TH-400 question

[smokeyco75]

In my quest for a transmission I came across a fellow selling a couple of TH-400 transmissions that came out of RVs. Problem is they have that big drum thingy on the back of them. My question is can these be removed? Is the tailshaft and housing different? He only wanted $250 apiece but they are useless to me unless I can change that mess on the back of them. Thanks for the input folks!

 

[Wyo George]

Without seeing it, my guess is that those are parking brakes and it's been 20 years since I dealt with one of those so I can't remember if it was a unique tail housing or if it was a simple bolt on.

Also, being in a medium / heavy chassis, it could be a TH475 and you probably don't want one of them. They are basically the same as a TH400, except with noisy straight cut gears and the only way I know of to tell the difference is to tear into it and look at the gears. Just something to check on.

 

[fletcherson]

In my quest for a transmission I came across a fellow selling a couple of TH-400 transmissions that came out of RVs. Problem is they have that big drum thingy on the back of them. My question is can these be removed? Is the tailshaft and housing different? He only wanted $250 apiece but they are useless to me unless I can change that mess on the back of them. Thanks for the input folks!

You need to look at the tail housing close to see if it was specific for the parking brake. If it was, the tail shaft can be changed, but yo need to make sure of the length and shaft size or you will have to change the main shaft, not worth the work. There should be a tag on the passenger side of the transmission with the model number on it. My experience with motor homes, etc is that the parts interchangeability is not favorable for much else.

 

[T-Test]

Useless unless you rebuild and replace out put shaft as well as the rear extension housing. Go with the TH350--cheaper and will be easier to rebuild/replace.

 

[fletcherson]

Useless unless you rebuild and replace out put shaft as well as the rear extension housing. Go with the TH350--cheaper and will be easier to rebuild/replace.

I agree. The th350 has more favorable gear ratios also.

 

[smokeyco75]

Cool thanks guys! Now if I can just find one around here lol.

 

[Mike]

I agree. The th350 has more favorable gear ratios also.

It is lighter and has less rotating mass, as well.

 

[Swampdog]

What is the difference in the horse power (torque) required by the 350 vs 400?

Is there really much difference in the 1st and 2nd gear ratios?

 

[Wyo George]

Not much difference in gear ratios.

TH350:
1st - 2.52:1
2nd - 1.52:1
3rd - 1:1

TH400
1st - 2.48:1
2nd - 1.48:1
3rd - 1:1

 

[Mike]

@Swampdog , the difference in rotating mass has the T-400 coming up at almost exactly 10 lbs. more weight (which is also the approximate difference in total, overall weights between the two). If we were to assume the T-400 had the same kinds of frictional losses and pumping losses as the T-350 (the T-400 pump does have more pumping loss), then you are still fighting that 10 lbs. Whilst it doesn't require much more power to maintain the rotation of the extra 10 lbs., you cannot escape the physical requirement to get that extra 10 lbs. turning. There's where the bite comes in, the need to accelerate the additional mass.

If we were talking about a 540" big block in a Monte Carlo, then the strength of the T-400 would be a factor. But when we're talking a mouse motor in a T-Bucket, the T-350 is more than strong enough to do the job.

 

[fletcherson]

@Swampdog , the difference in rotating mass has the T-400 coming up at almost exactly 10 lbs. more weight (which is also the approximate difference in total, overall weights between the two). If we were to assume the T-400 had the same kinds of frictional losses and pumping losses as the T-350 (the T-400 pump does have more pumping loss), then you are still fighting that 10 lbs. Whilst it doesn't require much more power to maintain the rotation of the extra 10 lbs., you cannot escape the physical requirement to get that extra 10 lbs. turning. There's where the bite comes in, the need to accelerate the additional mass.

If we were talking about a 540" big block in a Monte Carlo, then the strength of the T-400 would be a factor. But when we're talking a mouse motor in a T-Bucket, the T-350 is more than strong enough to do the job.

I can attest that there is a noticeable positive performance difference going from a 400 to a 350 behind a sbc. With a high hp motor the difference would likely be negligible and the added strength of the 400 may be realized. The kick down is different as well. 350 has a detent cable, 400 has a solenoid.

