Nov. 01, 1994 By Jay Kopycinski
This information details a V6 conversion performed on a 1985 Toyota SR5 Extra Cab 4x4 truck in late 1991. The truck was originally equipped with a multi-point fuel injected 2.4 liter four cylinder backed by a five speed transmission and the two speed transfer case (2.29:1 low range). Axle gears in the truck are 4.88:1 and the truck has approximately 4" of suspension lift. The engine swapped in is a 1988 Chevy 4.3 liter throttle-body injected (TBI) and the transmission chosen is a late model GM 700R4 4 speed automatic with 30% overdrive. Fourth gear engine speed is about 2500 RPM at 65 mph using 32" tires.

A view of the engine as it sits in the truck.


A 1988 engine was used that could provide both TBI and a single, serpentine fan belt for the engine and all its accessories. The 4.3 liter GM engine provides 165 hp and about 205 ft. lbs. of torque. It uses the same block design as the small block Chevy less two cylinders. Pistons, rods, bearings, etc. are all interchangeable. It also uses roller lifters. The only flaw in this engine is a slight imbalance condition at about 1200 rpm. About 1992 or 1993 Chevy revised this engine by adding a crank driven counter-balance to completely eliminate this slight vibration. The 3.8 liter Buick engine is often used in Toyota conversions. There are many parts available for the Buick motor and it is a bit smaller and lighter than the Chevy V6. However, the Buick has a worse vibration problem and tends to run a bit on the hot side. There is also a Turbo version of the Buick engine, but is best avoided for any application where the truck will have slow or limited cooling air flow.

The small block Chevy V8 is about 4-5" longer than the V6 and weighs about 80-100 lbs. more. The V8 can be used in a conversion but will require firewall modifications due to the added length and the rear mounted distributor. Installing the V8 may also require that the front grill sheetmetal area be modified and the radiator relocated forward somewhat. A stiffer front suspension is also required with a V8 conversion. Any of the above engines will fit well between the Toyota frame rails and should allow use of a regular air cleaner under the stock hood. The one exception is the Turbo conversion which will require a 2-3" body lift in order to clear the air intake components. All conversions need a 3" or more suspension lift in order to have oil pan clearance and retain adequate front suspension travel on a live axle truck. The 1985 and up 5 speed transmissions are supposedly quite strong and able to withstand sane amounts of V6 and V8 power provided a sturdy aftermarket clutch is used. However, rebuild cost for a blown 5 speed transmission is high. Toyota transfer cases are bullet proof and will easily withstand V8 power applications. The Toyota axle shafts are very strong and strength-wise lie somewhere between a Dana 44 and Dana 60.

The weakest links in the front axle are the Birfield joints of the stub axles. One risks grenading these when getting lots of air while running large, high traction tires and then slamming the front end to the ground. The weak link in the rear drivetrain is usually the driveshaft U-joints. These can also be destroyed by violently slamming a truck to the ground while under high power. The only time strength is of real concern in the differential is when 5:1 or greater gears are run. At this point, the number of pinion gear teeth is reduced significantly and ring/pinion strength suffers. Overall, the Toyota drivetrain is very strong and reliable, and can withstand V8 power easily, if not abused. In general, an engine can be legally swapped into a vehicle if the engine is the same year as the truck, or newer, and all stock engine emissions gear is retained. However, swap laws do vary from state to state.


The stock motor mounts were removed from the truck frame and custom motor mounts for the new engine were fabricated from 2" x 3" x 3/16" box tubing. These mate to the stock GM rubber engine mounts and were welded to the truck frame. A single bolt on each side secures the two mount halves together. An out-board stabilizer was also constructed of steel and uses a urethane bushing to prevent the engine from torqueing excessively. An alternative to the above method is to use bolt-in aftermarket mounts or make custom mounts and use aftermarket urethane mating mounts. The urethane would provide a firmer engine mount and remain more stable during hard acceleration.


The transmission is a late model GM 700R4 4 speed automatic that provides 3.08:1 first gear and 0.70:1 fourth gear ratios. It is built with manual 3rd and 4th gear lockup. To tune the shift points to the particular gear ratio/tire size, a TH350 governor cluster was used along with a TH400 governor gear. A stock TV cable was used to connect the transmission to the engine TBI unit. A B&M Megashifter 4 speed floor shifter was mounted to a custom recessed floor mount plate which was in turn screwed down to the floor of the cab. A cable connects the shifter to the transmission and a neutral safety switch was wired in to prevent the starter from engaging in any position except Park or Neutral. The transmission was mated to the stock transfer case using an Advance Adapters transmission output shaft and case adapter. Fluid lines were run to a cooler in the new radiator and then to an additional cooler mounted behind the grill. A dip stick, tube, inspection cover, and TV cable came from a Chevy S-10 were used.


The original Toyota transfer case was used along with the original cross- member. The cross-member was left in the stock location, however, the transfer case was relocated approximately 1 3/4" rearward. (The transfer case does not have to be relocated if a TH350 transmission is used in this configuration.) A custom cross-member extension was fabricated from 6" x 2" steel channel and was welded to the cross-member. The transfer case was then bolted to this extension piece. The floor of the truck cab was cut out to accommodate the repositioning of the transfer case shift lever. The floor plate was modified to fit the new hole in the cab floor. The speedometer cable remained at the stock location in the transfer case.


