How to Modify a 4.3 Chevrolet Motor


Some V6 engines in American cars derived from popular V8s, with the Chevrolet 4.3-liter being a prime example. Essentially a 350 with two cylinders lopped off, this “3/4 small block” draws from its older brothers’ enormous aftermarket support to build the kind of power traditionally reserved for much larger powerplants. This engine has come in dozens of GM cars and trucks produced from 1978 to present. The turbocharged LB4 versions used in the Chevrolet Typhoon/GMC Cyclone are known to produce well over 500 horsepower with aftermarket parts, which is a clear indication of the best possible approach for making power with this engine.

Step 1

Install a set of four-bolt-main steel main caps designed for a 350 small block in place of the 4.3-liter’s center two two-bolt-main caps. This procedure involves bolting the caps to the block, drilling two new holes through the main cap holes and into the block and tapping them for the additional bolts. Complete the bottom end reinforcement with chrome-moly main studs and a stud girdle. Have the cylinders bored 0.030-inch over and have the block align-bored afterward.

Step 2

Install a steel, split-pin, stroker crankshaft with 3.75 inches of stroke and the appropriate steel I-beam connecting rods and forged 9-to-1 pistons to bump displacement up to about 4.65 liters. The resultant and oddly familiar-sounding 283 cubic inch displacement is an awful lot for a V6, so have the assembly balanced to reduce the inevitable shaking. Have the machine shop install and align your piston rings, then install the piston/rods into the block. Bolt them to the crankshaft, install the main caps to hold the crankshaft in place and torque the bolts to spec.

Step 3

Acquire and install a set of true Vortec cylinder heads (casting numbers 772, 140 and 113). Vortec heads flow better than set of ported stock heads right out of the box, with another 100 horsepower in potentia after some port work. The Vortec heads are a direct bolt-on replacement for the stock units; bolt them to the block using a set of chrome-moly studs and Cometic head gaskets to seal the chambers. Install a reverse-flow water pump and connect the Vortec steam vents to your coolant overflow tank and you’ve got a perfect set-up for step five.

Step 4

Install a camshaft with about 0.280-inch lift at the cam lobes to yield 0.445-inch lift at the valve when uses with steel, roller-tipped 1.6-to-one rocker arms. Vortec heads work best at under 0.500-inch lift, and flow actually begins to drop off over 0.550. That aside, the 4.3-liter’s hydraulic lifter link bar won’t allow you to run a cam with more than 0.350-inch lift at the lobes. Shoot for a cam with about 216 to 220 degrees of duration at 0.050-inch lift. If you can find one, look for a split-pattern camshaft with two degrees more duration on the exhaust side. The cam will slide into the block without any machine work, but you’ll need to double-check that the cam’s drive gear is compatible with the distributor you’re using.

Step 5

Install a pair of turbochargers. These blowers are the reason for the fairly tame split-pattern camshaft, relatively low compression ratio and reverse-flow heads. Don’t use just any old boring turbo for your super-six–use a pair of gigantic turbos designed for the 6.0-liter Ford Powerstroke diesel. Normally, even a single turbo like this wouldn’t begin to spool up until 4,000 rpm on the relatively tiny 4.3-liter, but the tough Powerstroke turbo uses a variable-vane exhaust turbine that will allow you to get a pair of these massive huffers spinning at well below 3,000. Top end power potential for a pair of them comes in at about 1,100 horsepower, which should suffice. Bolt an aftermarket turbo header to each side of the block and bolt the turbos to the manifolds. You don’t need a wastegate; the variable vane turbo controller will modulate boost without one.

Step 6

Bolt an aftermarket LB4 turbo manifold to the heads, and drill and tap the manifold runners for an additional injector per cylinder. Bolt an aftermarket crank-trigger ignition system to the crankshaft to detect crankshaft position. You’ll need to buy an aftermarket fuel management unit to network the twelve injectors and crank-trigger system, but this arrangement will give you ultimate control over your engine at all rpm ranges.

Step 7

Plumb your turbos to the intake manifold via the largest intercooler that will physically fit in your chassis. The intercooler installs just like a radiator in that it bolts to the radiator core support, but you’ll need to purchase a front bumper cover with a lower intercooler duct to provide adequate airflow. Connect the turbo’s to the intercooler with aluminum tubing, silicone tubing connectors and V-band hose-clamps to ensure airflow through the system.


  • Consider looking for used turbos to save some money; the problem-stricken 6.0-liter Powerstoke was likely to blow up long before its turbo wore out, so used turbos shouldn’t be hard to come by.

  • To make your truck cheaper to run and more versatile on the street, connect your second set of injectors to a alcohol or methanol tank and program them to kick in under boost only. This way, you can run on 93 octane pump gas most of the time without having to dial boost back to prevent engine detonation.

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