CLcutlass

I am going to swap out a 350 rocket with a 455 and am wondering is it better to stay with the TH350 or go with the 400. I aksed around and got mixed reviews

GAOldsman

The 350 can hold up if it isn't too much horse power. Say less than 400 HP for a standard TH350, and then you wouldn't have to modify your driveshaft. But a 400 is better in my opinion.

citcapp

You can build a 350 to live behind a 455 and it is a lighter transmission so you will save weight but it does cost a lot. A stock 400 will hold up to a 455 with no modifications depending on how much hp and torque you build into the 455. If is stock or medium build no problem.

Other tranny experts will chime in with other opnions

TripDeuces

Buddy of mine must have gone through 3+ TH350 behind his 455. I have to admit he's a bit hard on them but regardless I'd go with the TH400. I've beaten the snot out of mine behind a 455 and now it's behind a 330 and still going strong.

oldsy

out of the factory they all came with a th400.i think

z11375ss

X2 on the 400. I went through at least 3 in my Chevelle when I was a kid. Very hard on them. I switched to a 400 and NEVER had a problem. That thing was just STRONG! You can really feel the fullness of the shifts. I know that sounds corny but its true.

dsolomon

A TH400 will reduce horsepower to the rear wheels. If your 455 is a mild build I would still consider the 350.

matt69olds

A properly built TH350 will live behind a stout 455 no problem. Machine the direct drum piston so you can install 5 clutches, dual feed the direct drum (either with a shift kit or removing the center seal), install a hardened intermediate sprag race, and pay attention to the clearances and you will be fine. I have built several TH350 that run in the high 10 with the above modifications. A TH400 is obviously a stronger unit, its also much heavier.

z11375ss

How much heavier? I found this on a Chevy site.

I measured a TH350, TH400, and 4L80E all on the same scale on the same day.

Weights were

TH350 short tail- 125 lbs.
TH400- 134 lbs.
4L80E- 178 lbs.

My opinion is that going with a turbo 350 is like trying to drive a golf ball with a 3 iron. Yeah, you can do it, but you have to hit the snot out of it and it won't go quite as far. A turbo 400 from a junk yard would probably live in front of most motors.

CLcutlass

Thanks guys...good info.

Whats involved with switching from a kickdown cable to electronic

citcapp

Not hard at all. B & M makes a kit that I used in my 57

joe_padavano

Of course, on a Cutlass you can just bolt in factory parts.

455man

I had a hard time finding factory parts at salvage yards for my kickdown switch so I just made my own bracket on the pedal, bought the factory switch and wired it myself. Works great.

I killed a well built TH400 behind a fairly stock 455.
A TH350 would be best for weight/power but I would not expect it to last.

joe_padavano

FYI, look at the accelerator pedal on early 80s G-body cars. Many of these used a very similar switch to disable the A/C compressor at W.O.T. These should be more readily available at wrecking yards.

455man

Good to know. Thanks.

380 Racer

I use a th350 in my car. I obviously abuse it. Installed it to reduce weight and increase HP over the 400s I had been using. It wasn't cheap but it has done very well. Here is a tech article for ya:

http://www.dragracingonline.com/tech..._7-olds-1.html

Chesrown 67 OAI

If its a stock motor and is to be street driven only, and you are going to purchase a trans, then you should seriously consider going with an automatic overdrive unit. The 200 - 4R is great for fuel economy on the road!! It is still even OK on a mild Street/Strip application!

CLcutlass

Hi Guys,

MY TH400 is out of a 68 delta 88...it looks like it is a long shaft. Any issues using this on my supreme. I have it in now and had to drill a couple of holes for the cross member. I am just a little concerned when i lay the body back on.

I know I have to shorthen the driveshaft

ELY442

Long time ago, Joe Mondello told me to get that TH350 out of there and get a TH400 on my 455. One thing i like about the TH350 is the low gear. It can really launch out of the hole.

