Words and Photos By Richard Holdener

We all know when it comes to performance, the airflow that goes into your motor must also find a way out. The exit part of the performance equation is where the exhaust system comes into play. Like the intake manifold, the header plays an important part in tuning the power curve, as primary length and diameter offer more than just changes in flow. They actually help scavenge exhaust flow out of, and thereby improve intake flow into, the combustion chamber. We all know headers offer this tuning effect, but what about the rest of the exhaust system? For most street cars, it is impractical, not to mention illegal, to run open headers and/or un-muffled exhaust on the street. The question now becomes, are we loosing power with the installation of a typical street exhaust system? To find out, we set up Project LS Ex — with the Ex obviously short for exhaust.

It should come as no surprise to find out open headers, or even open headers feeding short collector extensions, offer the ultimate in exhaust flow. Combine the proper header components (proper primary length, diameter, and collector size) for your engine configuration and you will maximize power, which is all well and good on the dyno, but what about the street? Open headers might be a possibility for an occasional street race or strip action, but the noise is hardly acceptable for a daily driver.

This test was designed to find out not if an open exhaust made more, but how much more, or to phrase it another way, how much a full exhaust might cost you on your hot street application. The test procedure was a simple one, build a suitable motor (check), order up a complete exhaust system (check), and run it against an open-header system (check) to determine the difference in power. Luckily, our dyno facility (Westech Performance) featured an engine dyno room capable of running a full-length exhaust.

The recipe for our 383 was to start with a 4.8L or 5.3L iron block, then add a forged 4.0-inch stroker crank from Scat combined with a set of 6.125-inch rods from Speedmaster, followed by JE forged, flat-top pistons (and Total Seal rings). Note also the use of Fel Pro MLS head gaskets and ARP heads studs.

The 383 stroker featured a hot hydraulic roller cam from COMP Cams. Perfect for our stroker, the 281LRHR13 featured a .617/624 lift split, a 231/239-degree duration split, and 113-degree lsa.

 Not wanting to rely on the (well-worn, high-mileage) factory damper, we stepped up to the ATI Super Damper. We made sure to torque all nine of the retaining bolts to the specified torque settings prior to use.

 Obviously, our stoker motor required plenty of airflow, so we installed a set of CNC-ported GenX 225 heads from Trick Flow Specialties. According to Trick Flow, the Gen X 225 heads flowed enough to support over 660 hp on the right application.

The test motor was our tried and true 383 LS stroker that featured an iron block (4.8/5.3L) bored and stroked to accept a Scat crank, Speedmaster rods, and a set of JE forged pistons. Toss in a hot COMP cam, TFS Gen X 225 heads, and a FAST LSXR induction system and you have the makings of a healthy test mule. All three of the different exhaust systems were run with a FAST XFI/XIM management system using a set of FAST 75-pound injectors and 102-\mm Big Mouth throttle body. Also present was a pair of billet fuel rails, ARP head studs, and Fel Pro MLS head gaskets.

The plan was to run the motor first with the complete 2.5-inch stainless steel Blackheart system supplied by Hooker. Originally designed for swapping an LS into a C10 truck application, the exhaust featured an X-pipe, a pair of stainless mufflers, and all of the necessary clamps and hangers to install the system. Installation in the dyno was simple, and the system sounded great once installed. All three of the exhaust configurations tested were run with a set of 1 7/8-inch Hooker long-tube, LS-swap headers. It was necessary to run 3-inch collector extensions to allow fitment of the system on the dyno. These 17-inch long extensions were shared with the open header test. The air/fuel mixture of each system was dialed in using the XFI/XIM system, but the timing values remained constant for all three tests.

For this exhaust test, we relied on a FAST 102mm LSXR induction system that included the intake, 102mm Big-Mouth throttle body, and billet fuel rails.

 Fuel for the stroker came courtesy of a set of FAST 75-pound injectors.

