# Automatic Transmission Education!



## Shooter (Nov 7, 2015)

Hi all,
I'm trying to get myself educated on automatic transmissions for a GTO. Specifically, the Turbo400 and the Turbo 350 (and variants!). What the heck is the difference?? What about Pontiac turbo transmissions versus plain ol' GM turbo transmissions? How significant is the short shaft vs. the long shaft? What went in the GTO? What went into other Pontiacs?

I speak Muncie and clutch pedals. Have never messed with an automatic and want to get smart. Starting from square zero.

Not trying to rebuild one in this thread, just want to know what they are and what belongs in a Pontiac. What are the problems with using one vs. the other? What are the benefits of one versus the other? I can't find a thread on this and need someone with a lot of experience to do some philosophizing. 

Thanks!

Shooter


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## dd68gto (Nov 14, 2012)

dd68gto said:


> Shooter said:
> 
> 
> > Hi all,
> ...


Test


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## Shooter (Nov 7, 2015)

dd68gto,

Thanks! I saw this. But this only addresses Turbo400s? What about Turbo350s? Trying to narrow down what's what and then buy the correct book.


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## dd68gto (Nov 14, 2012)

Shooter said:


> dd68gto,
> 
> Thanks! I saw this. But this only addresses Turbo400s? What about Turbo350s? Trying to narrow down what's what and then buy the correct book.


https://www.google.com/amp/s/www.he...04/GM-TH-400-Transmissions/1796188.html?amp=1


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## dd68gto (Nov 14, 2012)

dd68gto said:


> Shooter said:
> 
> 
> > dd68gto,
> ...


It really depends on what you are planning to do. I see a lot of 350 on drag cars but also see the 2 speed use. I would say there was a reason most all swap to the 400 when it became available for the street . Tons of info out there just search and read


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## PontiacJim (Dec 29, 2012)

Shooter said:


> dd68gto,
> 
> Thanks! I saw this. But this only addresses Turbo400s? What about Turbo350s? Trying to narrow down what's what and then buy the correct book.


From the information I have, Pontiac used 4 different transmissions in the 1964-'74 Tempest/Lemans/GTO. They were:

ST-300 which was a Buick 2-speed used from 1964-69
Powerglide 2-speed (Pontiac code M35) from 1970-1973
Turbo-Hydramatic 400 3-speed (Pontiac code M-40) from 1967-'74
Turbo-Hydramatic 350 3-speed (Pontiac code M-38) from 1969-'74

The TH-400 was first introduced in the 1967 GTO and was used in all GTO's from that point on.

The Long tailshaft TH-400 was used in the full size cars. The GTO used the short tailshaft TH-400. You can use the longer tailshaft in a GTO with a few changes.

The TH-400 is stronger than the TH-350 in stock form, but heavier, uses more HP to turn it, and has a different gear ratio than the TH-350. You can build a TH-350 to handle the power of a Pontiac engine with the advantage of less HP to turn it and it is lighter.

The TH-400 used in the standard and higher output GTO engines were modified to shift harder & faster. They could be manually held in gear when shifting or placed in Drive to shift automatically. It has an electric kickdown switch that works off the gas pedal.

If you take the time to read through this thread, you may get a better idea about the TH-400. https://www.gtoforum.com/f122/th350-th400-117065/


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## bigD (Jul 21, 2016)

"...The TH-400 was first introduced in the 1967 GTO and was used in all GTO's from that point on..."


Wasn't the TH350 used in the 1974 GTO ? 

https://www.hotrod.com/articles/hppp-0602-1974-pontiac-gto/

I realize that some don't consider the the X-body version a real GTO. But, it had exclusive features which set it apart from all other cars.

Any of the trans mentioned here can be used in a Pontiac. You can even use a Chevy PG, TH350, or TH400, by using an adapter plate. 

2 of my last 3 bracket cars show the use of non-original type trans. Bought a '74 Ventura drag car which had a long tail TH400 in it. I'm about to put a short tail TH400 into an '80 Bird. Both cars came with TH350. 

As mentioned, any of these trans can be built for HD street, street/strip, or strip only service. Although many badmouth the ST300, my trans guy said he can build one that will work in my 455 bracket car. The main advantage of a 2-speed trans is less chance of traction loss, in low gear. Another advantage is that strip times can be more consistent because of having only 1 shift. But, most prefer a 3 or 4 speed for street-only driving.


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## Shooter (Nov 7, 2015)

To all - thanks for the info. I did order the book above. Still a lot for me to figure out but that's the fun of this!

Shooter


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## catmandoo62 (Dec 24, 2018)

and if you want another choice there are the 700r4 and 200r4,these can be built to take good h.p. and give you an overdrive and lower first gear.and the 200r4 can be found in big olds from the 80's with the dual trans pattern to fit both chevy and bop engines.


