The previous owner put a Chevy 350 in my 72 Skylark, and I want to replace it with a Buick 455. It's got a TH350 in good condition with a 3.42 positraction, but I'm wondering if I would be asking for trouble if I didn't also put in a 400 trans. It's going to be a street-only car. Also, are there things I could do to beef up the 350 to handle the 455 torque without spending an arm and a leg? Thanks for any advice, Dennis
Before you spend money beefing up the 350 buy a TH400. Look at the size of the input shaft on the TH400. They are tough.
Also, the TH 350 is probably a Chevy bolt pattern and won't fit to the Buick 455 anyway. So, there you go, justification for the TH 400.
The TH400 is considerably beefier than the TH350, and for the amount of money you'll have to spend beefing up the TH350, you'd most likely be able to find a TH400 for about the same price, and that will be able to handle a lot more, pure stock. I've been running with a stock TH400 with a shift-kit behind my decently tuned 455 making about 500-550hp for over a year, and it's just now that third gear is starting to slip a bit. I have no idea how much mileage the tranny had, since it was taken from a 76' Estate Wagon, so it might've had several 100k's worth of miles on it, but still trucking like a champ.
Re: TH350 trans with 455 engine? Dennis - for years I ran a 350 behind a 3000 converter - never had any issues - the car ran consistently low 12s and dipped into the upper 11s when I had good air...........I ran a 12 bolt 373 behind the tranny...go for it - the worst is you have to change out at some point. regards Buickstaged
Re: TH350 trans with 455 engine? General Motors would not have gone through the added expense of designing a bigger transmission, and putting it behind their bigger engines in heavy cars if it wasn't necessary. Having said that, back then, everything was over engineered. You can certainly use the TH350 behind a 455. How long it lasts will depend on lots of factors. First, how healthy the transmission is right now. Second, on how much power the 455 makes. Third on how the car is used. If it is raced with sticky tires, and you consistently are able to hook it up solid, the THM 350 will not last as long as the 400. You can certainly beef up the 350 transmission to handle the 455. The parts for that are out there, and lots of guys have done so.
Good advice. A short tail TH400 will probably replace the TH350 using the same driveshaft, with the shortest 400 yoke. Push the trans support back 6 3/8". Switch Pitch is also an option. That 76 trans is probably suffering from very old rubber piston seals. Once they start to go, you lose clutch pressure, and the whole thing can burn up. Don't wait for that to destroy everything in a trans. I sometimes just put in a $40 seal kit, but the more it slips, the worse everything else will get. good luck, Bruce Roe
Yeah it's currently in winter storage, going to rip the TH400 down before the season starts and give it a full run-through. Noticed it started slipping just a day or two before putting it off for the winter, so hopefully there won't be too much damage.
For what it's worth, I put a 455 in front of a 86,000 mile 350 and it lasted 3 years before it bought the farm. I never beat on it, but some medium hard accelerations. Never any burnouts or reverse drops or anything. I was just driving home from an interview and lost all power. Motor just revved up like it was in neutral. Replacing it with a 400. Just to much torque for the 350?
If that 350 doesn't have a Chevy only bolt pattern I would run it until it pukes. You might be surprised how long it lasts. I had a high mileage 350 behind a stock 455 that is still going strong after 5 years and the guy I sold it to isn't easy on cars. Bob H.
Excuse me for asking a possibly dumb question, but since the 350 is bolted to a Chevy, doesn't it have to have a Chevy-only bolt pattern? Otherwise, I like your advice, because the 350 seems to be in excellent condition and I don't need the added expense of a 400 if it isn't critical.
Not really a dumb question. There were some 350 turbos with a multiple bolt pattern that fit Chevy and BOP. Bob H.
A very reputable tranny shop in town advised me to keep the TH350 when swapping to a BBB. They said if youre going to build a performance trans to handle the torque anyway, keeping the TH350 would save the driveshaft and reduce hp loss compared to a TH400 and be roughly the same cost. Thoughts?
There has been a lot of discussion on the two transmissions. There is no doubt the 400 is the stronger of the two, maybe to the point of overkill in some cases. One thing for sure if a 200-R4 can be built to survive behind a BBB a 350 can also. To me the more desirable feature in the 350 is the lower 1st and 2nd gear ratio and its lighter. Bob H.
There is no doubt at all that a THM350 can be built to take the abuse a 455 dishes out. Greg Gessler uses a THM350C in his F.A.S.T. 72 GS which has run high tens at 127 MPH. Back then, everything was over engineered to take much more power than the engine put out. There was a bigger safety margin. When the oil crisis hit, and gasoline started going up in price, they started designing everything light weight for fuel efficiency. The transmissions were designed with much smaller safety margins. I have the 4T65HD trans in my Riviera. As long as you drive the car normally, and change the fluid on a regular basis, they last. Start modifying things and drive the car hard, and the transmissions usually go. I have 137,000 miles on my original transmission. I step on it all the time, but I don't abuse it. It still shifts mint. The 2004R transmission takes a lot of upgrades if you want it to last behind a stout 455. That goes for just about any of the newer transmissions out there. They can last, but it will cost you. The 350 is a great transmission. It takes a lot less to beef them up. The 400 in stock form can take most any abuse you care to dish out on the street. Burnouts don't really stress the transmission a lot, as long as you have adequate cooling. It's when you hook up big power in a heavy car with perfect traction, like in a track situation. That is when you need the upgrades to make it last. I don't consider the weight difference or gearing to really be an issue when considering the the THM 350 or 400. There isn't that much difference IMHO. The weight difference is only 15 lbs, and the ratio difference is only .04.
Seems to me like you're screwed no matter what with the bolt pattern, you're going to have to get a different trans either way, so there is no "I already have it" advantage to the 350.
I had an interesting experience with a T350 behind a 455 in my 72 SunCoupe with 2.56 gears. Runs high 12s. I was doing burnouts with drag radials or slicks at the track and took out the sprag 3 times. Trans guy asked what I was doing to it. Turns out they don't like doing burnouts in 2nd gear. He said either leave it in 1st or drive not 2nd during the burnout and coming out of the water. It didn't really like drive with the 2.56 gears so I use 1st at 4500-5000 and never had another sprag go.
good thread i have had the same wuestion for a little while now i think i will use the 350 on my new 455 untill it goes and then do the mods for the 400
