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post #1 of 6 (permalink) Old 03-19-2019, 12:29 PM Thread Starter
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Bought a 389 with automatic transmission attached. My question is: Can I convert to a manual transmission, and if so, how is this accomplished.
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post #2 of 6 (permalink) Old 03-19-2019, 12:57 PM
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If its the original transmission that came in a 66 its a super turbine 300 2 speed automatic transmission. its a long process to convert so bear with me.

Here is the list of parts you will need.
1 a new 4 speed transmission, look for a muncie, do some research as to if you want a wide or close ratio ( the gearing of the transmission) some are better for different gear ratio rear ends.
2. hurst shifter and all linkages
3. Pontiac bell housing
4. flywheel, clutch, pressure plate, alignment tool to install the clutch and pressure plate
5. throw out bearing and fork
6. Pilot bearing or bushing which gets installed in the back of the crank shaft and accepts the input shaft from the transmission
7. z bar
8. clutch push rod
9. clutch pedal assembly (ames part number R135A)
10. boot for clutch push rod (there is a space on the bracket where the steering column mounts to the firewall, you will have to cut a hole for this pushrod and boot, or replace the existing one with a new one (available from ames)
11.THIS IS THE HARDEST PART. you need a countershaft braket, also available from ames. you will have to perfectly position this and WELD it to your frame down near the firewall ( I can take a picture of this tonight for you as I literally just did this on my own car on sunday) This bracket aligns the z bar so it is in the proper position.
12. Ball pivot which mounts in the block for the other side of the z bar opposite the countershaft bracket
13. center console
14. Shift porch ( also available from ames part # R146 this part gets bolted to the transmission hump on the floor where the shifter comes through. youll also need the boot for it (R150) and the bracket to hold down the boot (R152F)
15 new drive shaft as the length of your transmission will change

This SHOULD be everything you'll need that being said this is not at all a cheap conversion. I'm deep in the middle of a conversion and so far has cost me about 2500 dollars, will probably be close to 3000 by the time is all said and done. but i know I wont regret it and you wont either

first step is removing the old transmission, and flex plate.
You will need to cut the hole in the floor for the shifter to come through using the shift porch as a template.
install the new pilot bearing in the back of the crank on the engine.
Install your new flywheel, clutch and pressure plate.
take your bell housing with throwout bearing and shift fork installed and bolt it to the back of your block. Next you can install the transmission ( without shifter for now) bolt it all up
how you can install the shifter putting it through the new hole in the floor. screw in your shift porch, boot and retaining ring. now your center console can go on.
Next we're going to the engine bay now. Install your new z bar pivot on the block, then you have to remove your inner fender well and weld on the new countershaft bracket. I can send pictures to show you how.
You can now install your z bar.

now another tricky part, you need to cut a hole in the existing plate where the steering column mounts to the firewall to get the new clutch rod boot through the firewall for the clutch pushrod. I removed my whole steering column and replaced this whole plate, you can just cut it and use an aftermarket boot. (part number R132K)

once this is done, swap out your old brake pedal for a new clutch pedal assembly, hook up your push rods, and linkages, install your drive shaft and hit the road.

this is a project that has so far taken me a full week of working every day after work, getting a couple things accomplished and I still likely have another week left, then again I used all OEM parts, some of which needed to be repainted and refinished. but if you know what you're doing and have a few days of time you can do it yourself. Let me know if you have any other questions.

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post #3 of 6 (permalink) Old 03-19-2019, 01:01 PM
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This video will help to explain the z bar procedure I was discussing including the modification to the plate which mounts the steering column to the firewall. In this video however, they use a bolt on countershaft bracket, the 1966 will need to be welded, it cannot be bolted.
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post #4 of 6 (permalink) Old 03-19-2019, 05:44 PM
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To add to all this, you will want to also get a different rear end gear depending on the selection of either a wide ratio or close ratio transmission. The wide ratio will be less expensive and better on the street due to its first gear ratio.

With the automatic trans, you may have a 3.08 or even something like a 2.93 gear. Too stiff an axle ratio with a 4-speed and you will most likely burn up the clutch prematurely - and have fun pulling away on a steep grade or hill from a stop. You might not be able to ride the engine down to a stop without having to push in the clutch before you get closer to a stop in the lower RPM's - the gearing will be too stiff and the car will start bucking because you are dropping the RPM's down closer to below idle RPM's or what your cam/engine wants to run at, basically stalling the engine. To counter this situation, you may find yourself pushing in the clutch at 10-12 MPH road speeds and rolling to a stop that final distance.

With a wide ratio transmission, a 3.23 rear gear will work as long as you use the stock 26" tall tires. Once you go taller on the tires, this reduces your gear ratio and you could be right back at a gearing just like using 3.08's. So if you go taller tires, you want a gear like 3.36 or 3.55's.

With a close ratio trans, 1st gear is not as low and you generally want a 3.36 or 3.55's. Again, taller tires will affect the overall ratio and you may need to move up to 3.55 or 3.90 gearing.

There are tire size/gear ratio/MPH calculators on line that can be used to figure this out.
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post #5 of 6 (permalink) Old 03-21-2019, 08:47 AM
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Here are the pictures of the bracket you have to weld to the frame, if you look at the pictures you can see on the block where the pivot point for the z bar screws in. I had just painted it when this picture was taken which is why its more glossy than the rest of the frame. Just make sure when you weld this piece on it is perfectly in line with the hole in the block for that pivot.
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post #6 of 6 (permalink) Old 03-21-2019, 09:46 AM
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If you have the budget for it, you can get a new Muncie with a 1st gear ratio that will work with the high 2's & low 3's ratio rear gears.

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