Back in our last installment of the Modern Mouse series, we took a step back in time and subjected the 383 stroker LS (technically an LM7) to a pair of carbureted intakes. For part 6, we decided to jump back to the future and take a look at an SSI EFI intake from BBK.
When it comes to induction systems, even the factory 5.3L truck intake has a lot to offer. This shouldn't come as a huge surprise since the intake manifold was part of a complete package. That package, incidentally, was not the induction system, but rather the entire motor—in fact the entire vehicle, as the various sub systems were all part of a much larger whole.
In the case of the intake manifold, the GM design team sought not merely to maximize the power output (in reality a rather simple task given their engineering expertise), but rather to satisfy a number of different design goals. Those goals would surely have included a specific power output (something less generic than "as much as possible"), but so too would cost analysis, noise considerations, and even ease of initial installation (on the assembly line) and replacement.
In addition to this, we must also consider component weight, strength, and longevity (to minimize the expense of warranty claims). While meeting these design goals makes for an impressive component (or components), it also paves the way for potential improvements in any one specific area—namely performance.
That the factory intake manifold was designed to achieve a specific power output (285-295 hp in the case of the 5.3L LM7) and meet the numerous other design goals, provided room for the aftermarket to design systems to further improve the power output, especially on modified motors. Before getting to the testing on one such aftermarket system, we need to take a more detailed look at intake manifold design theory. While many consider the job of the intake to provide airflow to the cylinder heads and ultimately the combustion chamber, this is at best an oversimplification. It is true enough that airflow provided to the motor must flow through the intake manifold, but maximizing the flow rate of the manifold is actually not the main design goal when trying to increase horsepower.
The internal combustion engine is a giant air pump, and as such, improving the power output is a simple matter of increasing the amount of air processed by the pump. Additional airflow can come from more efficient cylinder heads, wilder cam timing, and (in our case) a more effective intake design.
While it seems that we have contradicted ourselves, maximizing airflow through the engine is not the same as maximizing airflow to it. If big airflow numbers were the limiting factor, all we'd have to do to maximize the flow rate of the intake is increase the cross-section and decrease the length of the intake ports. I'm sure no one would argue that large, short intake ports flow better than long, narrow ones. The laws of physics are on our side when it comes to airflow, but unfortunately an intake designed with huge, short ports will not provide the best power output, at least not in the rpm range used by any street/strip LS application. There is a reason the factory truck intake is configured the way it is. One reason is obviously for fitment, but the other reason is that there is a great deal more to the equation than simple airflow. Both the length and diameter (or cross-section) of the intake runner determine the effective operating range.
Not surprisingly, the intake manifold runner length must be tuned to work in conjunction with the cam profile, head flow, and even exhaust primary tubing diameter (assuming a long-tube header). Failure to tune the intake runner length will result in less power rather than more, regardless of the airflow numbers of the port itself.
How is the runner length tuned to provide additional airflow? In addition to providing a simple airflow path to the combustion chamber, the intake runner (combined with the head port) provides additional cylinder filling by way of both an inertial and reflected wave ram tuning. The inertial wave cylinder filling happens when the downward movement of the pistons (and open intake valve) creates a vacuum, which initiates movement of the air column. If timed properly (determined by the length and inside diameter of the inlet tract), the inertia of the air column can improve cylinder filling beyond the vacuum created by the downward moving of the piston.
The reflected wave provides additional cylinder filing as well, but by a different means. The reflected wave happens 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. The negative pressure wave expands out into the open plenum. The expansion of the negative pressure wave creates a low-pressure area that 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.
While both inertia and reflected wave ram tuning improve cylinder filling, the key is timing. The pressure waves must arrive at the proper time to maximize cylinder filling. Obviously they must also not overlap or work against each other (where a positive pressure wave is effectively cancelled out by a negative one). The runner length (and diameter) help determine when (in the rpm range) these forms of cylinder filling become effective. Since the pressure waves run through the intake runners (and head port) at the speed of sound, altering the distance changes the travel time through the port. The travel time can also be altered (to a lesser extent) with changes in temperature and pressure. If the runner length is properly tuned, the motor will realize not only the airflow drawn in by way of the vacuum created by the downward moving piston, but also the mild supercharging effect that comes from the inertial and reflected wave tuning.
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, once again assuming the tuned frequency coincides with the effective operating speed of the motor. Note that these forms of ram tuning are irrespective of the airflow potential of the port and function even in boosted applications.
While we have simply scratched the surface of the intake manifold, it should be clear that the proper design is much more complicated than simply building runners connected to a common plenum fed by a throttle body. Obviously computer simulations can be used to design an intake manifold for a given application, but given the complex nature and sheer number of pressure waves involved (most intake modeling is done with a single-cylinder motor), the final testing almost always involves actual dyno testing. Though it is possible to determine (or design) the effective operating range of the intake manifold runner length, diameter, and plenum, what effect does the design have at other engine speeds? Is the area under the curve sufficient? Are there any undesirable dips or peak? These are questions that only the dyno can answer effectively. Thus a proper intake design should see plenty of dyno time before being finalized. Obviously it would be nice if the same intake fit properly, bolted up to all the original hardware and looking like a proper performance piece.
The guys at BBK understand what goes into the design of an intake manifold. Their Single Stage Induction (SSI) intake promised effective power gains, so we decided to put one to the test on Modern Mouse. You will remember that Modern Mouse had long since been converted from a stock LM7 to a 383 stroker courtesy of Procomp Electronics, Probe Racing, and L&R Autotmotive. The 5.3L stroker short-block featured a Comp LSR cathedral-port cam, and was topped with a set of Gen X 215 heads from Trick Flow Specialties. For our testing, Modern Mouse was also treated to an 80mm Accufab throttle body, a set of American Racing Headers, and Holley 75-psi injectors. The engine was also sporting a Meziere electric water pump, FAST XFI/XIM management system, and a fresh oil change with Lucas 5W-30 synthetic oil (and K&N oil filter). The 75lb injectors were overkill for this application, but we had them on hand for another round of forced induction. Look for boosted results in the next installment of Modern Mouse.
To establish our baseline, the motor was first run with the stock 5.3L truck intake and Accufab 80mm throttle body. Once the air/fuel and timing curves were optimized with the FAST computer, the modified LM7 produced 515 hp at 5,800 rpm and 506 lb-ft of torque at 4,700 rpm. Obviously tuned for truck applications, torque production exceeded 475 lb-ft from 3,600 rpm to 5,600 rpm.
Installation of the BBK SSI intake also required replacing the factory truck fuel rail with a billet unit from Wilson Manifolds. After swapping over the Accufab throttle body, the peak power numbers jumped to 524 hp at 6,100 rpm and 503 lb-ft of torque at 4,800 rpm. The BBK intake improved the power output of the 383 by as much as 15-16 hp at the top of the rev range, but it is hard to beat the torquey truck manifold in the lower rev ranges.
Note that both the peak horsepower and torque values with the BBK intake occurred at higher engine speeds than the truck manifold. We suspect the BBK intake would show even better results on a larger stroker or one with even wilder cam timing.