Exhaust Headers and Manifolds

For those of you who desire more performance, and we all do, the next level of performance requires the upgrade of your factory exhaust manifolds. Also known as turbo header for turbocharged vehicles, or "headers" for those with naturally-aspirated cars, or simply exhaust manifold, a proper unit flows more exhaust gases faster when compared to your factory components. A proper exhaust manifold will also be required if you desire to make more power that surpasses the flow limits of your factory manifold. If you want to maximize the performance out of your factory turbocharger, an upgraded manifold is the way to go. If you upgraded to a larger turbocharger and are running more boost in your purpose-built race car, you will need a proper manifold to support the flow. When compared to your factory heavy cast log-style manifold, aftermarket options have longer tubular runners that smoothly meet collectively at the mounting flange. No flow restrictions mean more power. A must-have for those of you who have substantial power goals in mind. 

 

WHAT ARE THE MAIN DIFFERENCES BETWEEN A FACTORY MANIFOLD AND AN AFTERMARKET EXAMPLE?

So the factory exhaust manifold is going to experience more heat than any other engine component outside of the engine. Because of these reasons, the majority of manifolds are made using heavy-duty cast-iron materials. Cast iron manifolds are extremely heavy. For example, ever seen or removed the factory exhaust manifold on an 5 STI? It is a literal boat anchor in terms of weight. However, it has to be durable to withstand thousands of miles of daily use, it has to flow enough to support 3 HP at 6 RPM from the factory so it is functional. But whether it be a Subaru STI, Mitsubishi Evo, or even a Chevrolet Corvette, there is much room for design improvements. 

Aftermarket exhaust manifolds whether they be headers for naturally aspirated applications or turbo manifolds for our cars here such as the Subaru WRX or Mitsubishi Evo will have longer primaries or a combination of larger and longer primaries. The primaries are the tubes that are fabricated into the manifold and will increase the flow for each cylinder port. A conventional cast manifold is short and sometimes referred to as a "log" manifold because it resembles a heavy log on the exhaust side of your head. Because of the short design that has been essentially machined out of cast iron, it can be restrictive, especially when trying to make more power as your modifications can surpass the limits of the factory exhaust manifold. 

The longer tubes in aftermarket designs will free up the exhaust flow by allowing for an unobstructed flow path for your turbocharger, or for the rest of your exhaust system in your naturally aspirated application. Aftermarket manifolds are also crucial when upgrading to a larger turbocharger that will require the extra flow needed for more boost. 

 

WHEN SHOULD I UPGRADE TO A LARGER EXHAUST MANIFOLD?

Opinions will vary, but whenever you are upgrading your exhaust system, you should always start from the back first. You don't want to create a bottleneck of flow leading out of the back. You should start with a CAT BACK EXHAUST SYSTEM, then move upward to your DOWNPIPE AND Y-PIPE, then work towards your exhaust manifold. If you installed a downpipe first and adapted it to your factory cat-back, it would be like having a large volume of water trying to escape through a small funnel. Always start from the cat-back system, then move your way up.

 

However, there are instances where you would have to upgrade the exhaust manifold, and that is if you upgraded to a rotated turbocharger on your Subaru WRX or STI, or perhaps you might have got an exhaust manifold with an external wastegate setup for optimal boost control. And there are times when you may have upgraded the turbocharger that has a different mounting flange on the manifold such as a T3 or T4 setup that will work for a large variety of turbochargers. In this case, these manifolds should include their own downpipe setup, and some fabrication will be required to complete the installation. 

 

In conclusion, these are some of the benefits and/or requirements when upgrading to an aftermarket manifold:

• Lighter
• Typically made with strong 34 stainless steel construction
• May come coated
• May include external wastegate flanges
• Flow more volume to make more power 
• May be required to work with other turbocharger setups
• Will help produce a more unique exhaust note
• May be required for earlier Subaru WRX models that need clearance for the STI oil pan
• May be required if you are planning on making substantial power levels that cannot be achieved through the factory manifold