Corvette Intake Manifold - The LS1 Goes SSI

When it comes to induction systems, the LS-series powerplants boast some of the best designs out there. This shouldn't come as a huge surprise, considering that these intake components were developed not as individual pieces, but rather as part of a complete engine package. In the case of the manifold and throttle body, the GM design team sought not only to maximize power, but also to minimize cost, ensure long service life, and ease installation and replacement. While meeting these divergent goals makes for an impressive all-around system, it also sets the stage for potential improvements in one specific area: performance.

More on that in a moment, but first, let's take a closer look at the intake manifold itself. While many consider the job of the intake to be providing airflow to the cylinder heads, this is at best an oversimplification. While it is true that all air to the motor must flow through the intake manifold, simply maximizing airflow through the manifold should not be the main design goal when trying to improve power output. Rather, the intake-manifold-runner lengths must be tuned to work in conjunction with the cam profile, the head-flow numbers, and even the primary-tubing diameter of the exhaust. Failure to tune runner length will result in less power rather than more, regardless of the airflow numbers of the ports themselves.

How is the runner length tuned to optimize airflow? In addition to providing a simple airflow path to the combustion chamber, the intake runner and head port provide additional cylinder filling by way of both inertial- and reflected-wave ram tuning. The inertial-wave filling happens when the downward movement of the pistons and open intake valve create a vacuum, which initiates movement of the air column. This air column continues into the combustion chamber, even past the point of intake-valve closure. This creates a buildup of pressure against the closed valve. If this pressure is present when the valve opens, additional cylinder filling is realized.

The reflected wave, meanwhile, occurs when the intake valve opens to create a negative pressure wave. This negative pressure wave travels out, away from the valve, until finally arriving at the common plenum, where it expands outward. This expansion creates a low-pressure area, which is quickly filled. The filling of the void creates a positive pressure wave, which then travels back down the intake port and into the combustion chamber, improving cylinder filling.

A third form of wave tuning is called Helmholtz Resonance. This deals with the resonance of the common plenum and tuned length of inlet tubing (from the air filter to the intake manifold). Once excited, the common plenum resonates at a given frequency. These resonance waves can be used to bombard the intake valve with additional pressure.

Finally, there's the matter of engine operating range. It's actually pretty easy to improve the power output at one end of the rev range by lengthening or shortening the runner length. Unfortunately, this has a negative effect at the opposite end. If you simply shorten the runner length, chances are you'll gain power at the top of the rev range, but you'll most likely sacrifice power in the low and mid ranges. Lengthening the runner, meanwhile, has the opposite effect. As a result, it is often difficult to improve power output throughout the rev range, but that is exactly what BBK claims to have done with its new Single Stage Induction (SSI) intake for LS engines.

To properly test the SSI intake, we built a suitable test motor. Our LS1 short-block was assembled by Coast High Performance. The stock block and crank were retained but were augmented by a set of forged rods and pistons. The forged flat-top pistons offered valve reliefs to allow us to run aggressive cam profiles without fear of piston-to-valve interference. The reliefs dropped the static compression slightly (compared with stock), but the missing compression was offset with more-aggressive cam timing and a set of ported heads.

With street driving in mind, we chose a mild cam profile for the LS1. The Comp XR265HR cam offers a 0.522-/0.529-inch lift split, a 212-/218-degree duration split at 0.050, and a 114-degree lobe-separation angle. Keep in mind that because the valve events initiate the reflected and inertial waves, cam timing will also affect the gains offered by the intake (in other words, the gains offered by the intake might be different if used in conjunction with the stock cam).

The motor was also treated to a set of CNC-ported heads from RHS, a set of Kooks custom step headers (1 3/4 to 1 7/8 inch) feeding MagnaFlow mufflers, and a FAST XFI management system. We also installed a set of 36-pound injectors, a Meziere electric water pump, and a fresh crankcase full of Lucas 5W-30 synthetic oil (after a proper break-in period, naturally).

The motor was first run with the stock LS1 intake and throttle body. Once the air/fuel and timing curves were optimized with the XFI, the modified LS1 produced 462 hp and 436 lb-ft of torque. Even more impressive, the engine offered over 400 lb-ft from 3,800 to 6,000 rpm. After we installed the BBK SSI intake and matching 85mm throttle body, the peak numbers jumped to 489 hp and 450 lb-ft, for gains of 27 hp and 14 lb-ft. The BBK intake suffered no loss in power at any rpm compared with the stock LS1 unit, offering gains from 3,000 to 6,500 rpm, with significant gains occurring past 4,000 rpm. With the BBK SSI intake, torque production from the modified LS1 now exceeded 400 lb-ft from 3,700 to 6,400 rpm.

Though not quite as light as the factory composite intake, the aluminum BBK piece does provide protection against a backfire (are you listening, nitrous users?), the ability to port the interior passages (thanks to a removable plenum cover), and an attractive finish that can be polished to a show-quality luster. It even comes configured to receive all the factory vacuum, PCV, and MAP-sensor lines. With so much going for it, the SSI looks to be a serious new contender in the LS-intake-manifold market.

Stock LS1 Intake vs. BBK SSI
As is evident from the graph, the BBK intake and 85mm throttle body offered substantial power gains over their factory LS1 counterparts. The BBK setup even out-powered the stocker down at 3,000 rpm, though the significant gains did not occur until the tach needle had swung past 4,000 rpm. Peak-to-peak improvements were 27 hp and 14 lb-ft.