overdirve transmission swap

[kramerdad1]

I want to swap my TH400 tranny for an overdrive transmission(TH200-4R or TH 700-R4) . Has anyone done this swap? can the trans be swapped out without pulling the engine? will the floor shifter work properly? are the slip joint splines different? what about the lock up torque convertor? thanks for any help.

[Gunslinger]

I can answer some of your questions. I have a 700R4 swapped in my '70 in place of the Borg Warner PowerShift.

You should be able to do it without pulling the engine. The transmission support may need to be relocated. The floor shifter housing needs to be replaced with one from an Avanti with an overdrive automatic as while they look almost identical...they're not. The shifter indicator in your car is held down by two screws. The indicator in an overdrive car is held down by one screw and a tongue and groove type hold down, so a different quadrant is required. The shifter itself will work, but won't line up properly with the letters and numbers on the dial. To make it line up correctly you should obtain a shifter from a later car.

You might need a different driveshaft or have yours modified. I can't say for sure on that when swapping from one GM tranny to another. Mine required a different driveshaft when changing from the BW to GM. I went with an aluminum driveshaft to reduce rotating mass since I had to change units anyway.

Some will say a 700R4 might require transmission tunnel modifications...mine didn't. That may be a car by car thing. What my car did need was a different neutral safety switch and backup light switch setup fabricated. Whether that was mandatory due to the 700R4 size or because I went to a 2 1/2" diameter exhaust system in place of the original 2" pipes I can't say...but the proximity of the exhaust made it necessary.

Some say the 200R4 is a better choice as it has a better spread of gears. The speed shop that installed mine only deals in 700R4's as they're stronger...not that a 200R4 can't be built to take lots of torque. I will say the low gear on the 700 is quite low and shifts almost immediately into second. It will smoke the tires in any gear if I try and it did take some time getting used to it.

[Avanti83]

I can answer some of your questions. I have a 700R4 swapped in my '70 in place of the Borg Warner PowerShift.

You should be able to do it without pulling the engine. The transmission support may need to be relocated. The floor shifter housing needs to be replaced with one from an Avanti with an overdrive automatic as while they look almost identical...they're not. The shifter indicator in your car is held down by two screws. The indicator in an overdrive car is held down by one screw and a tongue and groove type hold down, so a different quadrant is required. The shifter itself will work, but won't line up properly with the letters and numbers on the dial. To make it line up correctly you should obtain a shifter from a later car.

You might need a different driveshaft or have yours modified. I can't say for sure on that when swapping from one GM tranny to another. Mine required a different driveshaft when changing from the BW to GM. I went with an aluminum driveshaft to reduce rotating mass since I had to change units anyway.

Some will say a 700R4 might require transmission tunnel modifications...mine didn't. That may be a car by car thing. What my car did need was a different neutral safety switch and backup light switch setup fabricated. Whether that was mandatory due to the 700R4 size or because I went to a 2 1/2" diameter exhaust system in place of the original 2" pipes I can't say...but the proximity of the exhaust made it necessary.

Some say the 200R4 is a better choice as it has a better spread of gears. The speed shop that installed mine only deals in 700R4's as they're stronger...not that a 200R4 can't be built to take lots of torque. I will say the low gear on the 700 is quite low and shifts almost immediately into second. It will smoke the tires in any gear if I try and it did take some time getting used to it.

Bruce did an excellent job of overviewing the swap.

I'll add a couple of thoughts as my 83 came with a 700R4 and I replaced it with a 200R4. The 700 fit without any mods as the 83 came with it. One problem is the 200R4's are somewhat harder to find as fewer were built. They also went into midsize cars so some had lower torque ratings but they were standard behind the Buick Turbo V-6's so they'll hold up generally ok. Mine came from a mid-80's 442 and was rebuilt to stage 2 specs. So it's capable of handling the 400 or so horses from my 355 SBC. That rebuild was about $1300 as a benchmark.

Here's a list of auto tranny lengths so you can compare your T400 with the others. http://www.angelfire...428/tranny.html

Let me now say that your shaft might work with a 200 but it's to long for 700, so you are balancing rebuild cost against tranny cost.

If you are in the lowest reasonable cost mode, I look around for a 700 series that is in good shape, low mileage or recent rebuild. It would handle anything your engine could dish out as they were built for large cars and pickups. The drive shaft mod is about $150 or so around here. Then you would only need to move/slightly modify the rear crossmember and be good to go.

