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 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.
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.
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 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.
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.
Sources: Aeromotive, Aeromotiveinc.com; ATI, atiracing.com; COMP Cams, compcams.com; FAST, fuelairspark.com; JE Pistons, jepistons.com; Holley/Hooker, holley.com; Lucas Oil, Lucasoil.com; Scat, scatcrankshafts.com; Speedmaster, Speedmaster79.com; Total Seal, totalseal.com; Trick Flow Specialties, trickflow.com.