Subaru Impreza WRX & STi head pair

The Subaru Impreza has a well deserved reputation for excellent performance and handling and a very affordable cost. The performance to cost ratio is extremely good and there is plenty of scope for tuning. As with most modern turbocharged cars, significant power increases can be gained by Engine Management system changes. These changes can take the form of standard "chips' or custom made maps derived from hours on a rolling road. Larger intercoolers, free flow exhausts, up-pipes, down-pipes, dump vales, boost controllers can and do make significant differences to power output and delivery.

When we looked at the standard Sti head we were quite impressed. In standard form it ran at 170CFM inlet on our flow bench. That's pretty impressive for a standard head. We would have a tough job improving on that. During our development phase, we were also fortunate enough to acquire a pair of heads from a works WRC Subaru as used in the world rally championship by Prodrive. Obviously the first thing we did was check the flow on our flowbench. The result was, shall we say, rather suprising. Rather than showing higher than standard flow, it actually shows less. 159 CFM. Why the team would choose to use these heads over a standard head might seem strange but it pointed us to where the real weakness was with the standard head.

The Prodrive head was a one off casting. To improve the flow ratio between inlet and exhaust, the WRC head had smaller ports. Flow ratio is important as a means to measure efficiency of a particular head design. Improving the ratio helps to improve flow derived from the supercharging effect of exhaust "draw" on the inlet where pressure pulses in the exhaust tract helps boost inlet flow. This is particulary important with a turbocharged engine where it is important to keep turbine speeds up with an efficient exhaust tract. What they managed to do in this case was boost the exhaust flow and improve the ratio by a reducing inlet flow. Understandably, to reduce turbo lag for rally applications, Prodrive focused on improved inlet/exhaust ratio, however, this head is limited in terms of absolute power potential.

The standard exhaust flow of 104 CFM is nothing to shout about (A standard Ford Cosworth exhaust is 119.6 CFM) so improvements here will help turbo efficiency and turbo boost. The inlet CFM is is very good so we had to work hard to improve that. We managed to achieve both after a great deal of development work.

Our Subaru head offers better inlet flow than the standard head and the WRC head. Inlet is up 8% and exhaust is up 31%. It also gives 14% improvement in inlet/exhaust ratio over the standard head iimproving turbo response.

The summary below shows the ratio and derived power prediction for a normally aspirated engine with our head. We do not publish turbo power predictions because boost figures are variable. We can confidently say that this head will provide huge power (500bhp +) with the right turbo set up and improve turbo response over standard.

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Note : flow diagrams show flow against specific valve openings at 0.050 inch valve openings at 10" pressure. Therefore actual flow volume increase is measured as the area under the graph (indicated in yellow for inlet and blue for exhaust). Standard valve sizes are used on our heads unless specified as a big valve version. Tests shown here are carried out on a flow bench on a bare head without filters. Therefore the difference between a standard engine with standard inlet ducting and filters will be larger than shown here.

Click any image below to see the full reports as generated by our gas flow bench and Port Flow Analyzer software. Click on the images for full size image: