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Doing some archaeology on the 67 GTO...

The car was originally equipped with a TH400 M40 Heavy Duty, code PX-67.

I’ve found it to have a TH400 PB-65 installed. Looks like this trans was specd for a V8 2bbl. Car has a YS 400 4bbl (335hp) in it.

Any trans guys (transmission) shed light on the differences? Can the PB be upgraded to Heavy Duty?

Trans works fine, just doing underbody refresh, and digging around. Planning on engine rebuild this Winter, debated swapping to manual...but prolly will stay with the Auto.

Nothing radical planned for motor. Likely push into 400HP range. Rear end is a 3.55 open.
 

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There are a couple one-off TH400's like the th375 and th475, but for the most part, they're pretty much the same. I'm sure some of the folks here with deep knowledge of Pontiac options can tell you the specific difference. From a mechanic's point of view, any TH400 (save for the TH375 which can be identified by it using the TH350 yoke) is a plenty stout transmission.
They can be built to handle serious HP. I doubt 400HP is going to stress most any TH400. Yours probably wouldn't bat an eye as is. If you end up freshening it up, any qualified trans shop can easily set one up for almost anything :)
 

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The GTO TH400 is different than the standard non-GTO/High Performance TH-400, thus the different trans codes. The GTO TH400 has a higher line pressure that gives faster and much firmer shifts - not quite what Grandma wants in a transmission. It can also be manually held in gear and won't shift until you select another gear. I am not sure if the governor which controls the upshifts in Drive is different and raises the upshift RPM's. You can buy a kit that will accomplish this so you can tailor the RPM's you want the trans to shift when you put it to the floor.

So you can add an aftermarket shift kit that will give you the faster/firmer shifts and provide manual control of shifting through the gears, but still shift automatically in Drive. Trans-Go makes a good kit, but there are others. You can also install a valve body having all the mods included, but the shift kit is cheaper.

I had a '68 GTO with the TH400 & His/Hers shifter. With the non-posi rear end, and in the days of bias tires, I could wind out first and shift into second and catch a little rubber due to the fast/hard shifts the GTO TH400 has.
 

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The GTO TH400 is different than the standard non-GTO/High Performance TH-400, thus the different trans codes. The GTO TH400 has a higher line pressure that gives faster and much firmer shifts - not quite what Grandma wants in a transmission. It can also be manually held in gear and won't shift until you select another gear. I am not sure if the governor which controls the upshifts in Drive is different and raises the upshift RPM's. You can buy a kit that will accomplish this so you can tailor the RPM's you want the trans to shift when you put it to the floor.

So you can add an aftermarket shift kit that will give you the faster/firmer shifts and provide manual control of shifting through the gears, but still shift automatically in Drive. Trans-Go makes a good kit, but there are others. You can also install a valve body having all the mods included, but the shift kit is cheaper.

I had a '68 GTO with the TH400 & His/Hers shifter. With the non-posi rear end, and in the days of bias tires, I could wind out first and shift into second and catch a little rubber due to the fast/hard shifts the GTO TH400 has.
Yep, the good old days! Then going to the tire shop and buying one replacement tire. No thanks on the set, the other 3 have lots of tread left:) That was a blast!
 
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