Upgrading To A 700R4 Overdrive Transmission With TCI Auto

If you’ve been following along with my C10 Cheyenne upgrades, you know that our last installment saw a Moser Engineering 12-bolt get stabbed under the frame. If you are interested, you can see the install by clicking here.

As I stated in that article, I knew the 3.73 gears were a little steep for a daily-driver truck. However, I was planning ahead for the impending install of an overdrive transmission. The hydraulic gear changer of choice for me? A TCI Auto StreetFighter 700R4. The time arrived, and the transmission is now installed.

The forward clutches, forward race, and overrun clutch hub install. Stock 700R4s only have six clutch plates in the Third to Fourth gear shift pack. The TCI StreetFighter has nine and improves Third and Fourth gear clutch life as well as the Second-to-Third shift quality.

While I could have rebuilt a salvage yard transmission, when you figure in the cost of the core, rebuilding it, along with a new torque converter and fluid, the cost is not much less than the StreetFighter transmission, which is a performance-built unit. In fact, it is designed to withstand most non-supercharged engines with upwards of 775 horsepower. I don’t “plan” to exceed the 775hp number, so I decided, why not get the performance-oriented unit instead of a stock-rebuilt box?

The StreetFighter transmission “kit” comes with TCI’s manual/automatic valve body, which allows you to manually shift the vehicle into each gear, or leave it in Drive for fully automatic function. TCI’s 700R4 StreetFighter also features what TCI calls a Constant Pressure Valve Body (CPVB) to ensure the clutches and bands are applied firmly to eliminate slippage.

700R4

Installing the rear support that functions as a roller clutch and pressure plate for Low and Reverse.

Fluid pressure settings are crucial in a 700R4, and the CPVB ensures the internal line pressures stay at an acceptable level — regardless of the TV cable adjustment. While the TV cable still needs to be connected for proper transmission shifting, the CPBV provides more freedom when setting part-throttle shift points. With the CPBV, line pressure is fixed with no chance of encountering a low-line-pressure condition, and the shift characteristics are greatly improved. In other words, the CPVB has all but eliminated the possibility of burned Third- and Fourth-gear clutches caused by an incorrectly adjusted TV Cable. More on that later.

In this day and age, I could have just as easily selected an electronically controlled 4L60 or 4L80. But instead, I chose the 700R4. My reasoning? There are fewer electronics to contend with. By not choosing the electronic 4L60/80, I saved another several hundred dollars spent on a transmission controller and wiring harness. Going analog was a personal choice, but you might be more inclined to choose one of the electronic-overdrive units instead. Luckily, TCI Auto can supply whichever unit you want to use.

700R4

The next item is the Input Housing Assembly. This contains the Third and Fourth gear backing plate, clutches, and steel plates.

All 700R4s have an overall length of 30 3/4 inches (except for Corvette units, which measure 29.875-inches overall). That 30 3/4 inches is the same overall length as the Turbo 350 I would be removing. That means cutting the driveshaft is not necessary. Just a head’s up, the Turbo 350 could have one of three different length tailshafts (6-, 9-, or 12-inch). If your Turbo 350 has the 9-inch tailshaft, the 700R4 transmission will be the same length, and you will not need to cut or lengthen the driveshaft.

700R4

The new TCI pump assembly provides superior performance over not only the factory 10-vane systems but many of the 13-vane aftermarket kits as well. Machined from 8620 billet-steel a substantial amount of strength is added, so the rotor kit installed in this pump makes it the best transmission pump you can purchase for your late-model GM transmission.

Speaking of the driveshaft, the 700R4 utilizes an output shaft with a 27-spline count — which is the same as the Turbo 350. The truck’s driveshaft is completely interchangeable.

The TCI 700R4 comes with a Constant Pressure Valve Body. This alleviates common transmission issues by maintaining adequate line pressure at all times. While the TV cable still needs connecting to achieve proper transmission operation, the Constant Pressure Valve Body provides more liberty when setting part-throttle shift points.

One thing I did have to modify was the transmission crossmember. The transmission mount on the 700R4 is further back on the housing than the Turbo 350, so I did need to relocate the transmission crossmember. I moved it back 1 3/8 inches and drilled new holes in the frame to attach the crossmember. Also, the transmission pan of the 700R4 is larger and touched the crossmember. To remedy this, I took a die grinder and notched the crossmember to make room for the pan. I did reuse the transmission mount as there was nothing wrong with it.

The Turbo 350 I removed has a 2.52:1 First-gear ratio, while the 700R4 has a 3.06:1 launch-initiating ratio. The latter transmission’s higher ratio will make First-to-Second gear shifts occur somewhat quickly. But, the replacement transmission gives me the lower gear I want for both initial starts while towing, and the occasional hard launch from a red light or 1/4-mile starting line. (Spoiler alert: not only do I now get harder launches with the swap, but I have my proverbial cake while I enjoy a cruise at 65mph with the C10’s tachometer registering a leisurely 2,200rpm). Not only does the 700R4 give me low-RPM cruising while in Overdrive, but the torque converter lock-up function is an added benefit.

