TH400 question
Thread Starter
Racer
Joined: Oct 2011
Posts: 294
Likes: 0
From: Easton pennsylvania
TH400 question
I have a 1974 Convertible that I am learning more and more about as I start to pull the motor for a rebuild. From driving it, I knew it was a 3-speed auto.
For some reason, I thought it was the Hyrda-matic 350, but now that I have the pan down, I see that it is not... It is actually a TH400... What transmissions came in the 74's?
I dont know much about this transmission. Is it a good tranny? My goal with my engine rebuild is to get 315-350 HP out of the 350 with intake, carb, heads, cam, exhaust upgrades. It the TH400 up the job?
What are your suggestions on rebuilding it? My buddy is a mechanic and said we can do the basic maintenance to it ourselves. It drove fine when I parked it this fall. I am on a budget, so what are your suggestions. Pull it with the motor? Leave it in there and just drive it till something goes wrong?
One thing I did notice is that the pan has been repaired once before. There is an outward dent in the pan that someone filled in with brass.. All I see for replacement pans are chrome ones. Are they reliable? Does anyone make a stock depth aluminum one that will help with cooling?
Thanks for the info..
For some reason, I thought it was the Hyrda-matic 350, but now that I have the pan down, I see that it is not... It is actually a TH400... What transmissions came in the 74's?
I dont know much about this transmission. Is it a good tranny? My goal with my engine rebuild is to get 315-350 HP out of the 350 with intake, carb, heads, cam, exhaust upgrades. It the TH400 up the job?
What are your suggestions on rebuilding it? My buddy is a mechanic and said we can do the basic maintenance to it ourselves. It drove fine when I parked it this fall. I am on a budget, so what are your suggestions. Pull it with the motor? Leave it in there and just drive it till something goes wrong?
One thing I did notice is that the pan has been repaired once before. There is an outward dent in the pan that someone filled in with brass.. All I see for replacement pans are chrome ones. Are they reliable? Does anyone make a stock depth aluminum one that will help with cooling?
Thanks for the info..
Melting Slicks
Joined: Jul 2009
Posts: 3,264
Likes: 3
From: Vero Beach FL
I am not a expert by any means but believe all 74 and 75 had Th400 in them. The 400 is a pretty much been said to be "Bullet proof" very durable and can handle up to 400HP stock. The biggest drawback that I have heard is that they suck horse power to operate. If you are on a tight budget and it was working OK I would look at maybe just replacing the seals. I have read that some of the aftermarket aluminum pans require a spacer to clear the heaver lip in order for the shift cable bracket to fit correctly. 
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
It's a good transmission. Don't buy a chrome pan, buy a cast aluminum one with a drain. And, you will need a spacer for the shift cable bracket. I use a thin paper gasket for sealing the pan - get one from a trans shop, and do not use any sealant.
Be careful tightening th pan bolts! The tanny housing is aluminum so the threads can easily be stripped.
Be careful tightening th pan bolts! The tanny housing is aluminum so the threads can easily be stripped.
Last edited by JimT; Nov 24, 2011 at 10:19 AM.
Burning Brakes
Joined: Sep 2005
Posts: 895
Likes: 4
From: whitman massachusetts
Turbo 400
These are tough transmissions and work well in their stock configuration for cars in your projected horsepower range, without modification. I have put one behind my stroked small block and modified it slightly with a shift kit and higher stall torque converter, to take advantage of the increased power curve of my new engine. One other modification I made was to install a truck( these 400's came stock in GM pickups, cheap boneyard upgrade!) oil pan, which is deeper and allows for more fluid, which I felt made sense in a high performance application. I plan to add an external cooler, too. You may or may not realiize, also, that the stock rear end gearing from the factory with the 400's, was a 3:08. I upgraded to the performace option rear gearing, which is 3:36, and can be found everywhere for these cars, very cheap, to give you an economical boost in the seat!
Good Luck!
Willem
Good Luck!
Willem
Thread Starter
Racer
Joined: Oct 2011
Posts: 294
Likes: 0
From: Easton pennsylvania
Never had much luck with chrome accessories. They always seem to be made cheaply.
I was shocked at how loose my pan bolts were. They were not much more than hand tight.
So what are people using for spacers? I saw where some say that the factory bolts were to wide and they needed smaller headed bolts. Will washers work for the spacers?
I was shocked at how loose my pan bolts were. They were not much more than hand tight.
So what are people using for spacers? I saw where some say that the factory bolts were to wide and they needed smaller headed bolts. Will washers work for the spacers?
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
a T400 can handle almost any thing you could attach it too and way more than the posters projected horsepower......and a positive note, there the way to go if you need a quick Vette......
And on this subject of modifications, why or what is the subjective mentality of people that will buy a Vette and almost in the same breath have to change most of the parts.......never could figure that out.....
And on this subject of modifications, why or what is the subjective mentality of people that will buy a Vette and almost in the same breath have to change most of the parts.......never could figure that out.....
