700R4 Overdrive Transmission Rebuild - Overachieving Overdrive

No matter how impressive the parts, paint, or power numbers, a Pontiac that can't move under its own power is a lawn ornament. Although having a reputation for being reasonably reliable with the stock engine output of 190 hp, we knew we were tempting fate with the stock 700-R4 trans in our '88 GTA by subjecting it to repeated dyno flogs at 300 rwhp. We hoped to delay a trans rebuild until we had some respectable dyno sheets, but recent testing on MD Automotives' Westminster, California, chassis dyno resulted in the trans spitting fluid from the breather; then Second, Third, and Fourth gears mysteriously vanished like intelligent conversation at a political rally. It was decision time. Should we beef up this transmission or seek other alternatives?

For a street-strip car, the 700-R4 has three distinct advantages over replacing it with a more robust Turbo 400 three-speed automatic, like many Third-Gen racers have done. First, it has a 3.06:1 First gear, thus a 700-R4-equipped vehicle with 3.55:1 rearend gearing accelerates like a Turbo-400 (2.48:1 First gear) car would with a 4.38:1 rearend. Second, the 700-R4's 0.70 (Overdrive) Fourth gear provides a cruise rpm equivalent to a Turbo 400 (1:1 Third gear) with 2.48:1 rearend gearing-the best of both worlds. Third, the 700-R4 has a lockup torque converter feature that locks the drive and driven portions of the converter with a clutch above 38 mph, completely eliminating slippage and the significant heating of the fluid from higher-stall-speed torque converters.

We decided to retain the 700-R4, but seeing our Pontiac loaded onto a tow truck left us with the sinking feeling that a garden-variety rebuild wouldn't survive the planned dose of (hopefully) more power and yet more chassis dyno drubbings. Thus, we sought professional help.

Bowtie Overdrives' RecommendationsBowtie Overdrives (BTO) specializes in building high-performance hydraulic and electronically controlled GM overdrive transmissions. After hearing we were going to continue looking for more power above the current 450 lb-ft of engine torque output, and that we also intended to abuse the Bird on the dragstrip, BTO's general manager Chris Gardner suggested swapping in the company's Level 3 Extreme Duty trans ($1,595 outright, no core charge) for our broken 700-R4, along with a 2,600-stall-speed converter ($569, 2600 Extreme Duty) with built-in lockup clutch. This combination promised neck-snapping launches, solid shifts, and increased durability with minimal degradation of street driveability.

The Autopsy And ReanimationTransmission removal was quick for the BTO boys, taking about 20 minutes. Upon disassembly, we found a number of issues that caused its demise. A primary telltale sign was the burnt, brownish fluid and its color-matched varnish cooked onto the transmission's internals. Gardner says overheating is the No. 1 reason for transmission failures. He advocates not only an external trans fluid cooler, but a temperature gauge as well. He warns that automatic transmission fluid (ATF) starts to degrade (burn) above 185 degrees F and loses its ability to lubricate and flow properly, thus dramatically reducing transmission life. He also states that virtually all the heat comes out of the torque converter, and the higher the converter stall speed, the more heat it creates before it locks up.

Our 700-R4's unwillingness to shift seemed to be due to a sticking governor, which was coated by burnt trans-fluid varnish. BTO's first priority is to sanitize or replace all internal parts in every transmission the company builds. Even the trans cooling lines and radiator heat exchanger are hooked up to a cleaning/purging machine.