If you want to go for broke and rebuild the trans, then either the 200 or 700 would work. The 700 has a lower first gear to get a heavy car rolling whereas the 200 is much more evenly spread. If you have a 3:07 or so rear end then the 700 is probably better. 3:40 or so and the 200 could be better. My 83 has a 4:10 behind the 200 and it a lot of fun off the line.

I'd also be sure to get a 700 or 4L60 variant not the 4L60E or later as the early ones have the ability to add a speedo drive. The E series is electronically controlled. You can use an "E" series with an electric aftermarket controller which is not inexpensive but works well. It would require an electronic speedo or GPS version.

Lastly, The 200 and early 700 variants use a TV cable attached to the carb linkage to control shifts. If improperly adjusted, it will toast a trans so be sure you or whoever hooks it up does it correctly.

Good Luck, Bob

Edited March 10, 2013 by Avanti83

[PackardV8]

Spend the money for a BowTieOverdrive throttle linkage, pressure gauge and instructions. They charge a lot but without their stuff, I was in the dark about finetuning the throttle linkage and TV cable.

jack vines

[plwindish]

I had the THM 400 replaced in my 76 with a 200R4. If your car has the 327 or 350, then the yoke on the driveshaft will not need to be replaced. My car has the 400 motor and it had the larger yoke so it was replaced. The driveshaft did not need to be altered other than the yoke and new rear u-joint. A trans rear mount needs to be made for a 200R4. I got the rear mount, trans linkage and a new shift indicator plastic piece that shows PRNDD321 from Nostalgic Motors. I had a trans shop I had dealt with earlier with other cars rebuild a 200R4 and do the installation. That took care of all the TV cable adjustment for me. The shop also put in a Lokar trans fluid dipstick on the firewall as its really tight to use the standard dipstick in the regular location. The 76 has a 3:31 rear end in it. The rpm's dropped from 3000 @ 70 mph to between 2000 - 2100 rpm's with the 200R4. If you intend to drive the car very much, I would encourage you to do the swap. I have put 10,000 on the car since the swap two years ago and have had no trouble. MPG went from 12.5 up to a best so far of 22.5. I live in the Chicago suburbs and the total trans swap cost was just over $2,000.

[Devildog]

You will love the OD tranny, I have a 700R4 in mine. Good info from others, to fill in a few points:

1.Lock-up: virtually all 700R4 will accept the lock-up internal wiring (if not installed), the torque converter must be designed to use lock-up (a normal item). I just ran a power 12V wired to the tranny with a cut out switch on the brake light switch to ensure it unlocks when brake applied. I will sometime wire a toggle switch (ashtray) into the line so I can disengage manually. Tranny shop will recommend a vacuum cut-out switch but really dod not need.

2.Insulate: while your under there install fiberglass/heavy aluminum foil insulation in the tunnel and bell housing area to reduce cockpit heat.

3.Speedo gears: depending on the rear end ratio you have you may want to change the drive gear ($15) on the tranny shaft. I had a 3.23 differential which I just changed to a 3:70; however, I can not change the speedo cable gear enough to have a accurate...I need a different drive gear on the tranny shaft which is easy while out but impossible after installed. Suggest you put in one that would be ranged for a 3:55 differential...cable gear could be installed to reduce for a 3.23 or increased for a 3:70.

4.Torque converter: consult with your tranny shop, many 700R4 were used in Chevy Luv trucks and the like, and had a higher stall rpm converter to rev to a decent torque range. With your Chevy engine, you do not need this high stall speed.

Do not hesitate, do it. It makes the Avanti more nimble driving around at street speeds and the OD (with lock-up) on the highway makes it a modern day road cruiser.

Joe

[PackardV8]

One other tuning component is the governor. As mentioned, the 700R4 was used in everything from 4-bangers to V8 diesels. Along with the throttle valve, the centrifugal governor controls the shift points. Most tranny shops have a bucket full of GM governors. If yours shifts too soon, get one with lighter weights and stronger springs. If it shifts too late, get one with heavier weights and/or lighter springs. With a bit of tuning, you can get it to shift exactly where you want.

The nice feature is the governor can be changed externally in two minutes.

jack vines

[kramerdad1]

Thanks guys for the great feedback. As I look more and more into this some unexpected things popped up. What did you guys do about the collar on the steering column that locks the steering wheel when you go into park and the rod that runs to it ? Nostalgic told me I had to buy a conversion kit consisting of a butterfly lever, two rods ( shifter to trans and trans to the steering column collar) , a needed bushing and a different shifter assembly costing about $700.00. Do I need to do all this or can the parts from the TH 400 fit or can I eliminate the locking collar ? will the oil cooler lines from my TH400 fit the 700 or will they need to be modified? How did you solve locking the torque convertor ? I know TCI sells a kit for about $80.00 Is there a more cost effective way to do this without having to spend 800 bucks for a shifter kit and to lock up the convertor? Id like to reduce the engine noise on the highway by going to an overdrive but I dont know if its turning out to be cost effective or not. Im mean aside from reducing the wear on the engine by lowering the RPMs how many miles will I have to drive to justify the cost? Maybe the Gear Venders overdrive for the TH400 would be a better choice?

