This is GrimmSpeed's new Unequal Length Header. Grimmspeed believes that there has not been any major changes to the unequal exhaust manifold in some time, and this is due to the tight geometry and long cross-pipe between the two collectors. Although it provides the Subaru with that signature boxer sound, it is not optimized for flow or power. This design is great for low-end spool, but the factory exhaust manifold runs out of steam higher in the rev range. To relieve this issue, other manufacturers would design a more equal-length header with larger diameter runners to make up for the performance. Because of these reasons, GrimmSpeed has created a header that would offer the best of both worlds meaning excellent power delivery throughout the entire RPM range, while still delivering that flagship boxer rumble. 

GrimmSpeed presents a proprietary design that utilizes an innovative setup where each bank of runners is equal length. One set is 1.5 times the length of the other which provides the ruble sound while maintaining the performance of an equal length setup. This manifold utilizes a single-piece, mandrel-bent runners to minimize disturbances to the exhaust gas along with an extremely durable cast 304 stainless steel collector to keep each bank of runners truly equal length which is very difficult to accomplish with a 4-into-1 welded collector. The low-angle merge of this collector allows for a faster spool and unbeatable off-boost throttle response. To match the flow, the collector features machined internal diameters to perfectly match the 1 3/4" primary runners and 2" secondary pipe which eliminates unwanted "steps" in the exhaust flow. 

During the engineering process, GrimmSpeed scanned the underside of several EJ engines using a FARO arm which allowed them to design and simulate their header in a digital 3D space to find the optimal balance of runner lengths and angles for the best performance possible on an unequal length setup. They then 3D printed a number of different configurations to narrow the most optimal final design. They were able to form the header around a variety of different STI oil pan setups from factory to the most popular aftermarket options. Once they were satisfied with a design, GrimmSpeed put the manifold through months of punishing performance and longevity testing. When compared to the OEM exhaust manifold, GrimmSpeed had produced +25 HP and +25 FT/LBS with a quicker spool even when compared to the best unequal and equal-length aftermarket offerings.

HIGHLIGHTS:

• Full 304 stainless steel construction
• Innovative 1.5 to 1 unequal length runners - Equal length performance with an unequal length sound
• 4 into 1 low-angle merge cast collector ensures minimal restriction and maximum performance
• Smooth restriction-free flow throughout is accomplished with single-piece mandrel bent tubing welded to a single-piece merge collector to remove "steps"
• Substantial power and torque gains upon research netted +25 HP and +25 FT/LBS over stock
• Quickest spooling aftermarket header when compared to the competition 
• No tune required and safe to run
• Includes new gaskets and hardware for the installation 
• Designed to clear aftermarket oil pans
• Manufactured and welded in the USA