Turbo 400 to 700 r4 conversion on 72

[L88 crazy]

Can I change my 3 speed Turbo 400 to a 4 speed overdrive 700 r4 auto trans?

 

[photovette]

There's a lot of information on this site:

http://www.bowtieoverdrives.com/index.shtml

I've been looking into it myself; the TH200R4 seems right to replace a TH350 in my '77.

However, I'd like to know, from anyone who has done the conversion, what kind of gas milage improvement did you recieve on the highway? There's nothing wrong with my TH350, but I put in a godawful amount of highway miles and I'd sure like to improve over the 18.5 mpg I'm getting now.

Without slowing down to 55 that is!

 

[JENSHAMMER@HOTMAIL.COM]

I have done this conversion on my '80, the tranny is very nice and you will definitly notice a milage boost, I would guess 1-2 mpg increase on the freeway.. worst case scenario. Bowtie overdrives was pretty helpful, however I did have some trouble, had to have 5 crossmembers shipped out (part my fault, part theirs). I like the tranny and am happy to have switched. First gear gives a bit better accelleration than the T350 and obviously the OD with 3.08's is pretty insane. I do 75 mph at 2250 rpms or somewhere around there.

 

[mrvette]

Well, I have had a 700 in my '72 for about 6 years, and I can say it's a mess from the git go.....the 700 is apparently weak in the 3/4 clutch pack and not much if anything to be done about it....
unless you exceed something like 450 ft lbs of torque, the much smaller, lighter, and better designed for reliability ...is the 200 4r, check with the Buick GN turbo crowd, they know....

just wish that back in '97 I had known of the mods and availability of the 200 4r over that heavy POS the 700 is....too boot I would not have had to spend the money on shortening the driveshaft, and effort to widen the tranny tunnel by cutting the pass side floor to widen the tunnel.....

IMO the 700 is a typical GM experimental POS.....the 3/4 clutch pak suffers from inadequate cooling/oil flow, ...that much I remember from the web sites....and so my opinion was formed....and from all information, remains unchanged....

all it takes is ONE uncurable weak spot, and i'ts game over....

GENE

 

[JENSHAMMER@HOTMAIL.COM]

I should have been more specific, and used good numbers. I swapped in a 200-4r from bowtie overdrives. I do 80ish MPH at 2000 RPM, not 70 mph as previously stated. Thats with 3.08 gears. They bowtie overdrives level 2 (out of 3) tranny cost less that 2 grand all told, did the install myself, didnt take long. Could have been done in a weekend with good tools. This will handle 450 HP and I think similar lb/ft. The level 3 wont even blink an eye at 450 HP by all reports. If you want more info or more specifics, email me

 

[wipeout]

I know this sounds crazy but I have 1 87 caprice classic. Would the transmission from that fit in my 77 corvette. I think its a th700 it does have overdrive. Im using it as my winter car right now but dont know what im gonna do with it next summer. Might sell it. But im thinking if I can use the transmission

 

[like2drive]

THE REASON I'M READING THIS IS BECAUSE i ALSO AM CLOSE TO SWITCHING THE TH350 TO A 700R4. AM DISAPPOINTED AT GENES REPORT ON THE 700 BUT APPRECIATED THE INFO.

 

[patsnitrovette]

Gene and i have 2 different oppinions of the 700r4 and i respect his outlook.I really like mine in my 73,i love the low first gear especially with my 3.70's and might even swap back to 3.90s.Though the 200 is an easier swap,the driveshaft has to be shortened for the 700 install,also some guys have had to cut the passenger side tranny tunnel and refiberglass it back in to clear the yoke,i did ,but some dont.You also use a yoke form a turbo 350 car not the turbo 400.

 

[BrianB]

If there's a tax refund coming this year I'm going to have to replace my TH400. The highway speeds just kill me on gas. The speed limit is 75 but if you are out of the city limits tickets are fine only, no points! I'm always in the 4000-4500 range on those trips. I'm sure the motor is tight, but I don't want to push my luck. The low end help will be nice since I'm at 4300 ft.

 

[dfw81shark]

There's a lot of information on this site:

http://www.bowtieoverdrives.com/index.shtml

I've been looking into it myself; the TH200R4 seems right to replace a TH350 in my '77.

However, I'd like to know, from anyone who has done the conversion, what kind of gas milage improvement did you recieve on the highway? There's nothing wrong with my TH350, but I put in a godawful amount of highway miles and I'd sure like to improve over the 18.5 mpg I'm getting now.

Without slowing down to 55 that is!

18.5? You must be running some highway gear on the rear-end! I'm estimating my TH350, stock motor and 3.54 rear are putting out about 12mpg. Whew! I'm envious!

 

[KANE]

There's a lot of information on this site:

http://www.bowtieoverdrives.com/index.shtml

I've been looking into it myself; the TH200R4 seems right to replace a TH350 in my '77.

However, I'd like to know, from anyone who has done the conversion, what kind of gas milage improvement did you recieve on the highway? There's nothing wrong with my TH350, but I put in a godawful amount of highway miles and I'd sure like to improve over the 18.5 mpg I'm getting now.

Without slowing down to 55 that is!