Relocation of the transfer case required that the rear driveshaft be shortened about 1 1/2". Use of the 700R4 transmission required a new driveshaft made of smaller diameter tubing in order to provide clearance between the driveshaft and transmission pan. (The 700R4 does not have a corner notched pan like the TH350 transmission.) A custom driveshaft was fabricated using a GM type CV joint coupled to a GM/Toyota conversion yoke at the transfer case and the original Toyota yoke at the axle end. Tubing size is 1.25" O.D. x 0.120" wall.


Factory tubular exhaust manifolds from a 1985 full-size Chevy truck were modified to clear the truck frame rails. This involved removing the end flanges and lengthening the tubing sections before reattaching the flanges further down past the frame rails. A Y-pipe was fabricated to join the two sides and a single 2 1/2" turbo muffler added. Also, the pipe boss needed for the EFI oxygen sensor was removed from the original V6 Y-pipe and welded into one leg of the new exhaust.


An Advance Adapters V8 radiator was used in the stock location using the stock mount holes and bolts. (This radiator hangs low on the truck requiring a steel guard for protection. A guard was made from 3" x 3" x 1/4" angle to protect the lower portion of the radiator.) This radiator is expensive, but is a quality copper radiator that drops right in and provides excellent cooling capacity. Almost any other radiator will require cutting of the front cowl sheetmetal and the fabrication of custom mounts. The lower radiator hose is a shortened stock Chevy S-10 hose and the upper hose is a standard part #71428 (application unknown). The original Toyota radiator cap and overflow setup is used on the new radiator. A 17" reverse direction (due to serpentine belt drive) flex fan is used inside a mid '80s Ford Thunderbird 302 fan shroud. In a typical GM application, hot water for the heater core is taken from a tube located at the passenger side rear of the engine. Once the water circulates through the heater core it is replaced in the system via a tube connection at the radiator. For this application, water for the heater core is taken from the water outlet on the intake manifold (the water outlet used is similar to V6 stock but has a 3/4" pipe outlet on one side). Water from the heater core is returned to the cooling system via the tube on the rear of the engine. A 195 degree thermostat is used.


The original Toyota high pressure electric fuel pump was left in the gas tank and a second in-line Walbro electric fuel pump was added slightly ahead of the gas tank. The old Toyota fuel pump is not energized but can be used as a backup pump if necessary. Electric power for the fuel pump is supplied via the engine harness. A large canister type fuel filter was spliced in-line and mounted to the firewall in the engine compartment.


The GM alternator (internally regulated) that came with the new engine was used and was spliced into part of the original Toyota wiring, A new 8 gauge wire was run from the B+ terminal of the alternator directly to the positive battery post. The case of the alternator was grounded using the original black Toyota wire. Terminal 1 and 2 on the alternator were tied to the white (+) and yellow (charge) wires in the Toyota harness.


The GM power steering pump (integral fluid reservoir) that came with the new engine was used along with the original Toyota power steering box. The GM high pressure hose was used by shortening the steel end at the steering box, replacing the tubing flare nut with one from the original Toyota hose, and reflaring the tube end. The low pressure hoses were simply hose clamped and the original grill mounted fluid cooler was retained. GM power steering fluid is used in the system.


A Howell Engine Developments TBI engine harness was used to utilize all the engine sensors and cleanly integrate the new engine into the power system of the truck. This harness retains full function of the GM engine computer and easily mates it to all of the engine sensors, but also allows deletion of the Vehicle Speed Sensor functions and transmissions functions.


The original Toyota V+ battery cable was connected to the starter on the new engine. The ignition switched solenoid line on the truck cannot sink sufficient current to reliably engage the GM starter solenoid. It was necessary to tie the solenoid line to a relay and provide a switched V+ line directly from the battery. Voltage is supplied to the GM coil via the original Toyota harness wire. The new coil was also spliced into the original tachometer wire to display rpm on the factory tachometer. The tachometer was modified to correctly display the V6 rpm by adding a calibrated resistive shunt to the analog tachometer meter. The check engine light wire from the new engine was spliced into the original harness and works with the original in-dash light.


The original Toyota throttle cable is used and a custom attachment was made to mate the cable to the TBI unit. It was possible to retain the function of the original cruise control because the controller receives it's speed signal from the speedometer.


The GM compressor that came with the new engine was used along with all other original Toyota air conditioning components, The Toyota and GM hose ends were mated using various tube fittings and crimp connections. This was done at an AC specialty shop. A GM compressor electrical connector was spliced onto the Toyota AC electrical lines.


The original Toyota temperature sender was threaded into the passenger side head of the V6 allowing the stock gauge to monitor temperature. The original Toyota oil pressure sender was added to the V6 (original location) using a Downey supplied adapter. The stock gauge is used to monitor oil pressure. An aftermarket vacuum gauge was connected to one of the intake vacuum lines of the TBI unit. The stock voltage gauge was retained unchanged. The stock Toyota tachometer was modified and used also.


The engine conversion took place over several weeks and, as in any such swap, there were a few bugs that had to be ironed out. As of August 1996, the truck had 175,000 miles on it and 62,000 miles on the conversion. The engine and drivetrain has performed very well and provides a significant increase in the driveability of the truck, both on and off highway. The truck generally runs cool and mileage varies from about 16-17 around town up to as much as 20-21 on the highway, with the truck now running 33" BFG Mud Terrains. In the last six years, the truck has overheated twice, but was quickly shut down as soon as the guage needle went into the red. When summer temps here in Phoenix near 115 degrees, the truck cannot sit idle with the air conditioning on for more than a few minutes. The temp will slowly rise unless the truck starts moving again or the air conditioning is turned off. Newsletter
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