GrimmSpeed Unequal Length Header - 2004-21 Subaru STI, 2002-14 Subaru WRX


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GrimmSpeed SKU: GRM113026

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Classic Subaru Sound with Unparalleled Performance

Over the years, GrimmSpeed Engineers have proven that we are capable of manufacturing the highest quality, best performing exhaust components for the Subaru aftermarket. While we might not always be the first to release a certain product, our major advantage is that we can take our time in testing and development, putting the product through countless hours and thousands of miles of testing. This ultimately ensures that once a component is in your hands, it will be the most-refined version of that product available. The all new GrimmSpeed Unequal Length EJ Header is a perfect example of this.

Our Unequal Length Header for the EJ-Series engines is very much the same. There are countless options out on the market for high-performing equal length header, but if you want to keep that classic Subaru rumble, there aren’t many great options.

The OEM EJ header really hasn’t changed much in the past 20+ years and it shows. The tight geometry and long cross-pipe between the two collectors are not optimized for flow or power. While the smaller diameter and volume may be good for low-end spool, the factory header gets choked out higher in the rev range. Simultaneously, it's known that swapping to a more-equal length manifold with larger diameter runners offers additional performance potential. We set out to create a header that would be the best of both worlds, offering great performance and the unique sound that makes your car unmistakably a Subaru!

While other aftermarket unequal length headers use different runner lengths for each cylinder, our proprietary design utilizes an innovative setup where each bank of runners are equal length, but one set is 1.5 time the length of the other, providing the rumble sound that we all love while maintaining the performance of an equal length setup. We utilize single piece, mandrel bent runners to minimize disturbances to the exhaust gas along with an extremely durable cast 304 stainless steel collector to achieve keep each back of runners truly equal length which would be difficult to accomplish with a 4-into-1 welded collector. The low angle merge this collector makes for faster spool and unbeatable off-boost throttle response. To match this flow, the collector features machined internal diameters to perfectly match the 1 ¾” primary runners and 2” secondary pipe, eliminating unwanted “steps” in the exhaust flow.

As with all of our exhaust components, our Unequal Length Header features ½ inch flanges for both the heads as well as the up-pipe side. These combined with the 14 gauge runner pipes make for extreme durability and heat retention.

We started the R&D process by scanning the underside of of several EJ engine setups using a FARO arm. This allowed us to design and simulate our 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. Next, we were able to 3D-print a variety of different configurations that would allow us to narrow in on the most optimal final design. During this process, we were able to form the header around a variety of different STI oil pan setups, from the factory pan to aftermarket options like those offered by IAG and Killer B Motorsports. After we were satisfied, we made pre-production prototype units that were put through months of punishing performance and longevity testing.

To validate our simulated performance, we did extensive real-world dyno testing comparing our header design to the factory header as well as some of the most popular headers on the market, both unequal length and equal length. For our testing, we used a 2010 STI with the stock turbo and basic bolt-ons as well as our 2020 STI shop car with full GrimmSpeed Stage 3 bolt-ons, E85, and a GrimmSpeed BB500 turbocharger. Throughout all of our testing, we kept the same tune loaded as well, so the only variable was the header that was installed.

With the factory turbocharger, the GrimmSpeed Header does experience a loss in spool when compared to the stock manifold, but we still saw maximum gains of 23.1 ft/lbs and 10.3 hp while averaging 1.6 lbs of boost less than the stock setup. Overall, the GrimmSpeed Header shifts the power curve further to the right, so it's clear that the OEM manifold was designed for low-end response but falls off higher in the rev range.

The GrimmSpeed exhaust manifold exhibits a loss of spool when compared to the stock header by 100rpm as well as producing over a pound less boost. However, the engine is outputting the same amount of torque, showing that the low and off-boost characteristics of the collector design balance out the changes from the differences in the rest of the header. The GrimmSpeed exhaust manifold makes less boost everywhere, but more power everywhere as well due to the increase in efficiency. Overall, we were able to achieve maximum gains of 25whp and 25 ft/lbs of torque when compared to the stock header.

When comparing the GrimmSpeed UEL Header to other unequal length offerings on the market, we were surprised to see that other header options really didn’t offer any upgrade over the factory header in terms of power or spool. The GrimmSpeed Header was able to easily out-spool the competition by 200 rpm and hold an average of 2psi of boost overall allowing for substantial maximum gains of 75 ft/lbs and 57 hp.

The GrimmSpeed Header out spooled the competition’s equal length header by 50rpm while making similar boost levels. Our Header produces more torque than the equal length manifold right out of the gate, producing about 15ftlbs more throughout spool up. At peak, the GrimmSpeed Header was able to produce and extra 2 ft/lbs of torque and an additional 21 hp compared to the equal length setup.


  • Innovative 1.5 to 1 Unequal Length Runners - Each bank of runners is equal length, allowing for equal length performance with an unequal length sound
  • 4-into-1 Low-Angle Merge Cast Collector - Ensure minimal restriction and maximum performance
  • Smooth, Restriction-Free Flow Throughout – Single piece mandrel bent tubing welded to a single piece merge collector removes unwanted “steps”
  • Substantial Power/Torque Gains - +25hp and +25ft/lbs of torque over stock
  • Quickest Spooling Aftermarket Header - Faster spool and more power compared to the competition
  • No Tune Required – Safe to run, just bolt on
  • Gaskets and Hardware Included – Everything you need to get installed and on the road
  • Designed To Fit With Aftermarket Oil Pans - Fits with the STI oil pan or popular aftermarket options
  • Full 304 Stainless Steel Construction - 1/2" flanges and 14ga runners for extreme durability
  • Manufactured and Welded In The USA – For consistency and top quality control


  • 04-21 Subaru STI
  • 08-14 Subaru WRX
  • 09-13 Subaru Forester XT*
  • 05-09 Subaru Legacy GT*
  • 05-09 Subaru Outback XT*
  • 02-05 Subaru WRX 5MT (Requires STI Oil Pan and Pickup)
  • 06-07 Subaru WRX (Requires STI Oil Cooler)
  • 04-08 Subaru Forester XT* (Requires STI Oil Pan, Pickup, and Oil Cooler)
  • 04-06 Subaru Baja XT* (Requires STI Oil Pan, Pickup, and Oil Cooler)

*Unconfirmed, may require further modification