[PackardV8]

FWIW, the Gear Vendors will cost the same or more than a 700R4 conversion.

How much difference is dependent upon how much if any the owner does himself. I did one myself, taking a gamble on a wrecking yard tranny, for about $1000. To have a professional do the swap using a guaranteed rebuilt tranny will be $3,000 - $3500.

The Gear Vendors swap with a new unit is the same $3500.

jack vines

[plwindish]

The parts list from Nostalgic sounds like what I had gotten 2 years ago. The rods, butterfly, bushing and shift indicator work fine. The steering column locks just as the THM 400 trans did. Nostalgic tends to be pricey on their parts but Dan is a walking encyclopedia on anything Avanti. He knows what you need to make the trans a factory installation. The price I had listed earlier did not include what I spent with Nostalgic to get those parts. A trans shop can set the lock up a couple of different ways, by a switch or by speed. I had mine set by speed, but had it readjusted once last fall as it was locking up around 40. Cruising around town at lower speed, I usually keep the trans in D3 which keeps it from locking up. I had also looked into a gear vendors unit, but they are higher than the trans swap, plus you'll need to have the driveshaft altered. I kept the THM 400 trans in case I would ever sell the car and the owner would want to go back original, but after driving it with the OD trans, I don't know why anyone would want to go back to the 3 speed THM. The 200R4 is the best geared match to the THM, having the first three gear ratios very similar to the THM. The are several good articles on OD trans swaps in Bob Johnstone's Studebaker Resource pages. I'd do a little more research before deciding what to do. Having a highway cruising gear makes the Avanti much more drivable at today's highway speeds without getting "droned" out of the car listening to the motor. Good luck with your decision. I think most owners that have done the swap will tell you that after everything is adjusted and running right, it made their Avanti a lot better driver.

[kramerdad1]

again thanks guys for the information. I called Gear Venders, they said because of the cross member on the Lark frame I couldn't use thier unit, too much to cut out. I just bought a fresh 305 with attached 700r4 so I have everything except the shifter set up from Dan. looks like Ill be busy over the next few weeks pulling out the 350 th combo and stabbing in this set up .

[plwindish]

Good luck with the swap. You might miss some of the 350's power going with the 305 though. I take it that the 305 is pre -computer?

[Guest dapy]

My 1983 had an owner/mechanic 305-to 700R4 conversion 10 years ago. Seems to work fine after 10K miles. When done the owner did not do whatever was needed to make the speedometer work. Car just sold and I would like to advise the new owner how to make the speedo work. Is a modification (kit) available?

[Devildog]

My 1983 had an owner/mechanic 305-to 700R4 conversion 10 years ago. Seems to work fine after 10K miles. When done the owner did not do whatever was needed to make the speedometer work. Car just sold and I would like to advise the new owner how to make the speedo work. Is a modification (kit) available?

Dave, I will look for a name and telephone of a guy in Chicago that makes the cables. he did one for one of my Corvette when I put in a T-45 OD.

1.Type connection on back of speedo (maybe someone on forum can help with this) or just pull the old one out if still in the 83.

2.length of cable (old one plus 12-15 inches is probably about right)

3.look inside the 700R4 and see what color the drive gear on the shaft is

4. Differential ratio (3:55, 3.23. ???)

he will make it up and send it for about $100 as I remember

Joe

Edited March 19, 2013 by Devildog

[Avanti83]

My 1983 had an owner/mechanic 305-to 700R4 conversion 10 years ago. Seems to work fine after 10K miles. When done the owner did not do whatever was needed to make the speedometer work. Car just sold and I would like to advise the new owner how to make the speedo work. Is a modification (kit) available?

What does the speedo drive look like on the tranny. Early 700R4's had a speedo drive that took a cable and drive gear just like the older TH400's and TH350's. Later 700R4 and the 4L60 derivative had an electric sender powered by a driven gear that can be adapted by using the older 700R4 housing and gear.

If it's the 4L60E that is computer controlled then it's difficult but I doubt that you have that. What's in the speedo drive hole will tell you what it will take to make it work.

