Process West Intake Manifolds: What They Are, How They Work, and What to Know Before You Buy or Install One
Process West is an Australian-based aftermarket manufacturer known primarily for performance components used in turbocharged vehicles — particularly Subaru models like the WRX and STI. Their intake manifolds are among their most well-known products, designed to improve airflow, boost response, and overall engine performance compared to factory units. Understanding what these manifolds do, how they differ from OEM parts, and what the installation process involves can help you make a more informed decision about whether this type of upgrade fits your build.
What an Intake Manifold Does
The intake manifold is the component that distributes air (or an air-fuel mixture, depending on the engine design) from the throttle body to each cylinder's intake port. Its internal volume, runner length, and plenum design directly affect how efficiently air flows into the engine — especially under boost in turbocharged applications.
On a turbocharged engine, the intake manifold sits downstream of the intercooler and turbocharger. Boosted air enters the manifold through a single inlet and gets divided among the cylinders. The geometry of that distribution matters a great deal: uneven airflow between cylinders can cause inconsistent combustion, reduce power, and complicate tuning.
What Process West Designs Differently
Process West intake manifolds are aftermarket performance replacements engineered to address limitations found in the stock Subaru EJ-series and FA-series intake manifolds. Key design differences typically include:
- Larger plenum volume — More internal air storage volume smooths out pressure fluctuations and supports sustained boost at higher RPM
- Revised runner geometry — Some designs equalize airflow distribution across all four cylinders, which can improve consistency and tuning accuracy
- Top-feed fuel injector compatibility — Many Process West manifolds are designed to work with top-feed injector conversions, which flow more fuel than the stock side-feed setup on certain Subaru engines
- Thermal properties — Some versions are made from materials chosen to reduce heat soak compared to cast aluminum OEM manifolds
The specific features vary by model. Process West has produced several versions over the years, including designs for the EJ20, EJ25, FA20, and FA24 engines in both naturally aspirated and turbocharged trims.
How Installation Generally Works
Installing an aftermarket intake manifold is a significant job — not a quick afternoon task. Here's what the process typically involves:
- Depressurize the fuel system before disconnecting fuel lines
- Drain coolant if coolant lines run through or near the manifold
- Remove the intercooler piping and throttle body
- Disconnect vacuum lines, sensors, and electrical connectors attached to the manifold
- Unbolt the factory manifold from the cylinder head
- Clean the mating surfaces carefully to ensure a proper seal
- Install new gaskets — never reuse intake manifold gaskets
- Mount the new manifold and torque bolts to spec in the correct sequence
- Reinstall all components, including updated fuel injectors if switching to top-feed
- Retune the engine using an ECU reflash or standalone management system
That last step is critical. A larger intake manifold changes how air enters the engine. Without an appropriate tune, the factory ECU may run too lean, too rich, or struggle with idle control. This is not an install-and-drive part.
Variables That Shape the Outcome 🔧
No two builds using a Process West manifold will produce the same result. The factors that matter most include:
| Variable | Why It Matters |
|---|---|
| Engine type | EJ vs. FA engines have different port configurations, injector styles, and tuning needs |
| Turbo size | A larger manifold pairs better with a larger turbo; mismatching can reduce low-end response |
| Injector setup | Top-feed conversions require a fuel rail change and often larger injectors |
| ECU and tune | Open-source (e.g., OpenECU), piggyback, or standalone tune determines how well the engine adapts |
| Supporting mods | Intercooler size, boost controller, exhaust flow — all influence how much benefit the manifold provides |
| Installer experience | Subaru intake manifold work requires knowledge of torque sequences, gasket sealing, and vacuum routing |
DIY vs. Professional Installation
Experienced DIYers with Subaru-specific knowledge do install these manifolds themselves. However, the job involves fuel system work, which carries safety considerations, and the mandatory retune typically requires either a professional tuner or someone with hands-on experience using tuning software. A failed tune or vacuum leak can cause engine damage — misfires, detonation, or lean conditions aren't always immediately obvious.
Shop labor costs for this type of work vary considerably by region and shop type. A dedicated Subaru performance shop will have a different rate than a general mechanic, and tuning fees are separate from installation. 🛠️
The Missing Pieces
How much benefit a Process West intake manifold actually delivers depends entirely on your specific engine, its current state of modification, how it's tuned, and what the rest of the build looks like. On a lightly modified street car, the gains may be modest. On a built motor with supporting hardware, it can be a meaningful piece of a larger system. What your engine gains — or doesn't — isn't something that can be determined without knowing exactly what you're working with.
