Hey, do these look like performance exhaust manifolds for a modified Hemi?

By Richard Holdener

Like cam timing and induction theory, long-tube headers are one of the least understood aspects of the entire exhaust system. The confusion is understandable, as it stems from the fact that header design deals with much more than simple flow. Unlike mufflers and exhaust tubing, headers provide a tuning effect on the power band, much like an intake manifold.

Some enthusiasts (and tuners alike) seem to be under the misconception headers add power courtesy of increased flow over the stock exhaust manifolds. This is a half-truth at best, as the power gains offered by long-tube headers come not so much from absolute flow, but rather from the scavenging effect offered by the primary (and collector) length.

Our Hemi test mule began life as a 5.7L Mopar crate motor. Unseen are the ported 5.7L heads and COMP Cams beehive spring upgrade.

Not unlike the intake manifold, tubular headers improve the power output by increasing the intake flow into the combustion chamber. How you ask? The scavenging effect created by the exhaust, through both pressure wave and reflected wave tuning, enhances exhaust flow out of the cylinder. This in turn helps scavenge intake flow into the cylinder during overlap. The length of the primary tubing, combined with other design criteria, determine the effective operating range of the scavenging effect.

You don’t have to understand pressure waves, reflected waves, or even pulse tuning to appreciate the power gains offered by a good set of headers. It is important to know the gains don’t come from more flow, or we would all just install the biggest headers we could find, or better yet, Zoomies like a Top Fuel car, and call it a day. Unfortunately, bigger is not always better, and (like the intake) the exhaust must be tuned to maximize the performance.

Such is the dynamic equation that is the internal combustion engine, and our modified 5.7L Hemi was no exception. The test motor started out life as a 5.7L crate motor, but had since been upgraded with a COMP Cams 273H-14 cam. The healthy COMP XFI cam featured a .547/.550 lift split, a 224/228-degree duration split, and 114-degree lsa and was teamed with a set of beehive springs on the ported 5.7L heads. Finishing up the 5.7L was a single-plane, MP intake and four-hole throttle body. The combination of the COMP cam, ported heads, and single-plane intake produced a 5.7L willing and able to run up at 7,000 rpm.

The exhaust test on the modified 5.7L Hemi was a simple one. We first ran the motor with a set of stock manifolds from a Dodge Ram truck application. Though not quite as performance-oriented as the SRT8 versions, the truck manifolds were much more common and readily available. The stock cast-iron manifolds were run with collector extensions, as the length of extension had a decided effect on power production lower in the rev range.

After dialing in the 65-pound injectors with the FAST XFI/XIM management system, the modified 5.7L Hemi produced 458 hp at 7,000 rpm and 378 lb-ft of torque at 5,700 rpm. After installation of the 1 7/8-inch Hooker headers (for Magnum chassis application), the peak power numbers jumped to 483 hp (again at 7,000 rpm) and 403 lb-ft of torque at 5,800 rpm.

Modified 5.7L Hemi-Stock vs 1 7/8-inch Hooker Headers
We love it when a modification offers sizable peak power gains, but even better when the gains are consistent through the entire rev range. The stock truck exhaust manifolds used on the 5.7L Ram trucks were hardly designed for maximum performance. Equipped with the stock manifolds, the modified 5.7L (COMP cammed, ported heads, and single-plane intake) produced 458 hp at 7,000 rpm and 378 lb-ft of torque at 5,700 rpm. After installation of the 1 7/8-inch Hooker headers (designed for a Magnum application), the power numbers jumped to 483 hp and 403 lb-ft of torque. The scavenging effect of the long-tube headers improved the power output from top to bottom, with gains exceeding 30 hp and 35 lb-ft of torque.

Note the motor produced much higher power numbers, but at basically the same rpm. The long-tube headers helped discover hidden Hemi horsepower through the entire rev range. More power is always good, but more power everywhere is even better.

Sources: COMP Cams, compcams.com; FAST, fuelairspark.com; Holley/Hooker/Weiand, holley.com