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## Shooter (Nov 7, 2015)

catmandoo62,
Don't those Xmissions require a significant mod to the tunnel on a classic GTO?

Shooter


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## Shooter (Nov 7, 2015)

From the web:

"The turbo 350 and turbo 400 are GM transmissions that were used from 1969 through 1979 for the TH-350 and 1964 through 1990 for the TH-400. The TH-350 was a low-torque transmission that was used in heavy cars. With modification, the TH-350 could be used as a high-torque transmission in a light car. The TH-400 was a high-torque transmission that was used in heavy cars --- it was a much stronger transmission than the TH-350.

Locate the vacuum modulator. On the TH-350, it is located on the right front of the transmission. On the TH-400, it was located on the right rear of the transmission. Also, the TH-350 had a kick down cable, while the TH-400 did not. Both were designed to be used in Chevrolet V-8 cars and light trucks.

Look at the transmission pans on both transmission. Both transmission pans have 13 bolts, but they have a different shape. The TH-350 uses a square pan, with one corner cut off. The TH-400 uses an oblong-shaped pan with one side having a "hump" in the pan and one side is curved while the other two sides are straight. The TH-400 pan also has two circular indentations on the humped side of the pan.

Measure the transmission. The overall length of the TH-400 is 38 inches, while the overall length of the TH-350 is 33 5/8 inches, if both use the longest tail shafts available. The TH-400's body is 25 inches and the TH-350's body is 21 5/8 inches long. The tail shafts for the TH-350 are available in 6-, 9- or 12-inch lengths and the tail shafts for the TH-400 are available in 4-,9- or 13-inch lengths."

(All credit and footnotes to https://itstillruns.com/tell-turbo-350-turbo-400-8714820.html as the source)

This article talks specifically about Chevrolet vehicles. Obviously, there was some application across all GM vehicles. It also mentions 3 different sizes for the tail shafts. I've only heard about the "short" and "long" shafts. Is there a 3rd option? On the turbo 400, which one is considered the short shaft, the 4 inch, or the 9 inch?

BTW, received my "GM Turbo 400 Transmissions" book by Ruggles. Looks great but the book only addresses the turbo 400. Doesn't talk about turbo transmissions in general and certainly not the turbo 350. As expected.....

Shooter


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## bigD (Jul 21, 2016)

"...Is there a 3rd option?..."

Yes


The 4" is "short".

Cliff Ruggles also wrote a TH350 book.

https://www.amazon.com/GM-Turbo-350-Transmissions-Rebuild/dp/1613251890


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## 1968gto421 (Mar 28, 2014)

Cliff Ruggles posts primarily on the PY forum ( _ Pontiac - Street - PY Online Forums _ ) and is noted in the Pontiac community for his expertise with the Quadrajet carburetors and his rebuild and parts business as well as the TH trannys. He also has built TH400 and TH350 trans and has written some really good books on the two trans. True, they say how to rebuild and modify but Cliff talks alot about their good/bad points (like Jim did) as well as how they operate. I got his books and found them to be a wealth of info. I would suggest them strongly as reference books.

https://www.amazon.com/s?k=cliff+ruggles&i=stripbooks&ref=nb_sb_noss_2

Do scroll down as he also has done GM overdrive trans and has a book on them.

Best of luck with your tranny investigation.

(Can also get the books from Cliff directly. Here's the link, Just scroll down the page to the lower left.)

https://cliffshighperformance.com/Q...uadrajet-parts/category/quadrajet-bushing-kit


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## 1968gto421 (Mar 28, 2014)

bigD, you treed me!


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## Lemans guy (Oct 14, 2014)

Had an ST 300 in my 66 Lemans took that out and put in a TH350......it has a cable for the kick down, no electric. Had it beefed up with a Rachel Diode by Performance Torque Converter and dropped a Gearvendors overdriveunderdrive behind it. 

That gearvendors is one awesome thing and the trans will take a thousand ft lbs of torque with that diode according to PTC. It will never near that from my 460. But you want more than you need so you won’t break it.

Lot’s of info on the gearvendors site, they are expensive but man they shift smooth and fast changes the TH350 to six gears instead of 3...

Oh I have a dual gate shifter, automatic or manual,...just like a hurst dual gate....I had own racing set the trans up for that when I ordered it.

The gearvendors keeps your RPM’s low at highway speed but keeps your shifts up high in the band when accelerating.....

I like the TH350 as it was a bolt in with no drive shaft modifications. But when I went to the Gearvendors I used a GForce cross member they make specially for the gearvendors and had a new driveshaft made....

Lots of choice!:nerd::nerd::nerd:


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