If you decide that's a lot of work, Speedhut makes a GPS Speedo that needs no tranny connection and can be ordered to look much like the original speedometer.

Bob

Edited March 19, 2013 by Avanti83

[Devildog]

What does the speedo drive look like on the tranny. Early 700R4's had a speedo drive that took a cable and drive gear just like the older TH400's and TH350's. Later 700R4 and the 4L60 derivative had an electric sender powered by a driven gear that can be adapted by using the older 700R4 housing and gear.

If it's the 4L60E that is computer controlled then it's difficult but I doubt that you have that. What's in the speedo drive hole will tell you what it will take to make it work.

If you decide that's a lot of work, Speedhut makes a GPS Speedo that needs no tranny connection and can be ordered to look much like the original speedometer.

Bob

On a 700R4 if you pull the cable gear from the side of the tranny. There will be a plastic colored (mine is green) drive gear that slip[s over the tranny drive shaft in the tail cone.

Dave, contact http://www.transmissioncenter.net/speedometer they custom make the cables with appropriate cennect on each end to match dash speedo - tranny drive gear, and can give advice and new gear (if needed).

Joe

I did nothing...I am old school, I just made a correction card like any resourceful Marine aviator and pasted between the speedo and tach.

[Guest dapy]

Thanks guys. I will look into the GPS possibility. I have an inaccurate speedo in another old car and have likewise calibrated the RPM/speedo. Turns out there are cardinal 2000, 2500, 3000, etc. RPMs equivalent MPH.

[plwindish]

The speedo in my car reads 8 mph higher at highway cruising speeds than the actual speed. I had all the SW gauges replaced with Auto Meter when I had the engine rebuilt and before I had the THM 400 replaced with the 200 R4 so I don't know if the old speedo/THM400 combo was as inaccurate. I found the speedo error by driving with my Tom Tom GPS, so the 80 cruising speed on super slabs is actually 72, which does help me stay out of speeding troubles with "John Law". The trans shop that did the swap had played with the speedo cable switching a gear out of the trans to get it to only being 8 mph off. One of these days, I may try to get it closer, but its not a high priority since I know its reading fast.

[77 Avanti]

Well fellas,I am reading this again and again. Seems like alot of this was some 10 years ago. I removed my THM and installed the 200r4. I did buy the rods,shift housing,indicator,new boot and arm from Dan Boothe. I for one will never mention his pricing for two reasons,the first being he has the parts! Second. The man is a walking encyclopedia,and very generous with his knowledge. Unfortunately,when they finally lay him to rest, everything in his head is going with him.

My driveshaft did not need to be altered with the exception of buying a little longer front yoke and a different,two size,bearing u joint. As far as the speedo,I ordered a speedometer correction unit from California Performance Transmission in Huntington Beach for a $100.00 you unscrew your cable from the transmission, screw the corrector on and then the speedo cable goes into this. My speedometer read 46,when gps read 50 mph. Important to me because I ordered a Compushift mini computer to control my shifts,I wanted my overdrive lockup to lockup automatically,and,my overdrive to UNLOCK automatically. This is controlled,partly buy a speedsensor,included in the plug and play mini computer. That is why I bought the speedo corrector. The whole computer comes factory set,but it is bluetooth enabled to your ipad,or smart phone,do you can adjust the shifts,via the app. 

I did have to get a dipstick and tube,shorten and redrill my read transmission mount and get a TV cable and mount for my Edelbrock carburetor..

My rpms at 60mph have dropped down about 1000rpms. Less heat,less engine wear and noise, and gas mileage.

[wildfelr]

RQB3263 81/305    Whats left to be said.... I did this swap 4 yrs ago and was just one day of hard work with a strong friend.....my 700 has worked perfect ever since....i did re-tube the drive shaft but cant remember if I leightened it or shortened it ....(CRS)...the shift letters dont align up but I can count to 3 so its OK....the rear mount req mod ...pic attached ...a new speedo cable from Calif  6 inch longer perfect  and no speedo error...im tacing 1700 at 65 MPH... no mods to the tunnel....BILL RQB3263

[wildfelr]

The pics show the fabed  mount (simple 1/4 in plate steel with part of the original mount bolted to it)sitting on top of the X member....it was later moved to under the X member to gain clearance for the speedo cable .... that also lowered the tail shaft about 1/2 inch  etc to no effect....BILL RQB3263

 

[wildfelr]

One more pic..... reminder...The new mount plate is now bolted underneath the X member instead of sitting on top of it ....RQB3263