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.
1 Unlike the factory truck, LS1 or LS6 intakes, the BBK SSI featured aluminum constructio
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.
2 The BBK SSI intake features a throttle opening designed to accept an 80mm or 85mm BBK t
3 The SSI intake featured a dedicated port designed to accept the factory MAP sensor.
4 We like the fact that the SSI intake design featured a removable lower cover. The cover
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.
5 Positive intake port sealing was provided by provisions for factory O-rings.
6 Loyal readers will remember that our 5.3L test motor is now sporting a 383 stroker kit
7 Feeding Modern Mouse was a set of Gen X 215 heads from Trick Flow Specialties. The head
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.