This is not intended to be a definitive how to but more of a how I did it/how you could do it too/some advice to get you started on all your porting and polishing adventures.
I did this in about 4 hours from pulling intercooler and throttle body out to getting everything back in and the engine running. So it's not too hard.
*I don’t recommend knife edging the throttle butterfly for the faint of heart or inexperienced because it is a non serviceable part (you cannot buy a replacement from Subaru, only the entire TB). As was mentioned in this thread, if you take any material off the edge, you will get a higher than stock idle. I was fortunate enough to have a local fabricator to make me new throttle plates of varying thicknesses and sharpness to play with after finding out that Subaru could not provide me with a new throttle plate. An expensive lesson learned.
Recommended tools:
-10mm Socket
-12mm Socket
-Socket Wrench
-Torque Wrench
-Safety Glasses- you don't want aluminum shards in your eye or a broken grinding wheel to take one out
-A Dremel Tool or similar
-5/16-in. High-Speed Cutter
-5/8-in. Aluminum Oxide Grinding Stone
-120 grit Flapper Wheel
-Sandpaper 220 Grit
-Emery wheel (leaves a nice finish and it does come with most polishing kits)
-Cleaning and Polishing Kit (optional)
-A Small Ruler
-A Spot Light
-Can of Brake Cleaner
-Rubbing Alcohol
-Some Towels
To start, pull your intercooler out. Set it face down on a nice soft towel and remove the Y pipe from the back of the intercooler. 4-12mm bolts mate the Y pipe to the intercooler. They aren't torqued on there very hard at all (Remember this all you people who don't like to use torque wrenches).
Next, removed the throttle body. It is held on by four 10mm bolts. Here is the throttle body. Notice the plugged coolant passages.
![Image]()
Notice the inlet side of the throttle body. The ends are square off and rolled. On the throttle body, also notice the rim deep inside. Not a very smooth transition from a smooth silicone connector to pipe. See:
![Image]()
![Image]()
On the throttle body, I taped the inlets for the MAP sensor and TPS sensor vent (it’s some sort of vent, I’m not sure what it is) on the backside to keep metal shavings out:
![Image]()
To get keep an eye on how much material I removed and to keep everything even, I used a red Sharpie marker on the rim of the pipe.
Now it’s grinding time. After testing several different type/sizes of cutters and stones, I found the 5/16-in. High Speed Cutter grinds the quickest and most precisely. I then used an aluminum oxide stone to smooth things out. Followed by a sandpaper flapper wheel. Then a emery stone for final smoothing before polishing. I highly recommend using safety glasses because aluminum shards will fly everywhere.
![Image]()
Closeup of High Speed Cutter:
![Image]()
Use the Dremel Tool and Cutter to grind the surface from the lip back. Start at the lip and work backwards. This will inadvertently cause you to grind more off the lip side and less the further in you go. You can use the small ruler’s edge to make sure you are grinding at an consistent angle. I ground the inlet in four quarters, overlapping each side. This kept everything very consistent.
REMEMBER: YOU CAN ALWAYS GRIND MORE OFF BUT YOU CANNOT PUT IT BACK ON!!! TAKE YOUR TIME, THIS IS A CHEAP MOD THAT CAN TURN VERY EXPENSIVE, VERY FAST!!!
Remember to overlap and make sure the lip is a consistent thickness all the way around. I ground mine down almost to a knife edge on the Y pipe. Less sharp on the throttle body because it is more susceptible to damage if the intercooler outlet hits it.
After grinding with the high speed cutter:
![Image]()
After roughly grinding the inlet, use the aluminum oxide grinding stone to grind the surface smoother. Be careful as the grinding stone also has the ability to remove a lot of material. Use the grinding stone to remove any grooves you may have made using the cutter.
After using the grinding stone:
![Image]()
Now you are ready to smooth out the surface. Take the flapper wheel and sand down all the rough grinded area. Next, use the 220 sandpaper to smooth it out even more. Now use the emery wheel to get the surface really smooth.
After flapper and emery wheel:
![Image]()
![Image]()
![Image]()
![Image]()
The process is the same for the intercooler Y-Pipe
Now you have two choices: you can leave everything as is-relatively smooth. Or you can go one step further and polish everything. Polishing is time consuming but the end product is well worth it. I doubt it gives any sort of real, tangible performance benefits, but it won’t hurt. If anything it looks nice and still does to this day.
