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replacing 4-speed with an automatic in a 73 roadster

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Old 08-04-2018, 03:33 PM
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pe73454
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Default replacing 4-speed with an automatic in a 73 roadster

There dozens of threads about replacing an automatic with a 4-speed but I want to do just the opposite. Any help would be appreciated.
I know by starting this this thread I'm leaving myself wide open to a ton of negative replies, so before any of you jump to criticize me, let me tell you the whole back story of why I want to dump the 4-speed in favor of an automatic: I bought my red 73 454 roadster brand new in June of 1973 and drove it for 40 years until I suffered a stroke in 2011 which resulted in my left leg becoming weak and lacking feeling. This made using a clutch difficult at best and unsafe at worst. In 2012 I reluctantly sold my beloved 73 to a local fellow who spent 20K+ on it, mostly rebuilding the stock 454 with into a 400hp monster with a competition clutch behind it. Then a month ago he offered to sell me my original 73 back because he was tired of it and needed the cash. So to make a long story short I bought it back because I missed that car. Needless to say my physical condition hasn't improved and driving a monster 454 with competition clutch is even more difficult than driving the original stock setup. So, now I'll move on:

I still love my old 73. I hate the rebuilt engine and driving it borders on impossible for me now. With the engine ruined it's not stock anymore so I'm thinking I might want to pull the Munsie 4-speed and replace it with an automatic that I could at least drive safely again. Have any of you done this sort of conversion? If so what challenges am I facing? What automatic should I consider?

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Old 08-04-2018, 04:06 PM
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Around town or long drives? Anything using high gear more than a few minures and you want overdrive. So 700r4 is best candidate. Around town mostly? T400. In either case, you will have to redo the trans crossmember. With t400, driveshaft will work as-is. But this involved a swap, an extra hunnert bucks to have a shaft made or shortened is no issue. You might want to push the clutch pedal down and put a floormat over it instead of replacing pedal setup. It will also give left foot a bit more room. Then mebbe downgrade the cam to a more sedate grind... But first, GO SMOKE THE **** OUT OF THEM TIRES one last time while it is still a monster...
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Old 08-04-2018, 05:28 PM
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Mostly the old girl is a garage queen right now, so around town and a few short local trips is going to be this cars future I'm afraid but I still like the idea of overdrive. Great idea on smokin off them tires . I forgot to mention that the guy who had the car for a few years scrapped my Permicast mags and replaced them with rally wheels and awful looking red line tires. I hate those almost as much as I hate the engine rebuild. I'm Ok with the 700r4. I have that in my '82 Collectors. Will it stand up behind a big block? I'm OK with a drive shaft modification too. But, the 700r4 has a computer controlled lock-up torque converter though. Isn't that going to open up a whole other can of worms in a 73?
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Old 08-04-2018, 06:00 PM
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Your transmission vendor should be able to offer solutions for torque converter lockup control.
I got my overdrive transmission from Bowtie Overdrives in California and they sent me everything I needed.
I changed from a THM-350 to a 200-4R behind my SBC and I love it.

It sounds like a much more involved project to convert one from manual to automatic, but I don't blame you one bit considering the leg weakness.
Good luck with your project.
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Old 08-04-2018, 06:19 PM
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Thanks for understanding my situation wendellp601. As you can imagine, I'm starting from ground zero here. Back in the day I used to do all my own wrenching. I changed the clutch in my 73 twice myself, Always did my own mechanical work both major and minor. Now I'm at the mercy of so-called professionals that I pay to do everything for me. It's a whole new world for me. I think I'd have been better off telling the guy that wanted to sell me back my old 73, "NO!", but I was so emotionally invested in a Corvette that I drove off the dealer lot in 1973 and raised my kids and married my wife in that I brought it home without thinking for even a moment about what I was getting myself into.
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Old 08-04-2018, 06:36 PM
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Originally Posted by pe73454 View Post
Thanks for understanding my situation wendellp601. As you can imagine, I'm starting from ground zero here. Back in the day I used to do all my own wrenching. I changed the clutch in my 73 twice myself, Always did my own mechanical work both major and minor. Now I'm at the mercy of so-called professionals that I pay to do everything for me. It's a whole new world for me. I think I'd have been better off telling the guy that wanted to sell me back my old 73, "NO!", but I was so emotionally invested in a Corvette that I drove off the dealer lot in 1973 and raised my kids and married my wife in that I brought it home without thinking for even a moment about what I was getting myself into.
My dad survived a debilitating stroke in 2006 a couple months after his 60th birthday.
Today he remains fully paralyzed on his RH side and his speech was profoundly affected.
He walks with a "hemi-walker", but hasn't driven a car since the stroke.
I know what you're up against with mobility and sensory concerns.

