Exhaust "tuning" Naturally Asp.

[torxxx]

So I decided I was tired of smelling exhaust in the cabin of my lifted outback so I had a buddy bend me some tubing up and we completed the exhaust.  I left the stock manifolds on, aftermarket cat and then ditched the second cat.  exhaust had ended at the tail shaft of the tranny (so all the fumes came up through the shift boot thats not there anymore.

 

OFC I noticed the lack of back pressure.  It actually made the car a big below 4k rpms.  I started looking at the stock system and they ran 2" OD pipe so technically 1 7/8 ID (we use ID when doing exhaust)   The mufflers on these cars gave them TOO much back pressure and I think thats why a lot of the EJ25D's suffered from the exhaust whistle ~ 3800 RPMs.  it was exhaust being forced out the drain hole in the muffler.

 

So by removing the last 4 feet of exhaust, we decided to try out 1 3/4" ID pipe from the tail shaft of the tranny down the car exiting @ at 45 degree angle right in front of the rear tire.  Its cut back enough that theres no way of snagging it on a tree backing up, it had 2 solid rubber box bolt on exhaust hangers so I can still remove the system (I love how subaru put random 12mm bolt thread inserts all over the car)  

 

Now the car is a blast to drive.  I'm not sure if its the correct amount of BP for a 2.5L, but its way better that stock, its got that throaty sound on accel and a nasty crackle on decel (got that good old Jake brake sound hehe).  Got back tons of bottom range, actually looped out on my rally road on like the 3rd turn because normally at that speed and gear, there was no power.

 

So here is where it comes into the tuning part of it.  There is several sections in the stock system that would allow for bigger/smaller pipe.  I'm wondering if I added in say a foot of 2.5" inbetween the cat and the tail shaft and then narrowed it back down to 2" down to 1 3/4" back to 2.25" then back to the 1 3/4" tail pipe would I see any performance gains or would it just be acoustical changes?

 

Kinda what gave me the idea is they can tune the ports of an intake, and air intake volume and storage play a huge roll in how an engine accels/has power @ set ranges etc.   So I'm figuring maybe theres more we can do with our exhausts to get more useable power out of these N/A Engines?

 

 

 

[efseiler]

I recently got my exhaust patched and now there's noticably more pickup and it runs hotter.  The temp gauge (which normally is fixed like some kind of quasi-cosmological constant) is now a few hairs higher. 

 

 

Given the ethanol in fuels these days (which probably varies from station to station) it's probably difficult to gauge what's the best combo to maximize fuel efficiency.  You'd have to put your Carnot cap on and trust the gods.

 

 

Maybe the on-board computer is smart enough to make the necessary adjustments...it may need two cat converters to work best.

 

The mileage can vary greatly...some tanks all I sometimes got was 275 miles...others I've gotten closer to 370 miles.

 

--Damien

 

 

p.s. If you don't have a Carnot cap...you may need to go to college to get one.  :-P

[torxxx]

not going for mileage one bit.  None of my Subaru's get better than 20 mpg cuz of the way I drive.   Remember 32 redlines a day keeps the mechanic away 

 

This is more geared strictly to usable power.  I refuse to run forced induction because of reliability issues, but if we can figure out a way to tune the back half of the exhaust, a few extra HP makes a HUGE difference when you are only dealing with 165hp to begin with.  Not to mention the ability to change the RPM that the torque curve starts.   The EJ25D is a high RPM torque motor, where as the 2.2 and the EJ253 are lower RPM torque, so im thinking maybe some form of variable exhaust.  

 

as far as the two cats go, you only need the second cat if it has an 02 sensor behind it.  Oh and I've noticed zero temp change with the new exhaust.   Mine always runs a @#%^% hair under half temp, even when I'm holding it to the floor in a mudhole/snowbank/icetrack etc. 

 

Onboard computer may be getting replaced with Megasquirt.  I have enough running rigs that I can afford to park one long enough to setup Megasquirt.  That and Subaru's technology/computer logic kinda sucks anyways.  Look at how horribly the AWD function is written for the 4EAT