Automatic to Manual Transmission conversion. - Pontiac GTO Forum
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post #1 of 9 (permalink) Old 04-20-2012, 04:54 PM Thread Starter
 
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Automatic to Manual Transmission conversion.

Howdy, new here and this is my 1st topic, so my apologies if this is the wrong board to post this type of question on.

Anyways, I recently purchased a 1968 Pontiac LeMans, 400 ci V8 in it, and a THM400 automatic transmission, or so I'm lead to believe. The car drives, but the transmission slips in what I believe is '3', and while in '2', it doesn't slip, but it doesn't have any pull to it and takes a long time to accelerate to approximately 50mph, where it then ceases to increase acceleration.

I wish I knew how to explain that a bit better, or actually knew what I was talking about, but that's the best I can do with my limited knowledge on the matter, haha. Currently a 21 year-old AST student.

I'm faced with 2 options, I think.

Keep the automatic transmission, and get a rebuild kit. Or get a manual transmission conversion kit.

I've always been a fan of manual cars, automatic makes me feel bored. Also I don't intend to race or take this car to the track or anything, so I'm not awfully concerned about performance or lap times, etc.

I'm leaning towards the manual conversion, however I know next to nothing, nor do I have experience with doing any of the work involved with either option. So my question is, what all do I need to get in order to successfully do this conversion?

I see that there are kits available, such as this Keisler ClassicFit? RS® and TKO Kits | 1968-1972 | Chevelle - GTO | GM | Keisler Engineering, Inc | Keisler Engineering - KeislerAuto.com , but I'm not sure if that covers everything that I'll need. Or if that link is even showing the right things I need for my transmission altogether, rofl.

I asked a teacher what all I'd need to purchase to do something like this, and he listed:

The transmission itself
Hurst Shifter
Flywheel - Pontiac 400 ci specific
Clutch
Pressure Plate
Bellhousing - Pontiac 400 ci specific
Crossmember
Clutch Petal w/ related linkage
Shift Boot
Driveshaft
Yoke

He said that most of those components do not need to be/ don't come Pontiac 400 ci V8 specific and can be/are generic.

If anyone can verify that, and could point me in the right direction I'd be truely greatfull. I'm actually trying to learn the whats, hows and whys of this and will be doing all the labor myself with the help of other students, so I really need to understand whats going on so I don't &%^* anything up.

Sorry for the lengthy post, figured it was worth being thorough so people can give me accurate information and don't have to ask me questions later on. Hopefully I've listed everything correctly and given the right parts I have, haha.

Thanks for any and all help regarding this.
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post #2 of 9 (permalink) Old 04-20-2012, 05:07 PM
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The linkages and pedals are very vehicle specific. OEM factory used parts are the best choice. Expect to pay upwards of $3000 for all of the needed parts, excluding labor. Lots of guys using (or trying to use) the Kiesler units on the pyforums. The trans is extremely expensive, and floorpan modifications are needed, as well as some fabrication work. My recommendation would be to get a good used short shaft TH400 for $100-$500 , or to rebuild your current one for about $400 or less. A lot of people are cloning Lemans and Tempest vehicles into GTO's, and because of this, 4 speed and stickshift parts are expensive. For speed consistancy, durability, and value, a TH400 automatic can not be beat. That said, sticks ARE more fun!!!
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post #3 of 9 (permalink) Old 04-20-2012, 07:10 PM Thread Starter
 
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Quote:
The linkages and pedals are very vehicle specific. OEM factory used parts are the best choice.
Appreciated.

Quote:
floorpan modifications are needed, as well as some fabrication work.
Could you possibly give me a better idea as to what specifically will need to be cut/welded/etc? At the school we have all the tools and etc needed to do any of the work we could possibly need to do, I'd just like to know what all I'll need to do before diving into it.

Yeah, I knew getting into this project that it wouldn't be cheap. Unfortunately for me, manual isn't something I feel I can overlook. I NEEDS TO SHIFT!

Related note, I heard converting to a 5 speed manual with O/D will improve MPG. Is this true? If so, think its worth spending the ~3,000$ upfront to get this alledgedly better MPG?

I intend to make this car a daily driver, so fuel economoy is definately a priority for me. When I first got this car I put in 3 gallons of premium fuel (RIP wallet) so I could get to school and back to get it inspected. Well, I got there and back, and put maybe 20 more miles on it, for a grand total of ~40 miles to that tank before it decided to leave me on the side of the road.

I'm used to driving a little 1999 Mitsubishi Eclipse, 2.0L nonturbo. Needless to say, I'll have to budget gas better and plan routes with this LeMans, lol. Related note, also need to fix my fuel sending unit. Doesn't help the predicament not knowing how much gas I got left.

Thank you very much for your reply!
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post #4 of 9 (permalink) Old 04-21-2012, 10:15 AM
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Garage
I run the TKO 5 speed but supposedly Kiesler has a "slim body" 5 speed that requires no floorpan mods.

That said, to install my TKO I had to "pie cut" my tranny hump to raise it about 1" or so to clear the trans and weld in some patches to fill the gaps. Not really a big deal. Shifter in stock location and using the factory Hurst stick. If you did'nt know it was in there you'd never suspect it was a five speed. Also had to relocate my factory tranny crossmember, but that just required drilling some new holes. The driveshaft needs to be shortened (not replaced) I did my own but if you're not comfortable with that any competent driveshaft shop can do it for you and it should be reletively inexpensive.

