Simple Q. Just curious to know how much power the (76) t400 can cope with, before reliability becomes an issue. Add a 2500 stall converter in there too.... Reason being, Im looking at getting rid of my motor, and putting something more powerful in its place. And a 2500 stall for good measure. Thanks!
Stock from 1976, or stock rebuilt? Yes both would ultimately take the same amount of power, but if its stock from 1976 its got a lot of wear and tear. Ive got a stock rebuilt th400 except for manual valve body and i run 7.0 on the drag radials with full weigh gs.
When the 4L80E was engineered, they used a design spec of 885 ft-lbs. From what I understand the internals of the two transmissions are quite similar, but I'm not an expert on that part. Hope that tidbit helps... -Bob C.
it should i know it was on 3/4 ton and bigger trucks in the 80s so it should be tough as nails not to mention my gfs grandpa has one in a 83 c20 with a million miles on it
IIRC, the TH-400 series transmissions are rated at approximately 600 ft-lbs. of torque at the upper limit. There are so many versions that cange with application tag code that an exact number is not accurate. I think there is a note in Ron Sessions book about it somewhere when I read it. I would be a bit concerned about putting too much engine in front of a 30+ year old trans that hasn't been rebuilt. You are just asking for trouble. Take a look at the tag on the trans and post the code so I can give you a better answer from the book.
I Came across this article on Novak's website. They are JEEP drivetrain specialists who manufactures adapters to use TH400 in JEEPS. The TH400 is an automatic shift, three-speed, longitudinally positioned transmission. It is widely regarded to be a supremely durable and legendary transmission. The TH400 is conservatively rated at 450 ft. lbs. of input torque. Aftermarket building techniques take it well past that figure. I have personally seen a stock TH400 handle over 1000 horsepower on a dyno for two days of tuning (alot of hard acceleration) and hold up, then used on the streets for about a month with a heavy right foot before it started seeing a problem holding third during moderate acceleration on the highway, when hot. When the trans was cold it still ran like a race horse. The trans was swapped for a TCI competition trans and the old turbo 400 went into a beater that was used to haul parts and run around town a little, and that trans still works in that car till this day, with only third gear slipping, 1-2 shift is still strong, and holds first and second gear well.
About 400-450 HP/TQ is the limit on the stock TH400. With ONE almost free modification and some minor valve body calibration changes, 800 HP on an otherwise stock TH400 is not a problem if it's assembled well (good endplay, etc), and the 34 element sprag is a wise addition above 450 HP or any drag usage.
The TH400 was used in 1970 Cad and Olds, claiming 400hp for the big block. The transmissions and all the FWD TH425 versions I have seen, used a 6th clutch friction in the direct clutch, can be adapted by using the extra shallow piston. Bruce Roe
My 70 GS trans is a original 400 has run 11.70 with a mild cam,now with a wilder cam,it should be running in the middle to low 11's .I run it hard all the time,figuring about that I have 600HP or more .This was the original owner race car,stock as the owner told me it was running 12.00 with nitrous.When I got it, I changed the heads,took the nitrous out and it ran 11.70.Never touched the transmission.And I have a 3500 stall converter that I got from Jim Weis.And the car is over 4000 pds with me in it.Bruno.
I have built many 400 with stock internals (other than 34 element sprag) that had survived 900 dyno proven rear wheel hp. Around 900-1000 is where the stock 400 forward clutch hub starts to have problems. Around 1500hp, I have seen a couple stator supports tubes strip inside the pump, and input shafts strip from the forward clutch housing. Since the no-prep racing has become popular around my area, rolling the sprag in the direct drum has become more common. I’m guessing that is due more to driver error than actual part failure. People tend to peddle the car under less than perfect traction and the sprag gives up. Putting an aluminum drum in solves that problem. A 400 trans is pretty durable. People today are putting far more power thru them than the engineers who designed it 60 years ago could have ever imagined. I’d say properly built with the proper factory parts, and the proper hydraulic upgrades, 900 hp is no problem.
Heat, correct pressure, clean fluid, proper capacity, not letting off a second gear burnout, no neutral drops, no "J" hooks, no shifting into neutral at high road speed, and service according to the way you drive it. (More frequent if harsh driving). It takes a lot to kill a case, and more of that is from frame flex, (soft/rotted frame, bad body bushings), bad engine/trans mounts, bad pinion angle, drive shaft too long and the yoke jammed into the rear of the case, stuff like that. More than power. (if you can afford to make that much power, you can afford to build the transmission to handle that power). If you ever have any issues, or too hot, then pull the pan and inspect the fluid, and service it. If it starts slipping, running it low on fluid, etc. pull the pan, and if the fluid is dark, smells "burnt" (you will know) or clutch, metal, (looks like fine metal flake paint) better to tear it down and inspect. If there are no problems, the "rebuild kits" are about $200, and well worth not having a transmission fail and eat up the pump housing and hard parts, cuz that gets spendy faster than a twin turbo GNX on nitrous!
I did get some sound advice from a friend trans builder ,he advised me that when you are doing a burn out always shift gears,as more trans are lost right there at the line.It worked for me,as I never blew a trainy racing my car.Bruno.
HP input is only half the equation. I split the forward clutch hub at the splines, in an otherwise well built 400, with an engine producing only 540 ft lbs of torque. Because that car weighed 4110 with me it, and did 18 inch wheelies every time the transbrake released. Weight and gearing are a huge factor in trans longevity. Heavy, highway geared cars are brutal on transmissions. The vehicle the trans in question is in, has just as much to do with longevity as power input does. A fresh stock TH400, 16 element sprag and all, will live forever behind a 750 Hp engine in a 1800 lbs T bucket. The real key in this discussion is that as was mentioned, there are just a couple of minor areas that have to be addressed, to make the unit exceptionally strong. Because the huge shafts and drums don't disintegrate if you look at that sideways, like they do in some of the later GM offerings. JW
https://www.ckperformance.com/Tech-Blog Here are pictures of th400 basically small block vs big block variations in reinforced vs thin cases.
