires. This was supposedly a well built trans with good clutches, extra clutches and other love, not just a cheap shift kit but it was back in about 1983-84. It wasn't a full race trans. So, here are at least two weak points you should consider upgrading if you plan to put much in front of a SP400. One failure was rolling the sprag (I think) and it would only shift 1-3 but would engine brake if you moved it to 2nd. I was street tire racing a guy and she didn't shift at 6200 like she normally did. When I let off it fell into 3rd and I limped it home. My trans guy fixed that with a heavier sprag and I never did that again. The second failure was street tire racing a buddy that was driving my 70 340-4speed Cuda. I again hit second gear at 6200 RPM and she zinged with a loud bang. I thought I blew the motor but the motor, knock on wood, was fine. Had to tow it in. Took it to my trans guy and he found a stripped input drum. The SP input shaft was fine but it ripped the splines out of the drum. That's when I gave up drag racing. I got tired of finding weak parts.
I had a 4000 lb 66 Skylark with aprox 450 hp 455 and I ran a 150 shot of N2O through a stock sp converter on Mickey Thompson slicks and the converter didnt show any signs of issues after a season of racing every weekend. the transmission however did have the hd intermediate sprag, shift kit and other goodies
I have seen procedures of removing the SP splined input shaft by driving it out of the forward clutch drum, then driving it into a different one. My thought is this process probably weakens the splines, so I just keep the shaft in its original drum when swapping parts. I have also seen the shaft welded in place from the inside to add strength. Bruce Roe