I would recommend either a Mcloed (what I run now) or Centerforce clutch and pressure plate. Centerforce did'nt last very long in my car but then I'm pushing a lot of power through it.

Incredible on the highway, I run 2500 rpm at about 75 mph with 4:11 rear gears.

The only real "hassle" I ran into was I neglected to remove my Magneto ignition and demagnetized the coil by welding on the car. Not something most people have to worry about .
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post #5 of 9 (permalink) Old 04-21-2012, 11:26 AM
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You will need to weld a Z-bar bracket to the frame, which will require removal of the wheel house on the driver's side. You will need to cut and modify the floorpan. You should try to find factory pedals and linkage (the expensive part). If I were doing what you are wanting to do, I would absolutely go with a 5 speed for the overdrive benefit. Your fuel economy will improve a BUNCH. You're cutting and modifying the car anyway, so no big deal. The only hard part of this whole operation is coming up with the $$$$.
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post #6 of 9 (permalink) Old 04-21-2012, 02:42 PM
 
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on a 68, you can use the bolt on chevelle z bar bracket. The pedals are available aftermarket. If you stay with the standard 4 spd you will have an easier time. A better place to save on gas mileage is on engine mods, cam, CR, carb, etc. , not changing the trans unless you are running a very high rear end ratio, which a gear change is a better idea.
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post #7 of 9 (permalink) Old 04-22-2012, 01:49 PM
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"which a gear change is a better idea"
Speaking of which: with the 2.56 gear I swapped into my heavy '67 GTO convertible with a non-overdrive, standard TH400 auto trans, I am able to consistently pull down a smidge over 20mpg at 75-80 mph. No large amounts of time and money spent. Mission accomplished.
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post #8 of 9 (permalink) Old 04-22-2012, 02:30 PM
 
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It might not mean much or be relevant but I was recently in your boat with a TH350 mated to a 73 350 engine. I ended up buying a TCI Streetfighter TH350 with a saturday night special torque converter. Couldn't be more happier with it as it brought my car back to life.

I would say do a ton of research and really understand what your driving habits/intentions are going to be for your car. It will go a long way in terms of it feeling the way you want it to be....

Is it going to be a daily driver?
Track queen?
Are you looking for extra gears to get better gas mileage?
Do you want to be strong off the line or at upper rpms?
How much are you trying to truly spend?

Questions like that you'll likely have to be honest with yourself before determining whether or not its worth switching from auto to manual etc.
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post #9 of 9 (permalink) Old 08-02-2016, 04:38 PM
 
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Hello JESchulte. So I have a 1967 HT GTO that my mom bought brand new. The frame has over 500,000 miles on it easy! I have been driving the car "legally" since 1996. Stock the car came with the HO 400 and a TH400. Original motor and trans are long gone. Everything underneath the car is completely been redone. Its going through a frame-off-purpose-build. Anyways it sounds like for what you described, have the TH400 completely rebuilt to at least OEM specs, and I would recommend having the rear end rebuilt and a person below mentioned 2:56 gears. I ran 2:56 gears with a TH400 for like 15 years. Get a converter with almost no stall and you will set yourself up for decent gas mileage. Pontiac's don't breathe well stock so I would at least recommend putting long tube headers on it, and a high flow muffler. If you don't want super loud high's, I ran Flowmaster 3-chambers for about 15 years as well through turn downs and I set off car alarms everywhere. You will save a SHIT LOAD of money for the more work you can do yourself. Even removing the rear end and transmission to bring somewhere for rebuilding will save you a TON of cash. I was 18 when I got that car set up the way I previously described. FAST FORWARD to me at 38 years old today. I've been married and divorced twice. Won't get married again, no kids and my second divorce was caused by my WHORE exwife who thought it was a great idea to start banging a married coworker with 4 kids behind my back. Luckily I live in California where divorce laws are pretty much equal for both sides regardless of who did what. So instead of doing the smart thing like saving my money for retirement, I put a chunk into my GTO and wanted to take it the next level.

Currently the car is being converted. I have a freshly built 406 with Edelbrock aluminum round port heads and a host of other goodies. New suspension, 4 wheel disc brakes. The drive line will be a TRMEC TKO 600 5-spd connected to a CURRIE built Ford 9" packed with 3.90 gears and Ford's TRU-TRAK posi-traction system. I have rebuilt that motor from the block 4 times since I've had that car. I have always worked on it front to back. But to put a 5-spd in, there is a LOT of fabrication that needs to be done, and a lot of other things may "pop" up when you take on a task like that. Luckily I had the money and found a shop that I trusted to do the work. There is no aftermarket kit to put those parts on my car. It was all fabbed from scratch. TREMEC is wider than my TCI TH400 so they had to cut the transmission hump out, no more center console. My floors were rusted so I had them completely replaced so there is stablitly and more safe area to weld to. I ditched the stock transmission brace and used a custom made tubular steel one. Driveshaft has not even been cut yet because the tranny and motor are not in the car yet. My point is, to put a 5spd in realistically you will spend at least $7000 in parts easy and probably $10000 in labor. My car is going to be a road car, and drag car, but its my mother's first car so I did not flinch dumping a shit load of cash into it because now it will last another 50 years and when I sell it, someone will appreciate all the work that went into it. If you have other questions I would be happy to answer them the best I can. Cars are a money pit, save your money. You have your whole life to enjoy cars so if money is tight, spend wisely.
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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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