Sure- I did the 700R4 conversion on my old 78. I went with Bowtie Overdrives for all the parts. Great company and very helpful.

I was regularly doing 100mph at 3k rpm. It was great. Gas mileage went WAY up. I was ejoying nearly 26mpg on the highway with my TPI system.

 

[KANE]

I know this sounds crazy but I have 1 87 caprice classic. Would the transmission from that fit in my 77 corvette. I think its a th700 it does have overdrive. Im using it as my winter car right now but dont know what im gonna do with it next summer. Might sell it. But im thinking if I can use the transmission

Yeah, you could. But you may have to swap out the tail shaft, rebuild it, and still have to change the detent pattern on the shifter. For 1500.... you could get one from Bowtie Overdrives and still have the car.

 

[JENSHAMMER@HOTMAIL.COM]

if my memory serves me correct, swapping to the 700 requires a driveshaft shortening and rebalancing. I did the 200 swap, used to stock driveshaft and everything else bolted right in. It was, by all rights, a bolt in operation, and all I have ever heard (and now seen) about the 200's is that they are stout pieces of equipment. I have heard varying reports on the 700, bt dont have one so I cant comment reliably.

bowtieoverdrives.com

take a look, cant hurt

 

[like2drive]

Really appreciat the feedback about this upcoming project. If the local yokal :eyerole trans rebuilder is right, there are 2 different cases for the 700r4, with the smaller(& more $) one being specifically for the Corvette, which may be the reason for the necessity to cut out the tunnel. Also, in addition to Bowtie(which does have good info) I have found Fourth Gear, Inc info to support the Local Yokal's statement.

 

[like2drive]

DFW81Shark,
12mpg? Does a 3:54 hurt that much! Is it possible that the converter is not going into lock-up? I could believe it either way. Jim

 

[dfw81shark]

DFW81Shark,
12mpg? Does a 3:54 hurt that much! Is it possible that the converter is not going into lock-up? I could believe it either way. Jim

Nah, with 2.87 I was getting 13.5 or so. My mileage went south about the time I changed to a new distributor and new plugs. My smog stuff came out too about that time so I can't really nail it down to one thing. I don't think the carb got touched but I could be wrong. When I first got the car it was unbelievably sluggish but it got 15mpg. It could be coincidentally not going into lockup, it is hard for me to tell.

 

[photovette]

I had an '81 when it was about a year and a half old--it got 22mpg on the highyway no sweat. Of course, it was 55 mph limit in those days. Still something must be wrong to get that high a fuel consumption.


My '77 started getting about 10 mpg before I took action. I rebuilt the carb and it starting getting about 13 mpg. Then I installed a new Edelbrock Quadrajet and now it gets 18.5 mpg highway any day of the week and I have a 3.55 rear and no locking TC.

 

[dfw81shark]

I had an '81 when it was about a year and a half old--it got 22mpg on the highyway no sweat. Of course, it was 55 mph limit in those days. Still something must be wrong to get that high a fuel consumption.


My '77 started getting about 10 mpg before I took action. I rebuilt the carb and it starting getting about 13 mpg. Then I installed a new Edelbrock Quadrajet and now it gets 18.5 mpg highway any day of the week and I have a 3.55 rear and no locking TC.

How much tuning did you do on that Edelbrock? I may be getting one.. Do you still drive 55? I usually cruise about 75-80 and it is mostly highways around here (Texas).

 

[photovette]

I cruise at 75-80 mph (I don't normally use the cruise control--I get sleepy if I use it--but I'm real good at holding speed); aprx. 3500 rpm. If the tops are off and windows open, I drop about 2 mpg. I normally run the a/c.

As for settings, I installed the carb, warmed the car up, set the idle mixture, idle speed, a/c idle increase soleniod (whatever it's called), and secondary opening. Followed by choke and fast idle the next morning. It's very easy. This is something that everyone should be doing to their carbs every tune-up anyway--we've got spoiled with our modern fuel injected other cars, and forgot what a real tune-up involves.

This carb (1903) is indentical with the original--everything mounted and hooked up the same. Except for an extra bowl vent which I put a rubber cap on.

I looked into their other carb, and by the time you add all the stuff you need to get it to work on a stock 'vette (10 separate and distinct and individually priced parts), it was about the same cost as the new Quadrajet. I bought mine at Pace for about $435.00. Besides, the Edelbrock tech said that the Quadrajet was a far superior carb--I took their advice.

 

[like2drive]

dfw81shark: photovette's got good advice on the carb, I might pursue that myself, but I wonder if there will be complications since ours are '81s vs. photovette's 1977? What i really wanted to say also, is there should be 3 shifts, (or four speeds) if the lockup us working. I've found, (and continue to find), reasons for the lockup not to work. The 80 in California, and 81 all had th350C (for computer) auto trans. (unless of course you had the 4speed) The lockup depended on temperature, computer, and just yesterday found out a speed sensor on the back of the speedometer(?) If you are not getting lock-up, it would be worth it to correct this to improve gas milage and less strain on the engine. I'm probably going to switch to one of the overdrive trannys: the 200R4 or the 700R4. If I wasn't , I would probably bypass all of the above problems with a B&M harness that costs about $160 at Jegs.