Porting of Exhaust Manifold / Headers and Turbo

The standard exhaust manifold on an Impreza consists of a 3 piece design on a single scroll turbo equipped car. It is a well made item, flowing adequately for a standard production car – when coupled with a suitable uppipe, the standard item doesn’t become massively restrictive until tuning to around 400hp.

However, even at lower levels of tune, significant improvements can be made by extensive porting of the cast iron sections/collector.

A well ported manifold will increase gas flow, which will noticeably improve throttle and boost response, and turbo spool by at least 200-300rpm in most mildly tuned Impreza engines.

As can be seen by the carbon build-up in the picture above, there is plenty of material that can be safely removed to reduce turbulence and maintain gas speed as the exhaust gas leaves the cylinder heads and enters the manifold.

The bulk of the job is carried out using a die grinder and various carbide burr bits. The carbide burrs make light work of this labour intensive job, removing the cast iron material with ease. Various emery flapper style wheels are put to use in finishing and polishing work.

A standard exhaust steel gasket can be fixed to the manifold and used as a template for port matching the ‘downstream’ side of every join in the manifold. The design of the OE Subaru multi-layered exhaust gaskets (which use a fire-ring for gas sealing), makes it extremely important that the ports are not excessively enlarged to the point that the fire ring can no longer provide a seal, which can result in an incurable exhaust blow.

The port can then be fluted and progressively tapered down into the manifold, removing material and smoothing out each runner.

The ‘Upstream’ sections of every join are smoothed out, removing all casting imperfections and seams and then polished through.

The front cross-pipe and standard uppipe are of tubed design with welded on joining flanges. Any weld through can be ground away and blended into the pipe with an emery wheel. The exposed edges of the tubes can be knife-edged (with a steady hand! :))

It’s not possible to remove much material from these pipes, but they can be polished to promote good flow.

Once the porting job is complete, all gasket faces are cleaned off and made ready for reassembly.

Lambda sensor threads are chased out with a suitable tap, then the entire manifold is washed through with solvent, and blown through with an airline to remove every last piece of metal swarf.

Safely wrapped in bubble wrap, boxed up and returned to the customer for fitment. 

If required, all external lugs can be ground off to enable the manifold to be heatwrapped.

The same technique can be used to good effect in porting the exhaust housing of your turbo. The entry from the manifold uppipe to the exhaust housing can be gasket matched and blended into the housing. The wastegate aperture can be enlarged and radiused into the housing helping to prevent boost creep in hybrid turbo applications, and drop wastegate solenoid operating duty cycle, for any set boost level.

If you are interested in having your manifold, uppipe or turbo ported, please contact us to discuss your requirements.