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Header Selection and Average Power: A 350 Chevy Dyno Test

How 1 5/8-inch Speedmaster headers outperformed larger sprint-car headers where it matters most - across the usable rpm range

Written by
Marketing Speedmaster

Published on
July 16th, 2017

Peak horsepower sells engines. But on the track, acceleration is determined by average power across the usable rpm range - not the number at the top of the pull. This header test on a mild 350 Chevy Street Stock motor illustrates exactly that: how selecting the right header for the combination can reshape the power curve without sacrificing peak output.

Why Headers Matter Beyond Flow

Headers do more than move exhaust. The primary tubing diameter, length, collector size, and collector extension length create a tuning effect on the power band - similar to how intake manifold design influences induction. The scavenging effect generated by properly designed long-tube headers improves not only exhaust evacuation but also intake charge filling during valve overlap.

The timing of that scavenging effect - the rpm at which it is most effective - is a direct function of these design variables. Match them to the combination and power production is maximised across the range that matters.

The Test Motor

The small block 350 Chevy featured Vortec heads, a COMP circle-track cam, guided roller-tip rockers, and a dual-plane Eliminator intake from Speedmaster. It was tested with both a 500-cfm Holley 4412XP 2-barrel carburetor (using a Wilson 2-4 barrel adapter) and a Holley 650 XP 4-barrel to confirm the results were not specific to one induction configuration.

The dyno test started with 1 3/4-inch sprint-car style headers.

Two Headers, One Motor

The two headers differed in primary tubing and collector diameter:

Sprint-car style: 1 3/4-inch primary tubing, 3.5-inch collectors with extensions.

Speedmaster chassis headers: 1 5/8-inch primary tubing, 3-inch collectors with extensions.

2-Barrel Results

With the 500-cfm Holley and 1 3/4-inch headers, the 350 produced 382 hp and 399 lb-ft of torque. After swapping to the 1 5/8-inch Speedmaster headers, the numbers moved to 383 hp and 409 lb-ft. Peak power changed by 1 hp. Torque improved by over 30 lb-ft across the range from 3,000 to 5,400 rpm.

Graph 1: 2-Barrel Header Test - 1 3/4-inch vs 1 5/8-inch. The smaller Speedmaster headers improved torque production from 3,000 rpm through 5,400 rpm with no loss at the top.

4-Barrel Results

With the Holley 650 XP 4-barrel, the 1 3/4-inch headers produced 418 hp and 421 lb-ft. The 1 5/8-inch Speedmaster headers returned 417 hp and 424 lb-ft. The pattern held: significant torque gains through the low and midrange, no penalty at peak.

Graph 2: 4-Barrel Header Test - 1 3/4-inch vs 1 5/8-inch. Results mirrored the 2-barrel test, confirming the gains are a function of header design, not carburetor configuration.

What the Data Shows

In both configurations, the smaller 1 5/8-inch Speedmaster headers produced higher average power across the usable rpm range. An extra 30-35 lb-ft of torque from 3,000 to 5,400 rpm translates directly to improved acceleration off the corner - the phase of a circle-track lap where races are won.

More average power across the working range means better acceleration. Better acceleration means the car gets up to speed sooner and carries that advantage down the straight.

Component Gallery

After installation of the 1 5/8-inch Speedmaster headers: 417 hp and 424 lb-ft, with torque gains extending to 4,400 rpm.

With the 4-barrel Holley and 1 3/4-inch headers: 418 hp at 5,900 rpm and 421 lb-ft at 4,400 rpm.

The same header comparison was repeated with the Holley 650 XP 4-barrel to verify consistency.

With the 1 5/8-inch Speedmaster headers and 2-barrel: 383 hp and 409 lb-ft. Peak horsepower changed by 1 hp; torque improved significantly from 3,000 to 5,400 rpm.