 We relied on the FAST XFI/XIM management system to dial in the air/fuel and timing curves (kept constant) during the exhaust test.

All three of the exhaust systems tested were run with a set of 1 7/8-inch Hooker long-tube (LS swap) headers.

First up on the dyno was the 2.5-inch Hooker LS-swap exhaust. Run with the complete exhaust system, the 383 stroker produced 542 hp at 6,000 rpm and 503 lb-ft of torque at 4,600 rpm. The motor sounded great, but was amazingly quiet compared to our typical sessions using the open or even 3-inch muffled exhaust system.

First up was a complete exhaust system supplied by Hooker for an LS swap, C10 truck application. The dyno facility at Westech provided sufficient room to allow us to run the complete exhaust system.

 The 2.5-inch Blackheart exhaust from Hooker featured a dedicated X pipe and all the clamps, hangers, and 2.5-inch mufflers needed to mount the stainless system in the truck. The X-pipe sounded amazing at full song.

 Run on the 383 stroker, the 2.5-inch Hooker exhaust produced peak numbers of 542 hp at 6,000 rpm and 503 lb-ft of torque at 4,600 rpm.

 Next up was our 3-inch dyno exhaust feeding a 3-inch (straight-through) muffler. It is important to note that the length of the collector extension pipe (at 42 inches total with muffler) was shorter than the full Hooker exhaust, but longer in this configuration than the open pipe tested next. The collector length has a tuning effect on the power curve irrespective of changes in flow. Equipped with the 3-inch (shortened) exhaust, the 383 produced 548 hp at 6,300 rpm and 507 lb-ft of torque at 4,600 rpm. Compared to the full 2.5-inch exhaust, the 3-inch shortened exhaust was up by 5-6 lb-ft through the entire curve and as much as 9-10 hp out at 6,500 rpm. There was a gain from our dyno exhaust, but such a system would be VERY loud for a daily driver and impractical for street use.

Next up was the 3-inch dyno exhaust which consisted of a 3-inch system, 42-inches in length (including the mufflers) feeding straight-through mufflers. Given the larger diameter and shortened length of the 3-inch dyno (only) exhaust, we expected and received an increase in power over the street system. Run with the 3-inch muffled system, the power output of the 383 increased to 548 hp at 6,300 rpm and 507 lb-ft of torque at 4,600 rpm. The exhaust change netted improvements of 5-6 lb-ft of torque and a maximum of 10 hp at 6,500 rpm.

The final test involved running a 17-inch long, 3-inch (OD) collector extension on the headers with no mufflers.

 Run with the open exhaust, the 383 produced 551 hp at 6,300 rpm and 510 lb-ft of torque at 4,800 rpm. Compared to the test run with the longer collector extension and muffler, this open system offered slightly more power in the middle and top-end of the power curve, but lost out below 4,500 rpm. 

The final test involved running the open headers into a shortened (17 inches total) 3-inch collector extension with no mufflers. A race-only possibility, the change netted 551 hp and 510 lb-ft of torque.

Hooker 2.5-inch C-10 Exhaust vs 3-Inch Dyno Exhaust

It was obvious to us going in that the larger diameter, shorter and louder, 3-inch dyno exhaust would offer more power, but we were impressed by how well the full exhaust performed. Exhaust flow decreases with increases in tubing length, the number of bends, and changes on diameter, but the 2.5-inch system looked, sounded, and performed impressively on this powerful 383 LS stroker. Run at a total length of 42 inches (including the mufflers), the 3-inch dyno exhaust offered gains of 5-6 lb-ft and as much as 10 hp at 6,500 rpm, but would be too loud to run anywhere but a drag strip.

The change in collector length between the two 3-inch systems was responsible for the majority of the shift in the power curve, as the shorter collector lost power down low but improved things in the middle and at high rpm. For our street LS, we gladly give up a peak of 10 hp at 6,500 rpm (how often are we really there?) for being able to enjoy the power without setting off all the car alarms in the neighborhood every time we started the old girl up.