Polished Throttle Body Inlet:
![Image]()
Polished Y-Pipe:
![Image]()
Notice all the metallic dust in the previous pictures. That would be very bad if it were sucked into your engine. Now that you are finished grinding and sanding it is now time to thoroughly clean the inside of the y pipe. I recommend spraying the Y pipe with brake cleaner because it will degrease/clean the inside of the pipe. Then use a faucet to run water through the pipe and pull towels through each outlet several times. This will remove any dust/metal shards that are stuck in the pipe. Do this several times just to be safe. After a final rinse, pull dry towels through the y pipe to dry it out. The final, clean product:
![Image]()
![Image]()
On the throttle body, I recommend using rubbing alcohol to clean it. It’s a degreaser and it will not be harmful to the electronic parts of the TB. All clean:
![Image]()
![Image]()
![Image]()
![Image]()
Reinstall the TB. The service manual recommends replacing the gasket, that’s up to you. The tightening torque for the TB to manifold bolts is: 5.8 ft/lbs.
Now reinstall the y pipe onto the intercooler and reinstall the intercooler onto the car. Tightening torque for the Y pipe to intercooler bolts is: 11.7 ft/lbs.
If anyone would like to add anything/ comment/ chastise me, feel free. Thanks for reading!
BONUS!! Coolant Bypass Mod!
The coolant bypass mod is very easy. The outlet line is on the right side of the TB and connects to a nipple on a bracket. The inlet line is on the underside of the TB and is connected to the block (requires removal of the IC to get to).
The easiest and cheapest way to do it (when the engine is cold)is to take the coolant inlet hose completely off and discard it / put it in a drawer, disconnect the coolant outlet hose and connect the end that used to connect to the TB to the inlet nipple on the block. I found that the outlet hose is the perfect length and type of bends to run from the block nipple, under the TB straight, around the BOV return line, to the bracket with the nipple on it.
For grins and giggles, i blew some compressed air through the inlet/outlet on the TB to get all the coolant out, probably not necessary. I then took some plugs from Ace Hardware and stuck them on the inlet/outlet nipples on the TB, just so no crap gets in there.
After I did this, I didn't really see anything noticeable benefits. Someone may be able to add their input. I'm sure it keeps the intake charge a little cooler and it's so easy that it's worth it to me.
I'm not sure if there are really any performance benefits to doing this on a turbocharged car but i know the muscle car guys do this and actually see some results in a few extra horses.
The coolant running through the TB is there to keep the TB from icing up in cold conditions. Obviously, if you won't see very cold conditions, there’s no reason NOT to do this mod.
The plugs I used can be found at any hardware store in the plumbing section. I got mine at Ace Hardware.
PM me any questions and i can put up additional pics. Let me know!
Throttle Body Removed and Coolant Nipples Plugged:
![Image]()
Finished Product (Transmission is at the top of the picture):
![Image]()
Finished Product Alternate View (Transmission is at the bottom of the picture):
![Image]()
I did this in about 4 hours from pulling intercooler and throttle body out to getting everything back in and the engine running. So it's not too hard.
*I don’t recommend knife edging the throttle butterfly for the faint of heart or inexperienced because it is a non serviceable part (you cannot buy a replacement from Subaru, only the entire TB). As was mentioned in this thread, if you take any material off the edge, you will get a higher than stock idle. I was fortunate enough to have a local fabricator to make me new throttle plates of varying thicknesses and sharpness to play with after finding out that Subaru could not provide me with a new throttle plate. An expensive lesson learned.
Recommended tools:
-10mm Socket
-12mm Socket
-Socket Wrench
-Torque Wrench
-Safety Glasses- you don't want aluminum shards in your eye or a broken grinding wheel to take one out
-A Dremel Tool or similar
-5/16-in. High-Speed Cutter
-5/8-in. Aluminum Oxide Grinding Stone
-120 grit Flapper Wheel
-Sandpaper 220 Grit
-Emery wheel (leaves a nice finish and it does come with most polishing kits)
-Cleaning and Polishing Kit (optional)
-A Small Ruler
-A Spot Light
-Can of Brake Cleaner
-Rubbing Alcohol
-Some Towels
To start, pull your intercooler out. Set it face down on a nice soft towel and remove the Y pipe from the back of the intercooler. 4-12mm bolts mate the Y pipe to the intercooler. They aren't torqued on there very hard at all (Remember this all you people who don't like to use torque wrenches).
Next, removed the throttle body. It is held on by four 10mm bolts. Here is the throttle body. Notice the plugged coolant passages.
Notice the inlet side of the throttle body. The ends are square off and rolled. On the throttle body, also notice the rim deep inside. Not a very smooth transition from a smooth silicone connector to pipe. See:
On the throttle body, I taped the inlets for the MAP sensor and TPS sensor vent (it’s some sort of vent, I’m not sure what it is) on the backside to keep metal shavings out:
To get keep an eye on how much material I removed and to keep everything even, I used a red Sharpie marker on the rim of the pipe.
