Project GD STI: Getting More Power with Precision Turbo and Cobb Tuning! – Part 1


With everything removed the stock turbo gets lifted out the top.
The cold side of the Precision Turbo dwarfs the stock turbo.  This gives you a better idea of how a large wheel in the stock housing just isn't the greatest thing to do.  You might give up nearly all of the diffuser where the compression takes place.  This can cost you more than 10 points of compressor efficiency.
You can see how much bigger the turbine inlet is here.  It all helps to reduce backpressure. 
Looking at the turbo's hot side you can see just how much bigger the turbine wheel is than stock, how much larger the turbine housing's A/R is and how much bigger the wastegate flapper door is.  Just putting a bigger turbine in a stock housing means the the housing's flow still isn't going to be as good and turbine efficiency also suffers.  The Precision turbo is an engineered drop in turbo, not just simply a bigger wheeled one.
Since we were in there, we switched out the stock manifold for an MXP equal length tri-Y header.  A tri-Y header goes 4-2-1.  A tri-Y typically makes more bottom end power than a 4-1 with a slight decrease in top end power.  Although some people love it, we are not fans of the Subaru flat four busted motorboat sound and the tri-Y header will leave our car sounding more like a conventional 4 cylinder.  The MXP header is constructed of high quality 304 stainless steel with clean TIG welds.
The stock Subaru exhaust manifolds are not bad for flow but the pulse tuning is not there and they require low overlap to prevent cross contamination.  The MXP headers are pulse tuned and will work better with the camshafts we have planned for later with their longer runners.
The MXP up pipe is larger in interior diameter and freer flowing that stock.  It also shares the nice 304 stainless construction and clean welds of the header.

Leave a Reply

Your email address will not be published. Required fields are marked *