Good luck in your search for a reputable and competent shop to do this conversion.
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Old 08-05-2018, 06:17 AM
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There is one sure-fire cure for aging. Just ask Marilyn Monroe. She is still in her mid-thirties!
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Old 08-05-2018, 07:17 AM
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What a great story, and vey cool you got your 73 back :- )

I would no go for a TH400, I would go for a TH2004R or a TH700, do you know what your rear end ratio is?
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Old 08-05-2018, 09:48 AM
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I agree to change the cam out, first off.....second is to get the right rear ratio....3.36 is mine....I dumped my muncie for a 700 many years ago....wasn't paying 5 grand + for an overdrive, so went with a 1200 buck 700, did my own install, just cut the pedal off with sawzall....but I found that in typical suburban traffic driving that the gear drop from that silly 3.0 first gear to the 1.8x second was just horrendous, 1/2 way through an intersection from a light, and the slam was just silly.....so I grew tired of it and built my own 200 in the car today, Trans Go shift kit of course and super duty clutches...Kevlar?? at any rate it's been there over a dozen years now....I got an old HURST shifter lever at a car show, and made my own 8 ball on top, easy do.....just not cool unless you have an 8 ball on your shifter.....I got a trophy shop to etch a new plate for the console.....

there is NOTHING stock about my car, stem/stern/wheels/roof, nothing.....

see pix below....

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Old 08-05-2018, 12:07 PM
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Good plan on buying back your 73 and for putting in the auto trans so it is driveable for you.
Congrates on the purchase.
Performance mods aren't a bad thing if installed and set up correctly, so first change out the tranny so you can drive it, then see what modes you need to do, if any to the motor.
I would get a list performance components installed in the motor, with part numbers if you can from the prior owner so you have an idea what cam is actually in the motor which can effect what torque converter you need for the auto trans and you should have an idea what torque and horsepower the motor is putting out so you don't purchase a transmission that won't hold up against the output of your motor.
A 700R4 or a 2004R are not designed for a performance big block and if you decide on an auto overdrive you will need to purchase one modified to handle the tq/hp of your motor.
A good TH400 may handle the motors output and you will have a nice trans for all around driving, then set up your rear end gears to accommodate hwy driving if you need to.
You may need a new crossmember when switching to an auto as well.
I would have the pedals swapped out as the manual pedals are in high demand and don't bubba in an easy chop fix.
The brake pedal on an auto is very wide and if you have large feet you may find it necessary to trim about an inch off the right side of the brake pedal to make it easy to swap from gas to brake.
I had to trim mine.
There are threads on this as well, and I cut mine with a hatck saw blade in about two minutes and slipped on a pedel pad for a 2001 pick up and problem solved.
Good luck and we would love to see detailed photos and a thread on your swap.
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Old 08-05-2018, 12:42 PM
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Iím with many of the above in that Iíd go for a 200 4R. The higher HP favors a 700 R4 but I like the ratios better as well as the fit. No more offort to do this than a stock TH400. For driving around town, donít overdo the stall on the converter. A stock Grand National converter is around 2200 RPM. Itís what I use and has great manners. Should hold up to the torque of your motor. Iíd leave the rear end ratio until youíve had a chance to get a feel for the combo of motor and transmission.

As for the conversion to auto, thatís the real chore. Iíd say going this direction is a little easier than auto to manual but the biggest job is the same for both, replacing the pedal assembly. Itís a time consuming and awkward job. Read the posts on the auto to manual conversion as the task is essentially the same.

youll also have to add a transmission fluid cooler. Iíd suggest aftermarket rather than converting your rad.
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Old 08-05-2018, 08:15 PM
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Flywheel has to be changed to a flex plate.
Not sure if you can just take the clutch pedal off and live with the smaller brake pedal. or whether the pedals have to be changed.
Clutch rod hole through cab has to be blocked or covered
The shifter hole in the cab has to be enlarged for the auto shifter if you are using the 350 or 400. Not sure what has to be done for R4s
The shifter linkage and switches have to be added.
The exhaust system has to be changed out.
The cross member in the back of the transmission has to be cut off and a new auto cross member bolted in for the 350 and 400s. Not sure about the R4s.
The drive line on yours is a 2-1/2" diameter. The 2-1/2" diameter is needed for the automatic. SB sticks use a 2" and this would have to be upgraded. Not sure if yours would be the right length though.