Now it’s grinding time. After testing several different type/sizes of cutters and stones, I found the 5/16-in. High Speed Cutter grinds the quickest and most precisely. I then used an aluminum oxide stone to smooth things out. Followed by a sandpaper flapper wheel. Then a emery stone for final smoothing before polishing. I highly recommend using safety glasses because aluminum shards will fly everywhere.
Closeup of High Speed Cutter:
Use the Dremel Tool and Cutter to grind the surface from the lip back. Start at the lip and work backwards. This will inadvertently cause you to grind more off the lip side and less the further in you go. You can use the small ruler’s edge to make sure you are grinding at an consistent angle. I ground the inlet in four quarters, overlapping each side. This kept everything very consistent.
REMEMBER: YOU CAN ALWAYS GRIND MORE OFF BUT YOU CANNOT PUT IT BACK ON!!! TAKE YOUR TIME, THIS IS A CHEAP MOD THAT CAN TURN VERY EXPENSIVE, VERY FAST!!!
Remember to overlap and make sure the lip is a consistent thickness all the way around. I ground mine down almost to a knife edge on the Y pipe. Less sharp on the throttle body because it is more susceptible to damage if the intercooler outlet hits it.
After grinding with the high speed cutter:
After roughly grinding the inlet, use the aluminum oxide grinding stone to grind the surface smoother. Be careful as the grinding stone also has the ability to remove a lot of material. Use the grinding stone to remove any grooves you may have made using the cutter.
After using the grinding stone:
Now you are ready to smooth out the surface. Take the flapper wheel and sand down all the rough grinded area. Next, use the 220 sandpaper to smooth it out even more. Now use the emery wheel to get the surface really smooth.
After flapper and emery wheel:
The process is the same for the intercooler Y-Pipe
Now you have two choices: you can leave everything as is-relatively smooth. Or you can go one step further and polish everything. Polishing is time consuming but the end product is well worth it. I doubt it gives any sort of real, tangible performance benefits, but it won’t hurt. If anything it looks nice and still does to this day.
Polished Throttle Body Inlet:
Polished Y-Pipe:
Notice all the metallic dust in the previous pictures. That would be very bad if it were sucked into your engine. Now that you are finished grinding and sanding it is now time to thoroughly clean the inside of the y pipe. I recommend spraying the Y pipe with brake cleaner because it will degrease/clean the inside of the pipe. Then use a faucet to run water through the pipe and pull towels through each outlet several times. This will remove any dust/metal shards that are stuck in the pipe. Do this several times just to be safe. After a final rinse, pull dry towels through the y pipe to dry it out. The final, clean product:
On the throttle body, I recommend using rubbing alcohol to clean it. It’s a degreaser and it will not be harmful to the electronic parts of the TB. All clean:
Reinstall the TB. The service manual recommends replacing the gasket, that’s up to you. The tightening torque for the TB to manifold bolts is: 5.8 ft/lbs.
Now reinstall the y pipe onto the intercooler and reinstall the intercooler onto the car. Tightening torque for the Y pipe to intercooler bolts is: 11.7 ft/lbs.
If anyone would like to add anything/ comment/ chastise me, feel free. Thanks for reading!
BONUS!! Coolant Bypass Mod!
The coolant bypass mod is very easy. The outlet line is on the right side of the TB and connects to a nipple on a bracket. The inlet line is on the underside of the TB and is connected to the block (requires removal of the IC to get to).
The easiest and cheapest way to do it (when the engine is cold)is to take the coolant inlet hose completely off and discard it / put it in a drawer, disconnect the coolant outlet hose and connect the end that used to connect to the TB to the inlet nipple on the block. I found that the outlet hose is the perfect length and type of bends to run from the block nipple, under the TB straight, around the BOV return line, to the bracket with the nipple on it.
For grins and giggles, i blew some compressed air through the inlet/outlet on the TB to get all the coolant out, probably not necessary. I then took some plugs from Ace Hardware and stuck them on the inlet/outlet nipples on the TB, just so no crap gets in there.
After I did this, I didn't really see anything noticeable benefits. Someone may be able to add their input. I'm sure it keeps the intake charge a little cooler and it's so easy that it's worth it to me.
I'm not sure if there are really any performance benefits to doing this on a turbocharged car but i know the muscle car guys do this and actually see some results in a few extra horses.
The coolant running through the TB is there to keep the TB from icing up in cold conditions. Obviously, if you won't see very cold conditions, there’s no reason NOT to do this mod.
The plugs I used can be found at any hardware store in the plumbing section. I got mine at Ace Hardware.
PM me any questions and i can put up additional pics. Let me know!
Throttle Body Removed and Coolant Nipples Plugged:
Finished Product (Transmission is at the top of the picture):
Finished Product Alternate View (Transmission is at the bottom of the picture):
