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Old 03-11-2007, 08:47 AM   #2
shvrdavid
Scooby Specialist
 
Member#: 86078
Join Date: Apr 2005
Location: Central Pa.
Vehicle:
2005 Sti
4094R Twin Scroll

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As you have already guessed...
Porting and polishing is basically finding restrictions, and getting rid of them to increase the flow...
Porting the exhaust manifolds and pipes will not do much for hp numbers for the reasons you mentioned...
There isn't that much to gain there...
Typically, the gains that can be found in porting are in these places...
Throttle body and Intake, including Tumblers...
Cylinder heads (see my thread on that, http://forums.nasioc.com/forums/showthread.php?t=973620 )
And the turbo itself... There are a few threads on here about porting turbos...

There are things you need to remember when porting...

The first thing is to try and keep a constant diameter...
A perfect example is the exhaust side...
If there is some difference in size between two mating surfaces, there is probably a difference in overall diameter of the entire port...
You wouldn't want to do just taper the inlet of an exhaust manifold, you should enlarge the entire passage...
That is called extrusion honing...

Something else to keep in mind is that the mating areas of exhaust flow better if the area gets bigger as it passes a mating surface...
Think of step headers used in drag racing...
This step breaks up the shock wave returning to the valve...
The up-pipe and the crossover pipe (the flex joint) are examples of a diameter change to break up the shock wave on the WRX exhaust...

If you port the cylinder heads, your exhaust ports will end up larger than the ports they flow into...
The heads on my thread will require a least 2" id header just to keep from covering the port on the head...
The intake side of them, will require custom tumbler deletes and intake manifold to match those ports...

Too make matters worse in the porting department, you can do more harm than good, unless you know what you are doing...

If you deciede to port anything, research it first, and make sure you take your time doing it...
Taking metal out is easy, putting it back is another story....
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