RE: 700 R4 lockup:
TCI had a work-around for the overdrive. It was a switch that is activated by engine vacuum. Whether/how well it works depends on the cam you run.

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Old 08-09-2018, 10:24 AM
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Originally Posted by c3_dk View Post
What a great story, and vey cool you got your 73 back :- )

I would no go for a TH400, I would go for a TH2004R or a TH700, do you know what your rear end ratio is?
3:36 I think
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Old 08-09-2018, 11:57 AM
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Ask the PO what cam he put in it, then choose converter accordingly.
If it were me, try a smaller cam and hydraulic throwout bearing, perhaps you can keep shifting.
With the auto Id look into converter/cam decisions. Sounds like a small converter wont work for you if its got a big cam..Or his tune is way off.
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Old 08-10-2018, 02:05 PM
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OK, I have the cam and engine specs now. Unfortunately there was never a dyno run made. Here we go:

Comp Cams XR264HR.
Original 454 block bored .30 over new pistons brought it to 9.9 compression.
Q-jet and stock intake replaced with Carter carb.
After market electronic ignition.

wish I had actual performance specs like dyno HP and torque but it was never done. The guy that had the work done estimates it's somewhere around 390hp and unknown torque.
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Old 08-10-2018, 02:38 PM
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That helps.
I would think you could swap out the manual for a good TH-400 auto with the stock torque converter and enjoy your car.
The cam specs show it is a nice mild hydraulic roller and combined with 9.9 compression ratio, I wouldn't think you would have to touch the motor at all.
Reading your original posting on what you want the car for, it would be a good combo in my opinion.
You might be able to find someone on the forum to swap out your 4-speed and associated parts for their TH-400 and associated parts to save some money.
Get the trans rebuilt with quality parts and have fun.
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Old 08-10-2018, 07:01 PM
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A question for you automatic guys: Wouldn't the OP be better off driving the car and figuring out what stall speed it can handle? He has a running car. I would think that if you got it in 4th gear @t 1600 RPMs and got on it and the engine handled that without lugging, bucking or hesitating that would be the correct stall speed. I would keep going down until I found the lowest RPM the engine would pull at. Would this work?

To the OP: Recognize that the cam you have will give you about 500 ft-lbs of torque in the 3500 RPM range. As you make your decisions, remember that whatever you go with may ultimately have to handle that level of torque.
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Old 08-11-2018, 08:36 AM
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The torque an engine can produce is limited by the driver's right foot. From the sound of it, OP is not planning on showing local kids who is the baddest mother in the valley.
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Old 08-11-2018, 10:59 AM
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Originally Posted by 2mnyvets View Post
A question for you automatic guys: Wouldn't the OP be better off driving the car and figuring out what stall speed it can handle? He has a running car. I would think that if you got it in 4th gear @t 1600 RPMs and got on it and the engine handled that without lugging, bucking or hesitating that would be the correct stall speed. I would keep going down until I found the lowest RPM the engine would pull at. Would this work?

To the OP: Recognize that the cam you have will give you about 500 ft-lbs of torque in the 3500 RPM range. As you make your decisions, remember that whatever you go with may ultimately have to handle that level of torque.
Thanks, this helps. Knowing it's at 500fp of torque tells me where I need to be for a transmission build. I was actually considering a 700r4 but it might not take that torque. I'll be talking to my transmission shop a lot now that I have an estimate of the torque output of that engine.
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Old 08-11-2018, 12:43 PM
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700r can be built to handle that much, but like I said, if you push the pedal hard enough often enough you can destroy any driveline part. Also, standard tires just smoke instead of hooking up well enough to break things. That Comp 264 is actually a pretty mild cam. Standard converter should be ok. Another thought. Put a want to trade ad in parts section. You might find somebody close enough who wants your manual setup in trade for his auto.

Last edited by derekderek; 08-11-2018 at 12:54 PM.
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