Transmission update - Pontiac GTO Forum
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post #1 of 7 (permalink) Old 11-13-2017, 04:10 PM Thread Starter
 
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Transmission update

Good morning, (well its morning for me hear on top the world in Deadhorse AK)

I have a 68 GTO that needs a complete restoration, the motor and transmission are numbers matching. Its a YS motor (400 4bbl) the Quadra Jet is off a 79 Chevy motor, the rear end has been changed out from a 10 bolt to a 12 bolt, and the transmission is a TH400. I tell you all of this to say this (and I know most of you will cringe). I will be going back 90 percent original from the frame up. However, the engine will be a 461 stroker ( I'm leaning toward Performance Unlimited in FL to build the engine). I will also be installing a Curry 9" rear end. This is where your collective knowledge comes in. I'm looking to put a newer 4 speed automatic tranny back in it. Fist...what transmissions would fit without to much massaging and second, does anyone make a 4 speed dual gate shifter? I'm enclosing some pictures. I know I will get a lot of flack about not restoring my Goat back to original and I really don't mind. I'm building the car with modern suspension and modern drive train. After all, how many hot rodders back in the 60s left their car original.
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post #2 of 7 (permalink) Old 11-13-2017, 04:19 PM
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I was considering getting a monster engine for my goat but after talking with the more experienced members on the group I decided against it for one reason, the frame isn't designed for that much hp.
I would stick to something in the 400 HP range.
They already have a tendency to sag in the engine cradle area.
The weak link in your build will be the frame.
Just something to think about before you go whole hog on the drivetrain.
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post #3 of 7 (permalink) Old 11-13-2017, 04:20 PM Thread Starter
 
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post #4 of 7 (permalink) Old 11-13-2017, 04:29 PM Thread Starter
 
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Thanks for your input. The engine would actually weight less due to aluminum heads and intake. However I'm expecting to be running around 500hp and a little over 600 ft lbs of tq. I haven't really looked at the frame but was wondering if it would be a candidate for boxing it? These are the kind of inputs I was looking for. Thanks again.
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post #5 of 7 (permalink) Old 11-13-2017, 04:40 PM
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Originally Posted by Akhuntr View Post
Thanks for your input. The engine would actually weight less due to aluminum heads and intake. However I'm expecting to be running around 500hp and a little over 600 ft lbs of tq. I haven't really looked at the frame but was wondering if it would be a candidate for boxing it? These are the kind of inputs I was looking for. Thanks again.
It isn't so much the weight as it is the torque and twist created by the extra ponies.
There are kits available to box the frame like the convertibles come with from the factory, I don't know if they would stiffen it enough for the extra ponies.
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post #6 of 7 (permalink) Old 11-13-2017, 06:34 PM
 
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Akhunter - nice to see you are still alive and have the GTO.

461CI is a good choice. I suggest you do box the frame to add some strength to handle the big increase in HP/Torque. The Currie 9" is a good choice, BUT, I would also put some thought into beefing up the rear suspension mounting points, especially the lower control arm mount. These mounting points were not intended for the HP/Torque your engine should be putting out. I went with a 9" housing/axles/brakes from QuickPerformance and then pieced mine together using a mix of manufacturers to get what I wanted.

Pics 1-3 show the factory lower control arm mount - not much of a bracket. I made my own templates and then brought them down to a fab shop to make them - it was just quicker for them to do it than me. I added a flat plate to the top & bottom, and a formed plate on the side where the bolt goes through. Then welded them on & painted.

I did the top as well, but I probably went overkill as I also added a roll formed angle on top of the factory crossmember to add strength to it. Picture #4 . Made the 2 cutouts to match the frame where the bolts go through on the upper control arm, and then welded an "ear" on the outsides that fit into the scallop so my upper control arm bolt goes through the "ear" and then through the factory mounting hole - effectively doubling the thickness of the mounting point where the bolt goes though. I also welded a tab on the other side opposite the "ear" to double the thickness of the opposing side of the control arm mount. I did not want to weld the angle to my crossmember, so I used a series of grade 8 bolts to attach it. The upper control arm bolts will additionally hold it into place.

TIP - to make sure I got the control arm bolt holes exactly correct, I found that a foam ear plug was the exact size as the bolt holes. I then put my fabbed pieces where they should be set, put some white spray paint on the end of the ear plug, and then put it through the factory bolt hole and pressed it onto the fabbed metal. This left a perfect circle on my piece and I then drilled it to bolt OD size. Then I put my pieces in place, ran a bolt though to align the holes, clamped up my parts, & welded - bolts fit nice and tight through the holes with no mismatching.

If you plate the bottom mount, the UMI control arm brace would probably be all you need to tie in the upper control arm mount with the beefed up lower control arm mount. Pic # 5 I have it set in place while I was trial fitting/welding everything. The UMI piece comes with all the needed bolts and it is adjustable so you can get it fitted correctly.

Transmission wise, I would suggest that you purchase the book "GM Automatic Overdrive Transmission" by Cliff Ruggles. This will help you make your choice and it shows what is involved in the installation.
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post #7 of 7 (permalink) Old 11-14-2017, 09:24 PM Thread Starter
 
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As luck would have it my brother spent 28 years in the Air Force as a welder and machinist and the cars at his house in Spokane Washington. As soon as I can get him to finish his 65 C10 my Goat gets transformation started.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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