The Great Lock-Up

A torque converter is nothing more than a complicated fluid coupler. This coupling device transfers engine power to the transmission, and finally, the rearend. The basic function of a torque converter is to multiply the engine’s torque.

Unfortunately, torque converters are a truly inefficient connection, as “slippage” occurs inside the unit. This slippage not only misplaces a certain amount of power, it also creates unwanted heat. To regain that efficiency, a lock-up clutch is introduced.

The TCC lockup system is installed in all TCI Auto 700R4 transmissions. Only a single-wire hookup and vacuum line connection are required. The red wire must be connected to a 12-volt switched source. If you desire more control over the TCC, you have two options: 1) manually lock up the torque converter in Second, Third, or Fourth gear by running the green wire to a grounded dash-mounted toggle switch. 2) shut down the TCC system through a second dash-mounted toggle switch mounted between the red wire on the TCI external harness and the switched 12-volt source. That allows the user to manually turn the lock-up system completely on and off.

The 700R4 torque converter’s lock-up (clutch) system creates a physical connection between the impeller and the turbine, transforming the torque converter into a mechanical coupling. This internal clutch locks the converter when engaged by the Torque Converter Clutch (TCC) solenoid (a pressure switch inside the transmission to engage the lock-up). When the TCC is activated, fluid pressure is routed through the transmission’s input shaft into the converter clutch, thereby engaging the lock-up function.

700R4

If the Turbo 350 you are replacing has a 9-inch tailshaft, driveshaft length will not be an issue. However, if your 700R4 is a pre-1984 model, the spline count will be different, requiring a different driveshaft yoke.

TCI Auto ships all of its 700R4 transmissions with an already-installed lockup wiring kit. This kit allows automatic activation of the TCC when the transmission is in Fourth gear, and disengages the clutch when accelerating or downshifting.

The universal lockup-wiring kit is connected to an external vacuum switch that automatically locks and unlocks the torque converter when the engine vacuum reaches a certain level (8 to 10 inches of vacuum), and can be adjusted for operation at various vacuum levels. This vacuum operation makes the lock-up function hands-free. However, if wanted or needed, there is also a manual-override option that allows complete control over lock-up.

700R4

The transmission did not come with a speedometer-gear housing or gear, so you will need to acquire one. I found a complete assembly online.

Cable Conundrum

One of the main causes of stock 700R4 failures can be attributed to the Throttle Valve cable (TV) cable. The TV cable on the 700R4 transmission controls line pressure, shift points, shift feel, part-throttle downshifts, and detent (full-throttle) downshifts. If the TV cable is not properly adjusted, it can cause numerous transmission-function problems. Luckily, the TCI unit has a CPVB, so this should not be an issue.

The TV cable is connected to the carburetor and the throttle-valve plunger within the transmission valve body. The connection at the carburetor transfers the movements of the throttle shaft in the carburetor to the TV plunger in the valve body. This movement causes TV pressure to increase in relation to the throttle opening.

The proper adjustment of the TV cable is based on the TV plunger being fully compressed with the engine at wide-open throttle. When the TV cable is properly adjusted, the movement of the TV plunger in the valve body is calibrated to the movement of the fuel-delivery system. This means the transmission will always have the correct oil pressure and shift feel — regardless of engine torque.

700R4

Before installing any new torque converter, you must add at least 3/4 of a quart of fluid so it is properly lubricated during startup. TCI Auto’s Max-Shift synthetic lubricant is designed to help your transmission run cooler, prevent excessive wear, and reduce internal friction. Moreover, intervals between fluid changes can be extended, since the fluid resists breakdown much better than conventional oils.

For some reason, a simple adjustment of the TV cable seems to elude many, and hence, is the biggest reason for most 700R4 failures. While the Turbo 350 utilized a vacuum modulator to control shifts, the TV cable was introduced with the 700R4. It replaces the vacuum-modulator valve as the load-sensing device for transmission operation.

700R4

The pan on the 700R4 is larger than the pan on the Turbo 350, and the crossmember needed some clearancing.

In theory, this is a simple operation. But, with the availability of different aftermarket carburetors, EFI throttle bodies, and various linkage mounts, the opportunity for improper adjustment is a real concern.

The C10 has a Sniper EFI already installed, so I used the throttle bracket that came with the Sniper (left). If you need the bracket to mount the TV and throttle cables, TCI auto has them for most applications.

In the case of my C10, I already have a Sniper EFI installed. This also means I have the Sniper-supplied throttle bracket. Well, most of it anyway. When I installed the Sniper kit, I retained the bracket that supports the Turbo 350 (detent) kickdown cable. This was not a problem with the stock Turbo 350, as it was all factory. However, the TV Cable on the 700R4 is a very finicky item, and connections must follow specific criteria.

