Originally published in Muscle Mustangs & Fast Fords

Starting Point: Building the Normally Aspirated Foundation

The power output of any forced-induction engine is the result of boost pressure multiplied by normally aspirated output. A 300 hp engine at 14.7 psi of boost produces roughly 600 hp. A 500 hp engine at the same pressure reaches 1,000 hp. The math is straightforward. Getting the engine to cooperate in the real world requires more precision.

This 347ci stroker was built to maximize normally aspirated output before adding boost. The short-block used an aftermarket block with forged internals: a 4340 forged stroker crank and 5.40-inch rods from Speedmaster, paired with forged flat-top pistons from JE. A Crane hydraulic roller cam provided a .579/.595 lift split, a 236/244 duration split, and a turbo-friendly 114-degree lobe separation angle. AFR 205 Renegade heads delivered 330 cfm of airflow with a thick deck designed to seal under boost.

Topping the assembly was a Parker Funnel Web intake fed by a Holley 950 Ultra HP carburetor, with an MSD ignition, Milodon oiling system with windage tray, and Lucas synthetic oil completing the package. On the dyno, the normally aspirated 347 produced 519 hp at 6,900 rpm and 434 lb-ft at 5,400 rpm.

The Problem: T3 Turbine Housings and Uncontrollable Boost

The engine was originally configured to test a CXRacing 5.0L twin-turbo kit. The kit was comprehensive - exhaust manifolds, turbos, intercooler, wastegates, stainless exhaust, aluminum intake tubing, and all necessary clamps, couplers, and fittings. However, the supplied GT35-style turbos used T3 turbine housings matched to T3 flanges on the exhaust manifolds.

The small T3 exhaust housings made boost control nearly impossible on the engine dyno. No wastegate spring combination could adequately manage the rising boost curve. Removing the wastegates entirely only reduced boost response lower in the rev range without solving the control issue. The T3 housings were too responsive and too restrictive for this power level. Engine speed had to be limited to 6,600 rpm to keep boost below 16 psi, producing a less than ideal result.

The Solution: Properly Sized Turbos and Precision Boost Control

Two changes resolved the issue. First, T3-to-T4 adapters were fabricated to step up the exhaust manifold flanges, allowing installation of larger T4-based, 76mm turbos from CXRacing - each capable of supporting over 700 hp. Second, the supplied wastegates were replaced with 45mm Hypergates from Turbo Smart, providing the extra exhaust flow needed for precise boost management via a manual wastegate controller.

With boost under control, the jetting on the CSU blow-through carburetor could be properly dialed in. The Holley 950 Ultra HP was replaced by an 850 model modified by Carb Solutions Unlimited (CSU) with revised boosters, metering blocks, adjustable boost-referenced power valves, and a carb bonnet to direct boost through the carburetor correctly.

With air/fuel and timing curves optimized for race fuel, the twin-turbo 347 reached 17.4 psi and cracked 1,000 hp - producing 1,003 hp and 891 lb-ft of torque. The conclusion was clear: boost without control is worthless.

Dyno Results and Analysis

Graph 1: HP and Torque Curves - GT35 Style vs. 76mm

With the T3-based GT35-style turbos, the 347 produced 919 hp and 731 lb-ft of torque, but the curve shape told the real story. Eliminating the wastegates was the only way to achieve boost response, and even then, boost rose rapidly at higher rpm. After installing T3-T4 adapters, 76mm T4 turbos, and Turbo Smart wastegates, boost control was fully restored. The result was not just more peak power - it was more power everywhere across the curve.

Graph 2: Boost Curves - GT35 Style vs. 76mm

The T3 twin-turbo configuration started at just 2.5 psi, climbed slowly, then rose rapidly at the top of the rev range to 15.5 psi before the test was shut down. By contrast, the T4 configuration started at 14 psi, peaked at 18 psi, then settled to 17.4 psi at the power peak - a flat, controlled curve that allowed safe, repeatable tuning.

Build Details

The twin-turbo, carbureted 347 on the dyno with boost fully under control.

The 347 short-block featured forged flat-top pistons from JE. The 10.8:1 static compression ratio was on the high side for turbo use, but it improved normally aspirated output. For street driving, this combination requires high-octane fuel or reduced timing.

The bottom end: a 4340 forged crank and 5.40-inch rods from Speedmaster. Milodon main studs were installed to accommodate the windage tray, requiring bottom-tapping of the main stud holes prior to assembly.

Oil control is critical on a turbo engine. A complete Milodon oiling system - high-volume oil pump, windage tray, and pickup - handled the job.

The Crane hydraulic roller cam: .579/.595 lift split, 236/244 duration split, and a turbo-friendly 114-degree lobe separation angle.

AFR 205 Renegade heads secured with 1/2-inch ARP head studs and Fel-Pro 1011-2 head gaskets. The heads delivered 330 cfm of airflow with a thick deck to ensure sealing under boost.

The AFR Renegade heads featured a 2.08/1.60 stainless steel valve package with a spring package rated for the near-.600-inch lift cam profile.

On the dyno with a Funnel Web intake, Holley 950 Ultra HP carburetor, and 1-3/4-inch Hooker headers, the normally aspirated 347 produced 516 hp at 6,900 rpm and 434 lb-ft at 5,400 rpm.

For turbo use, the Hooker headers were replaced with stainless turbo manifolds from CXRacing.

The T3-to-T4 adapters fabricated to allow installation of the larger 76mm turbos.

The 76mm CXRacing turbos with 1.15 AR T4 turbine housings replaced the original GT35-style units.

The twin-turbo kit's air-to-air intercooler with dual-in, single-out end tank configuration. Aluminum tubing was modified to suit the carbureted application.

The CSU blow-through 850 carburetor with revised boosters, metering blocks, adjustable boost-referenced power valves, and a carb bonnet for proper boost direction through the carburetor.

With the adapters and 76mm turbos installed, the carbureted 347 jumped from 919 hp and 731 lb-ft to 1,003 hp and 891 lb-ft. On a turbo engine, boost control determines everything.