First step is cutting off the muffler and replacing it with a straight pipe. The noise is MUCH more than what I was hoping for, but the power is definitely increased. As an added bonus the car pulls all the way to about 5200 RPM now, up from about 4700. This means there is still some exhaust restriction or a flow restriction in the heads, intake or intake manifold. The next step will be improving the down pipe, I suspect that is where most, if not all of the flow restriction currently lies. Here are the actual results.
Removed Muffler
Peak Airflow 3.85
Fuel Injector Peak 84
1/4 mile ??
Thats a .2 increase in airflow voltage and 12% more fuel flow. Running fuel injector calculations show about a 20hp gain. Not bad for sawing off a muffler.
The noise was simply way too much to bare so a muffler had to go on the car. I wasn’t about to put that extra restrictive “turbo muffler” back on the car and following the nature of this article a $150 Borla is too much… in fact a $60 Dynomax Ultra muffler is too much. So I made one. To make it, I rolled a piece of 12X18 20gauge mild steel and welded it into a tube. I then traced out 2 16 gauge end caps and drilled out the middle with a 2.5″ hole saw. The part of exhaust which this slipped over was drilled with many small holes and the space in the muffler filled with steel wool. Sounds easy, but welding such thin metal is very time consuming and challenging. The process took about 2 hours start to finish.
The Results: Much quieter, no drones on the freeway and unless I’m on it, the car is very comfortable to drive. High RPM’s have a resonating sound, but that’s probably because there’s no resonator on the car. Just a 2.5″ turbo back with the built muffler. Flow should not be decreased, but I will check this to make sure and post up when I get the results. (Sorry, but I never tested at this stage)
Built Muffler
Peak Airflow ??
Fuel Injector Peak ??
1/4 mile ??
Down Pipe: Notice though the smooth radius inside the wastegate portion and the contoured edges. If you are already running my downpipe you can contour the edges going into the down pipe for much increased flow. Similar to how a bell mouth opening will increase flow by about 14%. You could also add the additional ‘divorced’ 1.5″ section. Another step to increase the airflow is to move the o2 sensor further down the pipe. I’m hoping this will keep the turbulence to a minimum.
High Flow Down Pipe
Peak Airflow 4.00
Fuel Injector Peak 101
1/4 mile 15.686 @ 86mph
Brakes: I was able to pick up a set of used low mile 2001 Impreza RS pads, calipers and rotors from NASIOC for $60 shipped. I also bled out all the old fluid and replaced it with DOT 4 Valvoline synthetic. This mod should have been done LONG ago. The pedal feel is still kinda squishy, but the braking has improved considerably. During the bed in process, I was able to lock the front wheels at 70mph… this is with a firm, but not HARD pressure on the brakes. The old brakes couldn’t lock up over 30.
Front Suspension: Since I had to replace the front wheel bearing and the steering rack needed a rebuild anyway, I decided it was a good time to do the front struts, top hats, and steering rack bushings. The front struts were $150 with delivery from Tire Rack. The top hats are used RS 30,000 mile pieces I got in exchange for installing a new suspension on an RS. The steering rack rebuild was $160 and the rack bushings were $30. What can I say, the front is much tighter now, but because so much has changed, I can’t really comment on the rack bushings.
Porting: I had the exhaust manifold off anyway and a few days to wait for the steering rack. That said, I head out with drill and grinding stones in hand to port the exhaust ports and exhaust manifold. I’ll copy what I said in a post to save myself some typing.
While I had the exhaust manifold off and a day at home, I decided I might try a little porting on the exhaust ports and exhaust manifold. I had previously done some minor porting on the exhaust manifold to get rid of some of the intrusion going in. Well, I finished it off on Sunday grinding off the entire lip making for a smooth transition, not to mention MUCH less turbulence going into the exhaust manifold.
The Heads: The heads stayed on the car so I was only able to clean up the ports a few inches up. I ground down the lip right at the exit and some casting marks paying special attention try and not make the ports any larger. Realize, I want to keep the original flow characteristics, just clean them up a little. After hitting it with a grinding stone, I tried a less course smaller stone which smoothed it out some, but no where near a “mirror like finish.” Realizing the smoother the finish, the less likely carbon deposits will stick, I grabbed some WD40 and 2000 grit sand paper. 400 grit probably would have been better, but it’s all I had. With that I smoothed out the ports by hand.
All in all, both the exhaust manifold and heads took about 2.5hours.
Results:
Hot DAMN!!! My goal was (is) to get a good smooth pull all the way up to the 6200 RPM redline and an extra 4mph in the quarter so I hit the lights at 90mph. Well, I can’t say anything about the trap speed, but the power band is MUCH improved. I got the smooth pull all the way up to 6200RPM whereas it dropped off at about 5600 before the porting. The surprise is the better throttle response and low end power. I found myself cruzing in 4th instead of 3rd. I have always had good low end power from about 2800 rpm or so, but now it’s from about 2400 RPM. Even better is I can now boost 5psi if I roll on throttle at only 2000 RPM. By 2600 I have the full 8.5psi of boost.