 

[PotvinGuy]

Ah yes, the kickdown business. When I built my T, I wanted the electrical kickdown of the 400 so I could tie it into the EFI. Once I had the car done, I realized that the torque/weight was so high I never needed kickdown, so haven't hooked it up! But the 400 has taken 20 years of abuse and I'm glad I got it.

 

[smokeyco75]

I think I agree that I would be better off with a t350. After all that is what the car is set up for. My stock 454 shouldn't hurt it in something as light as a T.

 

[Indycars]

Everyone has different amount of lead in their right foot and different amount of traction to abuse the trans with. I have the perfect trans for me ....... the 200-4R and 450 hp and MT's

 

[fletcherson]

I think I agree that I would be better off with a t350. After all that is what the car is set up for. My stock 454 shouldn't hurt it in something as light as a T.

Aa long as it is a strong unit and you select a favorable gear, you will be good to go. Just do your homework and figure out the right converter, rear end gear combo for your cam, tire size, and desired rpm range and provide a cooler. There are calculators available to make tha chore easy on various aftermarket sites and on desk top Dyno, etc...

 

[Bucketman]

My book says a th350 can be as dependable as a th400 with only a couple of upgrades. Number one is to install a shift kit. Makes the transmission run cooler. Second replace the intermediate over run clutch race with a heat treated and stress treated aftermarket race. The kit was $44 and the aftermarket race was $35. My rebuild kit was $255. I had very few hard parts to replace, just a couple of springs. I paid $150 for the used transmission.
Last but not least, if I can rebuild it anybody can. So I would go with the lighter th350.

 

[smokeyco75]

Well here is what I have so far; a 1996 S10 rear with open diff and 3.42 gears and a bone stock 454 from a 1978 chevy one ton dually. Tires will probably be 295/50 R15 or similar. The only planned engine upgrades are an aluminum intake and a Holly 650 carb. Might add gear drive timing. If I decide to upgrade the cam later it will be mild. I want it to be fun to drive, not trying to hold back the reins on an engine that would feel more at home in a drag car lol. Sure the thumpety thump cams sound cool but they usually suck on the street. I figure this setup shouldn't be too hard on a TH350 with a mild shift improvement. Just not sure on stall. Figure I can start at 2400 and if that doesn't feel right I'll go up or down from there.

 

[Wyo George]

With a stock big block in a lightweight car you won't need a high stall converter. I'd personally go with a stock or very mile converter and upgrade when other mods made it necessary. You're going to have a high enough torque:weight ratio that even with a stock converter it's gonna launch like you were rear-ended by a freight train.

 

[fletcherson]

Well here is what I have so far; a 1996 S10 rear with open diff and 3.42 gears and a bone stock 454 from a 1978 chevy one ton dually. Tires will probably be 295/50 R15 or similar. The only planned engine upgrades are an aluminum intake and a Holly 650 carb. Might add gear drive timing. If I decide to upgrade the cam later it will be mild. I want it to be fun to drive, not trying to hold back the reins on an engine that would feel more at home in a drag car lol. Sure the thumpety thump cams sound cool but they usually suck on the street. I figure this setup shouldn't be too hard on a TH350 with a mild shift improvement. Just not sure on stall. Figure I can start at 2400 and if that doesn't feel right I'll go up or down from there.

The converter is dictated by the cams power band and rear gear for launch. Sounds like you want a basically stock, mild motor, stay with a stock converter. The big equation is the relation between rear tire diameter and rear gear. Too much tire and too little gear will destroy clutches, wet or dry.

 

[Bucketman]

Don't forget the weight of the vehicle on figuring which stall you want. A stock convertor on a 1800 lb. car will make it hard to sit at a stop light without making your leg tired from pushing on the brake. I'm also building a T-bucket, just using a SBC 350. I'm going with a 2700 to 3000 stall speed and it'll probably hook up at 2000. Just going by what I've read, and heard, from fellow gearheads. (probably won't be running a stock cam though)