Words and Photos By Richard Holdener

The Exit Side of the Power Equation

Airflow into an engine must find a way out. The exhaust system is the exit side of the performance equation, and like the intake manifold, headers play a measurable role in tuning the power curve. Primary length and diameter do more than change flow rates - they scavenge exhaust from the combustion chamber, improving intake efficiency in the process.

Headers deliver this tuning effect, but what about the rest of the system? For most street cars, running open headers is impractical and illegal. The real question is how much power a complete street exhaust costs on a high-output application. That is what Project LS Ex was designed to measure.

The Test Motor: 383 LS Stroker

Open headers - or headers feeding short collector extensions - represent the maximum in exhaust flow. Match the right primary length, diameter, and collector size to the engine and power is maximised. But open headers are limited to the occasional strip pass. For a street-driven car, the noise alone rules them out.

This test was designed not to confirm that open exhaust makes more power, but to quantify how much a full exhaust system costs on a hot street combination. The procedure: build a suitable engine, install a complete exhaust system, and run it against an open-header baseline. Westech Performance's engine dyno facility provided the space to run a full-length exhaust.

The 383 started with a 4.8L/5.3L iron block, bored and stroked with a Scat forged 4.0-inch crank, Speedmaster 6.125-inch rods, and JE forged flat-top pistons with Total Seal rings. Fel Pro MLS head gaskets and ARP head studs completed the bottom end.

The cam was a hydraulic roller from COMP Cams - the 281LRHR13, with .617/.624 lift split, 231/239-degree duration split, and 113-degree LSA. A strong match for the stroker displacement.

The factory damper was replaced with an ATI Super Damper. All nine retaining bolts were torqued to spec prior to testing.

The stroker displacement required serious airflow, so a set of CNC-ported GenX 225 heads from Trick Flow Specialties was installed. According to Trick Flow, the GenX 225 heads flow enough to support over 660 hp on the right application.

Supporting Components

The complete test combination included the Scat crank, Speedmaster rods, JE pistons, COMP cam, TFS GenX 225 heads, and a FAST LSXR induction system. Engine management was handled by a FAST XFI/XIM system with 75-pound FAST injectors and a 102mm Big Mouth throttle body. Billet fuel rails, ARP head studs, and Fel Pro MLS head gaskets were also present. All three exhaust configurations were run with 1 7/8-inch Hooker long-tube LS-swap headers.

The FAST 102mm LSXR induction system included the intake, 102mm Big-Mouth throttle body, and billet fuel rails.

Fuel delivery came from a set of FAST 75-pound injectors.

The FAST XFI/XIM management system controlled air/fuel and timing curves, which were held constant across all three exhaust configurations.

All three exhaust systems were tested with 1 7/8-inch Hooker long-tube LS-swap headers. Three-inch collector extensions (17 inches long) were required for dyno fitment and were shared across the open-header test.

Test 1: 2.5-Inch Hooker Blackheart Exhaust

The first configuration was the complete 2.5-inch stainless steel Blackheart system from Hooker, originally designed for an LS-swap C10 truck application. The system included an X-pipe, a pair of stainless mufflers, and all necessary clamps and hangers. Air/fuel was dialled in via the XFI/XIM, with timing held constant across all three tests.

The complete Hooker exhaust system installed in Westech's dyno facility.

The 2.5-inch Blackheart system from Hooker: dedicated X-pipe, 2.5-inch mufflers, and all mounting hardware for the C10 application.

With the full 2.5-inch system installed, the 383 stroker produced 542 hp at 6,000 rpm and 503 lb-ft of torque at 4,600 rpm.

Test 2: 3-Inch Dyno Exhaust with Muffler

The second configuration was a 3-inch dyno exhaust feeding a straight-through muffler. Total collector extension length was 42 inches (including the muffler) - shorter than the full Hooker system but longer than the open-pipe configuration tested next. Collector length has a tuning effect on the power curve independent of flow changes.

With the 3-inch muffled system, the 383 produced 548 hp at 6,300 rpm and 507 lb-ft of torque at 4,600 rpm - gains of 5-6 lb-ft across the curve and up to 10 hp at 6,500 rpm over the full street system. The larger diameter and shorter length improved flow, but the noise level made this configuration impractical for street use.

Test 3: Open Headers

The final configuration ran the headers into a shortened 17-inch, 3-inch collector extension with no mufflers - a race-only setup.

With open exhaust, the 383 produced 551 hp at 6,300 rpm and 510 lb-ft of torque at 4,800 rpm. Compared to the 3-inch muffled system, the open configuration offered slightly more power in the mid-range and top end but lost ground below 4,500 rpm.

Results: What the Numbers Show

The full 2.5-inch Hooker Blackheart system performed within 9 hp of the open-header baseline at peak. Exhaust flow decreases with tubing length, bend count, and diameter reductions, but the 2.5-inch system delivered strong results on this 542-hp stroker.

The 3-inch dyno exhaust - at 42 inches total including mufflers - added 5-6 lb-ft and up to 10 hp at 6,500 rpm, but would be too loud for anything outside a drag strip. The shift in collector length between the two 3-inch configurations accounted for most of the power curve movement: the shorter collector lost output below 4,500 rpm but gained in the mid-range and at high rpm.

For a street-driven LS build, the 2.5-inch system represents a measured trade-off: 10 hp at 6,500 rpm - a range rarely used on the street - in exchange for a complete, street-legal exhaust that sounds and performs well across the entire curve.