The Relationship Between Torque and Horsepower

From a mathematical standpoint, making more horsepower requires shifting torque production higher in the rev range. The formula is direct: HP = TQ x RPM / 5,252. Anything that increases either torque or the engine speed at which it occurs will improve power output.

A practical example: a 383 producing 450 lb-ft at 4,500 rpm equates to 385 hp. Produce that same 450 lb-ft at 5,000 rpm and the result is 428 hp. The math is simple. The engineering question is how to move that torque curve in practice.

The answer lies in the three primary power producers: cylinder heads, camshaft, and intake manifold. To isolate the effect of each, we tested them individually on a Speedmaster 383 stroker.

The Baseline Engine

The test mule came directly from Speedmaster. The 383 featured a Speedmaster 4-bolt block with a forged stroker crank, 6.0-inch H-beam rods, and forged flat-top pistons with generous valve reliefs.

The Speedmaster 383 short-block: 4-bolt block, forged stroker crank, 6.0-inch rods, and forged flat-top pistons.

The baseline top end: Speedmaster as-cast 190cc cylinder heads (190cc intake ports, 64cc chambers, 266 cfm at 0.700 lift) paired with a COMP XR270HR cam (0.495/0.502 lift, 218/224 duration, 110-degree LSA).

The bottom end was completed with a Speedmaster oil pan, pump, and pickup, along with an aluminum front cover and neutral SFI-rated damper.

Speedmaster oil pan, pump, and pickup secured the bottom end. The aluminum front cover and SFI-rated damper were also Speedmaster components.

The mild street combination was completed with a Speedmaster dual-plane Eliminator manifold, hardened pushrods, and aluminum roller rockers.

A Holley 950 HP carburetor and MSD distributor completed the induction and ignition systems.

Five quarts of Lucas 5W-30 conventional oil were added before two computer-controlled break-in cycles.

In addition to the as-cast heads, 218/224 cam, and dual-plane intake, the 383 was configured with a 950 HP Holley carb, MSD distributor, and 1-3/4-inch dyno headers. All testing was conducted on 91-octane pump gas with Lucas 5W-30 synthetic oil.

In baseline trim, the 383 produced 388 hp at 5,200 rpm and 453 lb-ft of torque at 3,800 rpm. This represented a solid street combination with good idle quality, drivability, and a broad torque curve.

Upgrade 1: CNC-Ported Cylinder Heads

The first swap replaced the as-cast heads with Speedmaster CNC-ported units. The order of upgrades matters; testing these three components in a different sequence would change the individual gains, though the combined result would remain the same.

Baseline result with as-cast heads, mild COMP cam, and dual-plane intake: 388 hp and 453 lb-ft of torque.

The as-cast heads were replaced with Speedmaster CNC-ported units.

Installation used hardware and gaskets supplied by Speedmaster.

The CNC-ported heads included a spring package matched to the hydraulic roller cam. The head upgrade added nearly 40 hp.

With the CNC-ported heads installed, peak numbers moved to 426 hp at 5,700 rpm and 459 lb-ft at 3,900 rpm. Peak power rpm shifted by 500, while peak torque moved just 100 rpm. Below 3,400 rpm, the ported heads actually produced slightly less power than the as-cast units. The additional airflow was most effective in the upper half of the rev range.

Graph 1: Head swap comparison, as-cast versus CNC-ported.

Upgrade 2: Camshaft

With the ported heads in place, the mild XR270HR cam was replaced with a more aggressive XR300HR grind: 0.562/0.580 lift, 248/254-degree duration, 110-degree LSA.

The mild XR270HR cam was replaced with the XR300HR: 0.562/0.580 lift, 248/254-degree duration, 110-degree LSA.

The damper and front cover were removed to access the camshaft.

The cam swap increased peak power from 426 hp to 466 hp and shifted the torque peak from 3,900 rpm to 4,300 rpm. However, peak torque output dropped by 10 lb-ft, from 459 to 449 lb-ft. The gains above 5,000 rpm came at the cost of equal losses below that point. At 6,000 rpm, the engine gained 44 hp; at 3,200 rpm, it lost 44 lb-ft. The cam choice is fundamentally a decision about where in the rev range power is most useful.

Graph 2: Cam swap comparison, 218/224 versus 248/254.

Upgrade 3: Single-Plane Intake

The final swap replaced the Speedmaster dual-plane Eliminator intake with the single-plane Shoot-Out intake. With the CNC-ported heads and aggressive cam already shifting the power band upward, the single-plane intake was expected to extend that trend.

The Speedmaster single-plane Shoot-Out intake replaced the dual-plane Eliminator. With the Shoot-Out installed, the 383 produced 486 hp and 444 lb-ft.

The result: 486 hp at 6,400 rpm. Peak torque dropped by 5 lb-ft (449 to 444) and shifted 400 rpm higher (4,300 to 4,700 rpm). The intake swap followed the same pattern as the cam: gains at the top of the range traded against losses at the bottom.

Graph 3: Intake swap comparison, dual-plane versus single-plane.

The Combined Result

Across all three upgrades, the combination shifted peak engine speed by 1,200 rpm. Peak torque dropped by just 9 lb-ft but moved 900 rpm higher in the rev range. The net effect: nearly 100 additional horsepower, from 388 hp to 486 hp.

Each component contributes differently. The heads added airflow capacity across the range. The cam determined where in the rev range that airflow was most effectively used. The intake refined the top-end delivery. The takeaway for any engine build is that these three components must be matched to each other and to the intended operating range. Selecting one without considering the others leaves performance on the table or, worse, creates a combination that fights itself.