Within those tubes of steel running underneath and hidden from sight is where spent exhaust gases exceeding 1,200 degrees F move at the speed of sound, interacting with what lies at the other end. It's here where an exhaust system's ability to purge an engine of all those fumes will determine just how much power can be made and how much noise there'll be while doing it. And according to the people at Bisimoto Engineering, as far as Porsche's '75-'89 911 is concerned, there's a whole lot of room for improvement—and in a most unconventional way.
THE BISIMOTO SOLUTION
The 911's rearmost exhaust bits are heavy. They're also restrictive, and a few decades later, they don't look all that great either. Forget about the dangling tube hanging off the side of Bisimoto's Pulse Chamber Exhaust system for a moment and the most obvious difference you'll recognize is its size. It's small. So small in fact that it weighs just 6.5 pounds and occupies a fraction of the space the original muffler had. But that's not what makes it so special. Without any supporting mods, the modest little bolt-on's good for an easy 15 whp. How it manages that 15 hp is the part you'll care about, and it starts with this exhaust's ability to harness the power of pulse wave.
IT'S ALL ABOUT SCAVENGING
The namesake of Bisimoto's deceivingly simple-looking exhaust is indicative of its ability to take advantage of an engine's exhaust pulse waves. According to Bisimoto's founder and lead engineer, Bisi Ezerioha, here, kinetic energy generated by resonant exhaust gas pulses can be harnessed to increase scavenging.
Exhaust gas scavenging doesn't make sense. How else can you explain how a bunch of stationary metal tubes are able to affect the speed of which the air/fuel mixture's drawn past the intake valves and across the pistons and then blown out the other end? Understand inertia, though—the tendency of any moving object to resist a change in motion—and you'll begin to understand scavenging. Once the exhaust valves open, a high-pressure stream of exhaust gases forced out by the upward-moving piston begins exiting the cylinder head, creating a pressure pulse that travels throughout the entire exhaust system. Once that pulse of energy reaches the end of the headers' primary tubes, for example, a reverse wave travels back up, creating a low-pressure void near the exhaust valves that can elicit a suction effect.
The hard part is figuring out a way to not just increase that negative pressure but to control it so that it occurs within the exhaust ports at exactly the right time. Wave resonance tuning by means of calculated primary tube lengths allows this low-pressure pulse to occur at exactly the right time—during the cylinder's overlap period that happens as the exhaust stroke concludes and the intake stroke begins. (In case you've forgotten, overlap happens when a cylinder's intake and exhaust valves are both open for a brief period of time.) When coordinated properly, the results are that suction effect that evacuates additional exhaust gases for a cleaner intake charge and, in some cases, can even help draw the air/fuel mixture into the combustion chamber from the intake side. Do it wrong, though, and that energy pulse will simply bounce off of some of those closed exhaust valves, reflect in the opposite direction, and ultimately dissipate into a whole lot of nothingness.
The more efficiently those energy pulses are able to move past those headers and throughout the exhaust, though, the better they'll be able to encourage the trailing air/fuel mixture that's yet to pass by those intake valves to speed up. It's a concept most often associated with headers that—because of their closer proximity to the cylinder heads—means they're better capable of doing all of this. However, according to the people at Bisimoto, the right exhaust system can be almost as effective. Similar to what occurs within an engine's headers, the same effect can be created when that pulse of energy encounters a muffler or even the atmosphere at the end of the system.
IT'S ALSO ALL ABOUT RESTRICTIONS
No exhaust system is good for making more power all by itself. That'll always be determined by the amount of air and fuel made available for combustion. But the right exhaust can make things a whole lot more efficient, and efficiency often leads to more power. You already know that your engine's just a really big air pump and that the faster you can evacuate those spent exhaust gases from it, the more efficient it'll be. Get rid of those restrictions—also known as its pumping losses—and you'll find more power by way of increased volumetric efficiency.
If all that exhaust system of yours had to do was direct those gases away from your engine, though, making power would be easy. But it also has to keep things quiet, which means a muffler's involved—which means those free-flowing gases aren't so free-flowing after all. We're talking about backpressure, and anytime it can be minimized—provided the rest of the engine has been tuned accordingly—there's power to be had by way of increased pumping efficiency within the engine. Look inside Bisimoto's Pulse Chamber Exhaust and you won't find any of the baffles, louvers, or packing material you'd expect, though. It's a straight shot all the way through with no interruptions. You couldn't make things any more free-flowing if you wanted to.
PULSE TUNING AND A QUIETER EXHAUST
And that handlebar-looking tube hanging off the side...turns out it's got an important job. Understand the properties of wave cancellation and you're one step closer to knowing what that pipe's doing over there. It isn't responsible for anything you'll see on that dyno sheet, but it is what keeps exhaust noise at bay. Ezerioha explains its purpose by first reminding us that air is a fluid, similar to water, but with different properties. Consider any conventional exhaust system and its fluid-like exhaust pulse waves that travel through its tubes and out its other end in one fell swoop. Introduce a closed-end tube like Bisimoto has, though, and you've just introduced the system's ability to bounce soundwaves back and forth that when timed properly by means of the appropriate length tube cancel out many of those very frequencies. In other words, that little tube is responsible for making things a whole lot quieter.
In terms of sound, Bisimoto's Pulse Chamber Exhaust is anything but unreasonable. Still, an available arrestor is offered that reduces all of that noise by an order of 18 decibels and yet without any power loss. That power isn't decreased after bolting the arrestor into place doesn't necessarily make any sense at first. According to Ezerioha, that's because of a slight richening effect that takes place at wide-open throttle on fixed fueling systems like the 911's once the arrestor's in place. All you need to know is that this exhaust has the ability to be slightly quieter and without any power loss.
Bisimoto's Pulse Chamber Exhaust is a simple upgrade if there ever was one. According to Ezerioha, double-digit weight savings are yours for the taking along with 15hp gains that can be realized on their own and even more should you make the appropriate intake upgrades and ECM tuning.