diehrd

In a perfect world maybe .... But your carb is going to be the issue , at 1200 rpm your hardly off idle yet suddenly that rpm is 45mph. I have the 3.55 rear gears and the 700R4 and it really is a nice combo . It took me a while to dial it all in but being set as it is now I have a really great ride.

And to my knowledge no O/D was put on cars with out some type of fuel injection. And I think most rpm calculators may not be considering the lock up converter.

redwingvette

I do. I have one on my 73 with the 200-R4. There are two types, one for mechanical speedo and one for electric speedo. My is the mechanical type. It comes with a short speedo cable that attaches to the trans in front of the stock cable. Two wires come out of the sensor and attach to the control module. The module has a control **** that allows you to set the speed you want the controller to engage.
B&M controller from Jegs.

scottyp99

How do you like it? Have you experienced any issues with it? I'm thinking I could mount the speed control module in the compartment where the jack is kept, I don't think it would be all that important to adjust it while driving. What do you think?

Scott

scottyp99

I think the rpm/speed calculator just crunches numbers and does a mathematical equation, and the rpm number that spits out at the end is a theoretical one that does not take any kind of drivetrain slippage into account.

Now, if I decide to use TCI kit, and wire a switch to allow lock on command, (as long as your in 4th) or used the B&M kit, with a minimum speed setting, what would happen when shifting into 4th at lower speeds? Would the torque converter allow some slip at that low rpm, and make it somewhat driveable? And what would be the effect of driving around at low speeds without the lockup engaged? Will it overheat the transmission?

Scott

lionelhutz

That's the part I was wondering too.

I set my lockup to only engage above a certain road speed in 3rd or 4th (different for each gear). Then, I have it set to disengage when the MAP rises (or manifold vacuum drops) a little above the cruising MAP signal level at each speed following a curve MAP vs speed from 40mph up. It will drop the lockup with a little extra throttle and come up on the stall to mildly accelerate then give even more extra throttle and it kicks down and really goes. It also will not lock below about 40mph so it doesn't lug the engine.

That's why I suggested both a vacuum switch and speed sensing since that best emulates what the computer allows. You pick a minimum speed so you don't lug the engine and then you have the vacuum switch that drops the converter out if you step on it to accelerate. Of course, adding all these parts is hell on the limited budget.

I used to have an older TBI system with a 700R4 that locked by vacuum only. Just as others have predicted, it had to be downshifted to drive around in town because it would lug the engine. The 700R4 will shift into 4th at low speeds when using little throttle and then it locks up and lugs the engine.

scottyp99

I am beginning to think about the TCI kit, with a switch on the dashboard, and a brakelight switch. I am concerned about the effects of driving the 700r4 in 4th gear without the lock up engaged, even at low speeds. Is it a problem? Or is it just extended time at highway speeds that are a problem?

Lionelhutz, I understand what you are getting at now, and it sounds like it would work swell. I could do the same thing, and maybe save a little green, by using the TCI kit with a switch on the dashboard, just not as convenient. But I think I can handle pushing a button.

Scott

scottyp99

Looking for someone with a 3.07 diff gear and a 700r4 to add some info.

Scott

redwingvette

I like the B&M system and have had no issues with it. I set mine to engage above 45mph but if I am just driving local I keep the trans in 3rd. Even when I use OD in local traffic it does not have any issues that I can tell from the drivers seat.

lionelhutz

If you search around you should be able to find a vacuum switch for much less than the TCI kit costs.

diehrd

With a low gear you can also go with a NON-Lock up system .. And keep her in 3 until you hit the open road and then shift up into O/D.

It is not that you can not put the O/D in a car with a carb and a low gear you can as long as your aware of its driving limitations and work with them it will be fine, but to me with such a low gear already and the infrequent amount of time you may use O/D I would stick with a solid well built 3 speed with lock up unless you plan for steeper gears in the future.

hugie82

My 82 is close with a 2:78 rear and a 700R4 . I can tell you it rips off the line because of the very low 1st gear and overdrive has me doing 80mph at 2000 rpm exactly
I would love 3:08 because the 2:78 has the 700r4 jumping out of overdrive often between 60-75 mph. I think 3:08 would have it in the sweet spot in every gear

lionelhutz

I had a S-10 with 4.3L engine and 3.08 gears. It worked fine with those gears and it's had less engine and likely more weight than your C3. It was a 4L60e so the shifts were more controlled, but a decently set-up 700R4 should work too.

scottyp99

I think I'm going to order the TCI kit. I'll wire it so that it goes through a switch on the brake pedal, and another switch on the dashboard. When I get out on the highway and up to speed, I'll just flip the switch. I could probably save a few bucks by scrounging up a vacuum switch, but the kit comes with instructions, and it'll be convenient. Thanks everybody for the help, may it be returned to you seven-fold!

Scott