Danny Asks:
I have an '89 Pontiac Firebird. I'm going to swap its factory 305 LO3 TBI engine to a 350ci/385hp Fast Burn crate motor. What is the best option: Keep the factory 700-R4 transmission or swap to a TH350 with a 3,000-rpm-stall-speed converter? How can I do the swap and keep the factory torque arm?
Marlan answers:
The Chevrolet Performance 385 Fast Burn crate motor (PN 19201331) is a non-computer-controlled small-block that's delivered with a traditional carburetor and HEI distributor, so I'm assuming you'll be trashing the factory computer (Electronic Control Unit or ECU). That calls for extensive rewiring, so I highly recommend springing for the complete factory service manual and wiring schematics (Helm is one source).
Your trans choice really boils down to intended usage combined with any budget restraints. By 1989, the 700s' early durability problems had been resolved (especially the versions used in top-line performance apps like the Trans Am, Z28, and Corvette). Many even stronger hard parts from the 700's beefed up descendant—the late-'90s-and-newer 4L65E—can be retrofitted, and there are plenty of Godzilla parts and pieces available from aftermarket sources as well. So, assuming your existing 700-R4 is in good condition, there's no reason not to use the trans for a basically street-driven combination. You get a lower First gear (3.06:1 versus the TH350's 2.52:1) plus a 0.70:1 overdrive. Chevrolet Performance has a kit that lets you run the 700-R4 without an ECU. Other noncomputer TCC lockup solutions are available from leading aftermarket trans specialists.
B&M’s TH350 third-gen Camaro/Firebird changeover kit includes a mount adapter and a torque arm mounting bracket that uses the TH350’s existing extension housing-to-case mounting holes. Besides the kit, you’ll also need to purchase a different GM outer torque arm isolator bracket (part numbers vary, see B&M’s instructions included with the kit).
On the other hand, if your 700 is toast, a beefed TH350 may be more economical. It's also lighter, with a more consistent ratio spread between each gear. This is an advantage if drag racing is the primary intended use. Note that the Fast Burn 385 has a fairly mild hydraulic roller cam (PN 10185071, 0.474/0.510-inch lift, 208/221 degrees of duration at 0.050, 112-degree lobe-separation angle), so your projected 3,000-rpm-stall-speed converter is overkill. In fact, the stock converter should work fine; certainly you won't need anything with a stall speed greater than 2,300 rpm.
The stock Camaro/Firebird third-gen torque arm is bolted to cast-in mounting bosses on the lefthand side of the transmission extension housing. A TH350 lacks such mounting provisions, but B&M offers a TH200/700-R4 to TH350 changeover kit (PN 30299) that comes with a bolt-on torque-arm mounting bracket for use with the stock GM torque arm. Performance Automotive and Transmission Center (PATC) sells a more racing oriented conversion that includes a new crossmember (NHRA-legal driveshaft loop optional) plus its own adjustable torque arm. This setup (PN 61XXX) is said to accommodate the short-tail TH400, a TH350, or a 200-4R.
Connect an old-school auto or manual trans directly to a late GM electric speedometer without an ECU. As long as the trans still uses a traditional driven-gear with a stem and a sleeve with 7⁄8-18 thread, Stealth Conversions’ reed switch should bolt right up. Swap driven and drive gears to dial in the speedo just like the old days.
Other parts needed to complete the TH350 swap include a TH350-style detent (kick-down) cable and a vacuum modulator appropriate for your application. B&M offers a fully adjustable cable (PN 30287) and a vacuum modulator (PN 20234). The driveshaft must also be lengthened.
Your Firebird has an electronic speedometer (no stock mechanical speedo cable). If keeping the 700-R4, retaining the GM factory Vehicle Speed Sensor (VSS) screwed into the side of the trans and the VSS Speed Buffer behind the instrument panel should still allow the speedo to function, even if the ECU itself is no longer present.
An old-school TH350 never came stock with a VSS or modern electronic speedometer. Fortunately, Mike Knell's Stealth Conversions sells a four-pulse reed switch (PN 4PRS) with standard GM cable-driven speedometer 7/8-18 hookup thread on one end that converts an old-school transmission's mechanically driven speedo gear output to the GM electronic 4,000 pulse/mile square-wave Vehicle Speed Sensor (VSS) signal needed to run the electronic speedometer. In this scenario, speedo calibration is handled by changing as appropriate the drive or driven gears at the transmission end and/or using a custom-built in-line ratio adapter box. Knell says his part should be compatible with most GM electronic speedometers at least through the late '90s. However, there can be compatibility issues with antilock brake systems and/or electronic cruise-control setups.
Chevrolet Performance kit PN 24502513 allows running a 700-R4 or 4L60 (non-“E”-suffix version) without an ECU. It includes a TV spring for carbureted engines, a normally closed Fourth-gear TCC (transmission controlled clutch) pressure switch for locking up the trans, and torque-converter wiring connectors. John Elway Chevrolet is one source.
The Chevy TH350 driven gears are the same as those in most older, conventional Chevy manual and automatic transmissions except the TH400; select an appropriate driven and drive-gear combo as needed to dial in speedometer accuracy. Sources for drive gears, driven gears, and ratio adapter boxes include Stealth Conversions, Lance Martin Speedometer, and PATC.
Knell's device is nonpolarized; it has two white wires coming out of it, and it makes no difference which one will ultimately become the ground and which is used as the speedometer's sensing wire. Connect one of the two white wires to a convenient chassis ground near the trans; on your '89 Firebird, splice the other white wire to the stock light-green/black-stripe wire coming out of instrument panel connector A, cavity 14. Previously, the other end of this stock wire connected to the speed buffer, which is no longer used or needed with the TH350. Happy motoring!
