Drawbacks of keeping the TH400
Thread Starter
Instructor
Joined: Jul 2010
Posts: 108
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From: woodinville washington
Drawbacks of keeping the TH400
So I am still 6-9 months from making an decision but I want to gather as much info before any decision is made on the TH400's future. The car is a 75 coupe, and I am going through every system. Many here go to a 5 speed setup, or swap in a auto w/overdrive.
If I was to keep the TH400 in the car, am I loosing anything in performance, as compared to the other two options? If I do keep the existing transmission, it would be completely rebuilt with heavy duty parts, and a shift kit installed.
Aside from mileage and RPM's does the TH400 offer any other dis-advantages...Does it offer any advantages? The car will not see the strip, but I want something completely reliable. I would like to be able to drive this thing someday on a long trip without worries.
Thanks for your help.
Jeff
If I was to keep the TH400 in the car, am I loosing anything in performance, as compared to the other two options? If I do keep the existing transmission, it would be completely rebuilt with heavy duty parts, and a shift kit installed.
Aside from mileage and RPM's does the TH400 offer any other dis-advantages...Does it offer any advantages? The car will not see the strip, but I want something completely reliable. I would like to be able to drive this thing someday on a long trip without worries.
Thanks for your help.
Jeff
Last edited by JB46; Jul 20, 2010 at 02:12 AM.
Le Mans Master
Joined: Aug 2003
Posts: 7,720
Likes: 12
From: Chattanooga TN
TH400 is reliable and bulletproof, especially behind a stock engine. Parts are available.
On the downside it is heavy and a HP robber.
To answer your question, there is nothing wrong with a TH400 if you want simple and reliable. Long trips, no problem. If you want to go to the strip (you said you didn't)-no problem.
On the downside it is heavy and a HP robber.
To answer your question, there is nothing wrong with a TH400 if you want simple and reliable. Long trips, no problem. If you want to go to the strip (you said you didn't)-no problem.
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
My TH400 is a solid reliable tranny, but, I would love a 4th gear for cruising. Most of my driving consists of long drives and mountain roads. Weekend trips are usually three to four hundred miles.
Melting Slicks
Joined: May 2002
Posts: 2,431
Likes: 52
From: Florida
I believe it's attributed to the mass of the internal rotating components. If you don't have to have over-drive, it's the best/toughest 3 speed unit GM ever made. Needs no upgraded parts, just quality clutches & bands. If you need over-drive, a 4l80e which is much like a TH400 w/over-drive and electronically controlled is bullet proof but overkill. Most choose a 2004r as a lighter alternative that's an easier install. A 700r4 is another option but I don't like the large step between 1st & 2nd gear and its cable operated shift. A 4l60e is the electronic version of the 700r4 and does away with the cable but requires a computer like the 4l80e. If you're not a hot rodder a TH350C is a very good unit, 3 speeds with a locking converter. Better mileage than a TH400 but not as tough.
Thread Starter
Instructor
Joined: Jul 2010
Posts: 108
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From: woodinville washington
I realize that the majority of C3 owners probably just keep their existing transmissions. Would adding a modern 5 speed, or auto w/overdrive transmission take away from the value? I would imagine it would slightly increase the value, or make it more attractive during resale. Correct me if i'm wrong though.
I need some schooling on converters though, the TH400 has a non locking converter, what am I loosing in performance with this. What is the diifference between the locking/non locking converter? Sorry if I come across as ignorant here.
I need some schooling on converters though, the TH400 has a non locking converter, what am I loosing in performance with this. What is the diifference between the locking/non locking converter? Sorry if I come across as ignorant here.
Melting Slicks
Joined: May 2002
Posts: 2,431
Likes: 52
From: Florida
I realize that the majority of C3 owners probably just keep their existing transmissions. Would adding a modern 5 speed, or auto w/overdrive transmission take away from the value? I would imagine it would slightly increase the value, or make it more attractive during resale. Correct me if i'm wrong though.
I need some schooling on converters though, the TH400 has a non locking converter, what am I loosing in performance with this. What is the diifference between the locking/non locking converter? Sorry if I come across as ignorant here.
I need some schooling on converters though, the TH400 has a non locking converter, what am I loosing in performance with this. What is the diifference between the locking/non locking converter? Sorry if I come across as ignorant here.
A locking converter refers to a clutch that is applied internal to the converter though a fluid passage on the input shaft. It applies a clutch that mimics that of a standard trans. in that all slippage is removed from the torque converter reducing heat and increasing MPG. This clutch is about a 1/2 to 3/4 inch wide and roughly 4/5th's the diameter of the converter itself located inside converter on the side facing the flex plate and is not intended for towing or torque. It's usually activated by a vacuum switch that electrically engages a solenoid that directs fluid to apply the clutch and de-actived by another switch near the brake light switch or before mentioned vacuum switch.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
If you drive it on the highways for any significant amount of time, installing an overdrive automatic with internal lockup is probably a good move for you. If you do, just bag up the THM-400 (drain the oil and sit it on a skid...nose-down...to save space and store it). If a future buyer wants it to be numbers matching, give him the tranny, too.
Most folks buying a '75-'82 aren't worrying about the 'matching' issue; they are looking for an inexpensive cruiser for car shows and the road. Most buyers of those [notice I said most....not all] would welcome the fact that it had an overdrive/lockup tranny in it. I think that you would recoup about the same money for your car either way.
Most folks buying a '75-'82 aren't worrying about the 'matching' issue; they are looking for an inexpensive cruiser for car shows and the road. Most buyers of those [notice I said most....not all] would welcome the fact that it had an overdrive/lockup tranny in it. I think that you would recoup about the same money for your car either way.
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Thread Starter
Instructor
Joined: Jul 2010
Posts: 108
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From: woodinville washington
Thanks for all the info guys. My 75 is a car I plan on keeping for awhile. As I mentioned I am just going through everything on the car. It will never be a daily driver, but I want it to be totally reliable. I am leaning towards replacing the TH400. What started out as a brake overhaul is turning into much much more.
Team Owner
Joined: Jun 2000
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From: Southbound
Cruise-In II Veteran
Pro
Joined: May 2010
Posts: 737
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From: Pennington NJ
If you want to change to another automatic trans go 700r4 all the way. My aunts '91 pontiac firebird has a 700r4 and it gets magical fuel efficiency with the 5.0 tbi and it is extremely reliable no problems ever.
Will
Will