Nasty455

I like the 400,as I like to show off.
I down shift without always checking RPM.
The 400 will take more abuse.

MDchanic

+1.

The 400 will take an astonishing amount of abuse.

- Eric

ah64pilot

I have a stock rebuild 400 that I beat the snot out of behind a 13.001 second street tire car in high school. After sitting in the garage floor for 14 years it is now behind my 500 HP / 550 TQ engine and it is still going strong. I'll trade the power loss for the ruggedness all day long.

I'd say a $350 rebuild on a stock TH400 that last 20 years behind most anything is well worth it

ah64pilot

You won't have any issues but the short shaft models are a little stouter...If you don't have excessive horsepower I doubt if your engine even needs it. Go with what you have...

CLcutlass

Well...just realized that there is no speedo connection on teh TH400 out of the delat 88.

What is next ....can I add a gear adaptor ?

ah64pilot

If you have a block off plate blocking the hole that the speedometer gear housing would go into then you'll be able to find all the parts you need here:

http://www.transmissioncenter.net/sp...____va.htm#400

CLcutlass

I should get a pic but I see a plate but it is not in the very tail piece...it is right before the tail piece. Should there be a block off plate ?

ah64pilot

The speedometer gear housing is in the main housing on the driver's side, just in front of the tail housing assembly You're in the right place judging by what you described.

CLcutlass

Ok ...had to go get a pic. Is this the right spot.

ah64pilot

That's the right spot, does it have a freeze plug in it because it doesn't look like a bolt on cover.

If you look at my build page (see link in above signature) you'll see where I changed mine out. Its on page 4 I believe.

CLcutlass

its not a bolt on so I guess its a freeze plug...

I am not sure what i need to order...my TH350 had the orange gear with a 2.56 rear end. How do I know what to order for the Th400.

I am starting to think I should clean up my TH350 and put in back in.

joe_padavano

Be careful. Some of these transmissions had a pressed-in plug, but some never had the hole machined in the first place. Also, note that the hole for the retaining bolt isn't tapped, either.

CLcutlass

Thanks...that could be the case here. It did not look like a plug to me but I do have it painted. If it is not the 350 is going back in.

CLcutlass

After a closer look today...it looks like an untouched casting so it is not a plug. Thanks for the heads up...not sure if I can get rid of this thing.

Olds462

My TH 350 didn't hold up to my 455 that puts out 462 horse and 535 ft # of tq. I am deffinetly going with a 400.

droptopron

Others will opine but I thought the 1st gear was about the same.

TripDeuces

They are similar Th350: 1st - 2.52, 2nd - 1.52
TH400 1st - 2.48, 2nd - 1.48

I actually have a low gear set in my TH400 which I believe uses the TH350 ratios. It's noticeable but I wouldn't call it Earth shattering. I will say that 1st gear seems to fly by but that could be a function of the Switch Pitch also, dunno. Oh yeah, my 4.10s don't help the equation, lol

CLcutlass

I am going to stick with the TH350 for now. Can I keep the converter that was origionally on teh 455/TH400 combo or do I have to put the converter from the 350 on the 455.

oldsy

CLcutlass

sorry.... the torque converter

hotrodpc

Did you get this problem solved CL? You can use the Th400 Converter off of the 455 on your Th350 SO LONG AS, it wasn't a variable pitch Th400 unit. That Speedo isn't machined out because that trans was from one of the older boat cars that ran the speedo off the front, I believe, left wheel. Can you get rid of that Th400? Sure can. If you were close to me, I'd be asking what you wanted for it just for the hard parts in it, and even moreso if it were VP. So you didn't just toss it away or scrap it I hope. And its still a good case if you found someone running a Chevy or GMC 4x4 truck with BPO motor like I am inclined to do. All I'd have to do is use a Chevy output shaft to mate that trans to my Transfer case, and the speedo drive gear goes into the Transfer case on a a 4x4, not the transmission. So yes, even the case, it's useable to someone.