“You must be able to get the correct distance from throttle shaft to the cable end,” says Ondra Terry of TCI Auto. “I am not sure anyone can achieve this distance with an intake-mounted bracket. The throw on the cable is going to be off.”

700R4

To get the proper TV Cable adjustment, hold the throttle wide open, pull the cable, and then attach the TV-cable end and secure with the cable lock.

Therefore, I needed to use a 700R4-specific bracket. TCI Auto offers a complete bracket that retains both the throttle and TV cables, but I was able to install the rest of the bracketry that came with the Sniper EFI.

700R4

Although the cable will physically mount in the bracket for the Turbo 350 detent cable, do not use it. According to TCI Auto, the geometry will not be correct, and transmission failure could result.

If ordering just the TCI Auto transmission, it does not come with an installed TV cable (many purchasers are replacing a worn transmission and already have the cable). Still, a universal cable is available through TCI. My upgrade was a complete conversion to overdrive, so I ordered the StreetFighter Package, which comes with not only with the transmission, but also, TCI’s Max-Shift fluid, a StreetFighter torque converter, and the Universal TV cable.

To get the cable close to properly adjusted when using the appropriate bracket, first, position the throttle to wide open. I held the throttle open with springs. With the cable housing locked in the bracket, you will notice the cable has a D-shaped button where it attaches to the bracket. This button allows slight movement of the cable within the bracket without removing it from the bracket. When you depress the D-shaped button, you can slide the cable housing back and forth.

To begin, press the D-shaped button and position the cable housing at the middle of the housing’s available travel distance and then let go of the button. This will give you some adjustment of the cable when finally connected to the throttle linkage. To join the cable to the throttle arm, TCI Auto supplies different connectors for you to use depending on the application. The Universal Cable Kit comes with explicit instructions about how to connect and adjust the TV cable to your carburetor or throttle body. Follow them.

700R4

As a reference, TCI-approved pressures (at 2,000rpm). Confirming the line pressures in your transmission is the best way to adjust the TV Cable confidently. This will require you have a fluid-pressure gauge and a way to connect it to the transmission port location on the driver’s side of the case.

A properly shifting 700R4 should shift from First to Second gear between 15 and 20mph, from Second to Third at 25 to 30mph, and from Third into Fourth at roughly 40 to 45mph. You can raise the shift point by pushing the D-shaped button and moving the cable housing toward the firewall. This will increase throttle pressure, and also make the “kickdown” more responsive.

Only move the cable housing adjustment a small amount at a time (one click at a time). A slight change can make a world of difference. To lower the shift points (the pressure) and make the “kickdown” function less sensitive, press the D-shaped button and move the cable housing towards the front of the vehicle.

The Perfect Cruiser Lives

Unlike a new rearend, there really isn’t a required “break-in” requirement for an automatic transmission — it’s not gear driven, it’s hydraulic. That meant there was no procedure to follow before I could really enjoy the benefits of the new overdrive. As soon as I started down the road, I immediately realized I needed to make a TV cable adjustment.

The transmission was shifting too late for my comfort. I immediately pulled over and gave the cable housing a few clicks forward — that made it shift a little sooner. After this adjustment, another “happening” that concerned me while driving was the lock-up mechanism repeatedly bouncing in-and-out at light cruise (around 45mph).

Proper Indication

Pre-1983 C10 trucks did not come with an overdrive (700R4) transmission. That means if you do the swap, your shift indicator on your dash will not register correctly. The quality of any job rests in the details, so you should change the shift indicator to show what gear you are actually using at a given time. Luckily, Classic Industries has new shift indicator assemblies you can get. Swapping the indicator is relatively simple, and can be accomplished in an hour or two. Simply remove the dash bezel, then the screws holding the indicator in place. Next, disconnect the cable going to the steering column, and remove the indicator. Simply reverse the process to install the new indicator.

As I said, the vacuum-actuation switch comes factory preset to operate at roughly 8 to 10 inches of vacuum. If you find the torque converter lockup experiencing what TCI calls “business”(the rapidly locking and unlocking I mentioned) when at light-cruise conditions, it is probably because the engine vacuum is within the window of where the switch turns on and off.

This is an easy fix. All I needed to do, was locate the small Allen-head adjustment screw on the face of the switch. Adjusting the screw clockwise raises the vacuum required to close the switch and counterclockwise lowers it. The adjustment screw is sensitive, so it does not need to be moved very much to make a difference.

The TCI Auto StreetFighter has only been in the truck for a couple of weeks, but I am already enjoying the benefits — probably more than I should. In fact, my wife has already commented about me enjoying the spirited launches from stop signs and red lights a little too much, but the smile on my face each time lets her know it’s no big deal to me.

Article Sources

About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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