Subaru's horizontally opposed EJ-series engine has always been known to be a well-engineered powerplant that is both powerful and torquey, but Subaru engineers left a lot to be desired when it came to designing their unequal length exhaust header manifold. The unmistakable "boxer rumble" is the number one and perhaps only reason why Subie owners prefer to retain them rather than switching out to an equal-length header. The great debate on unequal versus equal length isn't much of a debate if you consider the facts, which we break down in this article. We review the Killer B Motorsport's Holy header and test it in a head-to-head comparison versus a Greddy 4-to-1 unequal length header with some surprising results.
We placed these two headers side by side and immediately noticed that they look nothing alike. We measured the Greddy unequal length versus the Killer B header's primaries and found the Killer B header sporting the smaller diameter runner between the two, but rest assured, Chris from Killer B states that he had full confidence in the performance of their unit. "So many people make the mistake of thinking that bigger is better. With headers that is not at all the case. There are plenty of manufacturers out there just bending tube without putting knowledge of testing behind it. We designed the Killer B Holy Header to offer improved boost response compared to long primary/double secondary designs. A difference you will immediately notice in throttle response and the ability to hold more power higher into the revs," Chris says.
Killer B offers the option to treat your header using Swain Tech Coating. This is the only coating Killer B recommend due to its thermal capability, ability to protect the base material, and durability. We had our unit coated using Swain Tech's White Lightning.
Sharp edges and rough corners can cause flow turbulence, which ultimately leads to a decrease in performance as shown on our "knockoff" header we had in the garage (shown on left). Compare that to the Killer B unit and the quality is evident.
Killer B Motorsport is proud of the fact that they are the only manufacturer that engineers their headers using full 321 stainless steel. Chris states that 321 stainless is less prone to cracking in comparison to 304 stainless steel due to strength and durability at higher temperatures. We liked the fact that Killer B also allows you to ceramic coat or wrap their headers without voiding their warranty.
Unequal Length Header
The unequal length header design will allow you to keep the Subaru Boxer rumble sound you've come to love, but other than the noise, they offer a poor choice for maximum horsepower potential. The different levels of exhaust pulses cause the exhaust gas temperatures and air/fuel mixtures to run at a different level.
All Subaru unequal length header design consists of a longer runner for cylinder No. 4, followed by the runner for cylinder No. 2 and shorter runners for cylinders 1 and 3. The unequal lengths creates inconsistent conditions from cylinder to cylinder in the form of various exhaust gas temperatures and fluctuations in air/fuel ratios, which is frequently the culprit in engine failures such as cracked ring lands or melted pistons due to excessive heat. Equal-length headers provide each cylinder's gas expulsion an even distribution of EGT, A/F ratios to and output all four cylinders. This results in a safer tune with significantly reduced probability of a cylinder having a lean condition (knock), more power at the same boost levels (or the same power at lower boost), improved responsiveness, and even improved fuel efficiency.
Typical flange type up-pipe connections rely on a gasket and are often prone to leaking. Killer B's header is designed with a V-band for fast, easy installation that requires no gasket, no leaks, and no disruption of flow.
If you seek quality and product longevity, we suggest you stay away from "type", "style", or "replica headers" at all costs. We've heard firsthand that too many people had issues with poor quality and fitment, as shown with this 1.5 twin scroll header we purchased online for $200. But its cheap you say? Sure, times are tough, and money doesn't grow on trees but we prefer to wait the extra amount of time and save up to purchase quality parts. There's no sense in doing the same thing twice, especially if it's not a fun job or costs you a bunch in labor. Inferior quality aftermarket headers might be appealing due to their price point but are a very problematic aftermarket upgrade. Sure, you can make huge horsepower numbers with a poorly designed setup as easy as the next guy, but that's not to say it's not an efficient setup.
Subarus are no strangers when it comes to exhaust leaks. Common occurrences can be traced back to how the exhaust flanges are machined, as well as the mating surface of the primaries. An unequal surface will guarantee the gaskets to fail.
A common issue with inferior designed headers is the welding processes many manufacturers use, which will actually weaken the headers at the welds. This problem can lead to cracking, which is often the case with inferior fabricated headers.
Even with flex joints on the primary length tubes, our "knockoff" header fitment prevented us from aligning the boltholes to properly secure to the engine.
Driven to the dyno facility, our WRX was currently outfitted with a VF39 turbo; STI top-mount intercooler; BC cam gears, valvetrain, and rods; JE pistons; Greddy equal length header; DC Sports exhaust; Cobb catted downpipe and tuned on a Cobb Accessport. Before we began our testing phase, it should also be noted that the WRX was tuned prior to our first pull and producing 223.8 whp and 214.1 lb-ft of torque at 16 psi.
Dyno Test: Unequal vs. Killer B Holy
As with any aftermarket part installed on our WRX, we tuned the car after installing the Killer B header while keeping boost levels identical between both pulls. The header performed like a true champ and collected some ponies from 4,300 all the way to the 7,100-rpm redline with a deliverance of consistent torque numbers of 10 to 12 lb-ft of torque from 5,700 to redline. The graph comparison at lower rpm show power numbers that were nearly identical but that's where the similarities abruptly end. At mid and high horsepower levels, the Killer B header quickly overpowers the unequal length header, with the most impressive gain of 15 hp and 12 lb-ft of torque at 6,350 rpm. In the end we saw a peak gain of 15.4 hp and 2.4 lb-ft of torque with power under the curve being the most impressive.