Instructor
Joined: Dec 2010
Posts: 149
Likes: 0
From: Southgate Michigan
TH400's tend to run a little 'warmer' temperature wise, so U may want to consider making sure your cooler part of the radiator is not plugged up & think about running a small external 'cooler' in front of the radiator.
If you really want to know how to tell if that car came with a TH400 or a TH350. Remember this, the TH350 comes with a manual 'kick-down' cable mounted to the carb linkage, whereas the TH400 has an electronic 'kick-down' switch mounted on the gas peddle linkage inside the interior, right near the firewall.
If someone were to put a TH400 into car that originally had a TH350, thay'd have to add this switch. Not too many will go through the trouble to do this. If the car was a TH400 & there is now a TH350 in it, chances are this switch, although disconnected, will most likely be still mounted in the car, where I said.
But, the '74 auto's came with TH400.
If you really want to know how to tell if that car came with a TH400 or a TH350. Remember this, the TH350 comes with a manual 'kick-down' cable mounted to the carb linkage, whereas the TH400 has an electronic 'kick-down' switch mounted on the gas peddle linkage inside the interior, right near the firewall.
If someone were to put a TH400 into car that originally had a TH350, thay'd have to add this switch. Not too many will go through the trouble to do this. If the car was a TH400 & there is now a TH350 in it, chances are this switch, although disconnected, will most likely be still mounted in the car, where I said.
But, the '74 auto's came with TH400.
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Thread Starter
Racer
Joined: Oct 2011
Posts: 294
Likes: 0
From: Easton pennsylvania
Great info guys.
As far as the gearing, is the ring and pinion all that I need? Also, what are you thoughts on the gear. Some say I won't see much of a difference with 3.36. They say go with a 3.55.".
As far as the gearing, is the ring and pinion all that I need? Also, what are you thoughts on the gear. Some say I won't see much of a difference with 3.36. They say go with a 3.55.".
Last edited by Xterrable; Nov 24, 2011 at 04:06 PM.
Race Director
Joined: Jan 2007
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From: Danville Illinois
2025 Corvette of the Year Finalist - Modified
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Have TH400 in my 75 with a shift kit, has lasted 37 years. 3.08 rear gear
B/M deep pan adds two more quarts fluid. Had a trans cooler on but didn't think it helped.
B/M deep pan adds two more quarts fluid. Had a trans cooler on but didn't think it helped.
Last edited by Mod75; Nov 24, 2011 at 07:19 PM.
Instructor
Joined: Dec 2010
Posts: 149
Likes: 0
From: Southgate Michigan
unless gas mileage is not a concern.What ya should do is toss in an OD trannie
Like a later 700R4 with the better front pump. Then you can 'bump' the gear ratio up to 3.73 & 'leap' off the line, but be able to 'cruise' a much lower RPM's, saving the engine wear & tear.
Even though the 2004R is just about a direct fit into the vette, that has a TH400, it doesn't have a lower 1st gear as the 700R4, but 2nd & 3rd, are 'closer' ratioed.
I put a 700R4 in my '72. There's more kits out there today, than a couple of years ago, to make it fit nicely, without too much 'hacking up'
Many places now make a real nice rear trans mount that fits nicely to the crossmember.
Either way, OD trannies are a good choice
Cruisin' Davey G !
Burning Brakes
Joined: Sep 2008
Posts: 805
Likes: 5
From: Naperville Illinois
I agree with the recommendation to get an aluminum pan; you will need to get two longer bolts for the shift cable bracket, and you will need to modify the bracket to get it to fit--I ground the bolt holes open so I could mount the bracket in the stock location using the aluminum pan. You'll see what I mean if you install an aluminum pan.
Thread Starter
Racer
Joined: Oct 2011
Posts: 294
Likes: 0
From: Easton pennsylvania
Thanks for the recommendations!
I wish I had a lift at home. This car is a P.I.T.A to work under. Even up on car ramps, you dont have very much room under there.
As far as mileage, everything is a concern. Budget, drive-ability, gas mileage. But the ability to put a grin on my passengers face (and mine to) is rated pretty high.!
I wish I had a lift at home. This car is a P.I.T.A to work under. Even up on car ramps, you dont have very much room under there.
As far as mileage, everything is a concern. Budget, drive-ability, gas mileage. But the ability to put a grin on my passengers face (and mine to) is rated pretty high.!
Instructor
Joined: Nov 2011
Posts: 183
Likes: 0
i like 700s and 200r4s in classic cars shift works sell gearshifter conversions to make it nice if u use the stock th 400 use a no walk case bushing a stock rebuild kit is all u need but get some thicker steel clutches to reduce clutch pack clearence new pupm gears are cheap so replace them and use a teflon coated pump bushing repace the wave clutches with flat steels i use a band m kit in all 400 so i can have a new seperator plate install all the check ***** drill the feed holes and remove the 2 3 accumulator spring and replace the kick down soleniod and a red stripe modulator that combo willl make a 400 shift fim but not agressive and last for a long time
