Words and Photos By Richard Holdener

More Airflow Does Not Always Mean More Power

An internal combustion engine is an air pump, and increasing power output requires improving airflow through the system. The distinction matters: airflow through the system - the engine as a whole - is different from increasing the flow of a single component. More throttle body, head, or intake flow may or may not increase power. The question is whether the engine can use the additional airflow, or whether a restriction exists elsewhere.

A throttle body capable of feeding 900 hp will show limited gains on a 450-hp engine. The issue is not the component - it is the combination. However, there are times when a single airflow device makes a measurable difference. This 427 LSX throttle body test was one of those cases.

The 427 LSX Short Block

The test was run on a 427 LSX short block built around a GM LSX iron block from Gandrud Chevrolet, stuffed with a Lunati crank, Carrillo rods, and CP pistons. A Total Seal ring package, Moroso oiling system, and BTR Stage IV LS7 cam completed the rotating assembly. ARP head studs and Fel Pro MLS head gaskets secured the top end.

Mast Black Label LS3 Heads

Mast supplied a set of Black Label LS3 280 heads for this test. Designed for engines running a 4.125-inch bore, these six-bolt LS3 heads flowed 390 cfm at .700 lift - enough to support over 800 hp on the right application.

Full CNC porting delivered 249 cfm on the exhaust side.

The 280cc intake ports featured 12-degree valve angles, a 2.20/1.60 stainless steel valve package, and 70cc combustion chambers. The CNC finish quality was first-rate. The .750-thick deck surfaces also make these heads well-suited for forced induction applications.

Top-End Configuration

Having previously tested cathedral-port and LS7 induction systems on this 427, the focus shifted to the LS3 platform. The Mast heads were finished with a Platinum valve spring package from Brian Tooley Racing. Due to the difference in valve guide diameter, the BTR springs required COMP 4682-16 spring seats. The heads accepted factory 1.8-ratio LS7 rockers using dedicated rocker stands supplied by Mast.

The BTR Platinum spring kit installed with COMP spring seats and dedicated Mast rocker stands.

Despite LS3 port locations, the Mast heads were designed to accept factory 1.8-ratio LS7 rockers.

A finned aluminium valley cover from Holley replaced the factory DOD cover, matching the LSX block finish.

Topping the heads was a two-piece CNC-ported single-plane intake from Mast, configured for LS3 and LS7 applications with a 4150 carb/throttle body flange. Mast also offered this intake with a 4500 flange. The EFI version included injector bungs. A previous test of the Mast LS7 intake on a 495-inch engine produced 810 hp, confirming the intake platform's capability.

Fuel and Management

Fuel delivery was handled by a set of 83-pound Holley injectors.

Injector flow rates were verified on an ASNU machine before testing. They checked out correctly despite sitting unused for several months.

All testing was run through 1 7/8-inch Hooker long-tube LS-swap headers feeding short collector extensions.

A FAST XFI/XIM management system handled air/fuel and timing for the 427 LSX.

Test 1: FAST 4150 Throttle Body

The Mast intake was flanged for a 4150 throttle body. The first test used a FAST 4150 unit.

With the 4150 throttle body installed, the Mast-headed 427 produced 669 hp at 6,600 rpm and 574 lb-ft of torque at 5,300 rpm. The numbers were lower than expected for this head and intake combination, suggesting a restriction upstream.

Test 2: Accufab 4500 Throttle Body with Speedmaster Adapter

The next step was a 4500-series throttle body from Accufab, offering a larger bore and higher flow capacity.

Mounting the 4500-series body onto the 4150-flanged intake required a 4150-to-4500 adapter from Speedmaster.

With the larger throttle body in place, the 427 produced 681 hp at 6,600 rpm and 578 lb-ft of torque at 5,200 rpm.

Results: Where the Power Was Found

The throttle body swap netted 12 hp and 4 lb-ft of torque. As expected with an airflow restriction, the majority of the gains occurred in the upper rev range where airflow demand is highest. The power curve with the 4150 body levelled off at 6,600 rpm; the 4500 body allowed the engine to continue pulling.

That the power gain came with no change to the intake manifold itself confirms the 4150-flanged Mast intake still had more to offer. The Mast Black Label LS3 heads - flowing 390 cfm on the intake side - were not being fully utilised by the smaller throttle body. A properly sized inlet allowed the combination to move closer to its potential.

The Speedmaster 4150-to-4500 adapter provided a simple, effective path to increased airflow without requiring a different intake manifold - a practical solution for builders working with 4150-flanged setups on high-output LS combinations.