Quiet(ish), High-MPG Axle-Back Exhaust

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2013 Mazda3 Skyactiv
Hey everyone, just got back from Castro Motorsport in NoHo after verifying that my design is physically possible, so I think it's time to start a thread on this sub-project of mine.

From the moment I got my Skyactiv I've been looking not to gain power, but to free up as much wasted power as possible in order to get maybe more power but improve fuel mileage and driveability. Mazda IMO did a fantastic job on the engine proper, but the airflow through the engine is hampered in two important places, the intake and exhaust, thanks to the usual reasons: it can't be loud, Mazda can't piss off its mainstream customer base, etc. etc. Well -- we're not the 99%, so we can get a boost in fuel economy simply by de-restricting the intake and exhaust.

On the intake side, I really have no complaints -- I installed a Corksport SRI and the free-flowing filter gives a nice 2-3mpg improvement alone. It's good enough, so leave it be.

On the exhaust side, however, the commercial options are lacking. Tanabe and Racing Beat make axle-back exhausts but they keep the factory exhaust path, aka a full 360 degree exhaust turn. No matter how you look at it, each turn increases backpressure, so you're not going to optimize performance with these two systems. Corksport makes a straight-pipe exhaust, which is great, but I strongly disagree with their decision to use 60mm piping (up from 50mm stock). My car spends most of its life putting to work and back: at anything less than high-rpm full-throttle, a 2.0L NA Skyactiv engine just cannot produce enough exhaust gases to honestly need anything more than 50mm pipe. For daily driving, all the larger piping can really do is reduce exhaust gas velocity, which you need in order to get low-end torque and partial-throttle fuel economy. My impression is that the Corksport system can't really hurt your mpg's all that much, but certainly it's not designed for gains in that direction.

Finally some of you guys just delete the muffler outright and run 2" straight pipe as an axle-back, I think this is the smartest approach. The only thing is that I find pure muffler deletes too loud, so I wanted to add a little sound deadening so I can keep being friends with my local police.

This is kind of moot since I ended up just designing an axle-back, but in designing any exhaust system purely for fuel mileage, you want to maximize exhaust flow velocity and minimize backpressure as usual, but you put more emphasis on low-rpm exhaust velocity than high-rpm backpressure. So my thought was to keep the stock 50mm midpipe -- it's big enough, the bends are fine, the resonators can stay and the cats will stay anyways -- but optimize the axle-back section to fit a muffler in there with minimal bends. Here's a rough diagram of the stock exhaust with its garbage 360-turn design:

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Exhaust goes from the midpipe, 90deg into the muffler, makes a u-turn inside there, and then comes back out with another 90deg before exiting out the rear bumper. Following the muffler-delete guys' inspiration, I spent a long time on the internet trying to find a good muffler short enough to fit between the rear subframe and crash bar, and finally came up with this pretty much straight-through design:

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The Magnaflow 11132 is a tiny thing, with 2" (50mm) inlet, 2.25" (57mm) outlet, and the can itself is 3.5"x7"x11". The space between the rear subframe and crash bar/tow hook is just about a foot, and as soon as I read the tape measure I figured that it would be stupid close, and I'd probably have to heat-wrap it to protect against vibration and burning stuff up, but it would work. Oriented the way it is, the 11132 is essentially a straight pipe with some fiberglass cladding around the sides. Not too shabby.

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Here is another view of how the system is designed, the stock exhaust wraps over the rear subframe before coming down, into the muffler, making that u-turn and then coming back out. It's a lot of metal.

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The new Magnaflow takes up the space that's left behind by cutting out ALL of the bends adjacent to the stock muffler, so you can see it is a very tight squeeze but it routes the exhaust gases properly, with very minimal turns, no baffles, and 50mm piping all the way to the very exit.

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My visit to Castro Motorsport today was to confirm that no, I'm not crazy, it actually fits in the space, and that yes, it can physically be assembled and it's not thermally risky or anything. Once the muffler and modified exhaust hangers are in, the last and easiest part is just to get a nice tip on the end, and for that I picked up a Magnaflow 35125 off eBay. Pretty classy looking IMO, and of course the inlet diameter of the tip is 2.25" to match the muffler outlet perfectly.

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So there you go, I've been promised an easy weld and install job next Saturday and am looking forward to a lot less weight in the rear, a little more power and hopefully a 2mpg boost to complement the Corksport intake. I think it should be obvious by now that I don't claim to be doing anything new, but I'm using it in a different enough way that I thought it'd be proper to post about it. We'll see what the result is next weekend. (detect)
 
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Exhaust is in!

To recount my experience at Castro Motorsport, the construction was simple in theory, execution was professional and high-quality, but damn if the fitment wasn't a little sketchy. This is going to be a massive post, so bear with me.

It started off innocuously enough, we cut off the the stock muffler a few inches past the location we thought we could fit the Magnaflow.
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Piece of junk.
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Of course, there were not inches of extra piping to play with here, so our technician started incrementally cutting off piping and re-measuring whether the muffler would fit.
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Turns out, we ended up cutting all the way to the rear subframe. Figures.
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Inlet pipe on the Magnaflow was then flared to accept (via slip-fit) the piping from the picture above.
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Now the first real test fit.
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Unfortunately, like this the muffler was jammed up against the tow hook, rear crash structure and several bits of heat-shielding material, none of which is really safe touching the muffler.
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Solution? They don't call it "The Persuader" for nothing, lol... but yes, we hammered away at whatever was protruding too much on the Magnaflow. The hammer was relatively light, and the case construction on the new muffler excellent, so none of the welds broke and no exhaust leaks resulted from the hammering.
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After a few rounds of hammering, we got the muffler to sit right and maintain about 5-10mm clearance between itself and all structural/shielding parts.
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Tack weld in place:
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And inspect what can only be described as a close call.
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On the subframe side there was a little more room to play with, however the angle of this transition isn't quite ideal. A fabricator with more time and more angled piping to play with should really cut off more of the stock axle-back, and weld in an angled section to then turn towards the muffler.
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Overview of the muffler positioning. It kinda gets in the way of rear spring perch adjustment, but as long as you lift up the entire rear of the car (or only work from the wheelwell) it's not a huge issue.
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You can also see the post for the stock rubber exhaust hangers, those would have been melted by the new exhaust setup, so at this point we elected to build a custom exhaust hanger too, just welding a rod to the outlet tubing of the muffler, into a new bracket that would bolt into the tow hook support.
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With the hard part finished, we mocked up the end tip fitment and tacked that in place. Starting to take shape.
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Then prepare to TIG weld the stainless steel together to form the final product.
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Another view of the axle side of things, the transition bend/angle is pretty close to perfect but not quite there if I were seriously trying to engineer a product. The piping does continue bending internally though, and in the correct direction, so I would think this outer appearance has negligible effects on performance.
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Master Chris at work. Very, very nice welds.
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My phone went out of juice at this point, so I couldn't capture the process (however simple) of making the exhaust hanger, but here is the final axle-back. As I planned out, it's nearly a straight path through from flange to tip.
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And the fitment in the car is flawless. I deliberately tucked the 3.5" outlet a little higher into the bumper to fill up space and to prevent any road contact from possibly messing up the piping.
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I had the exhaust hanger purposely place the tip a little bit to the right of perfect center (of the bumper cutout): I will be shimming this thing myself leftwards to get it perfect as I didn't trust myself to eyeball it on-site, while the car is 6 feet off the ground and I'm too short to see.
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So, final thoughts:

1. Again, hard to test MPG properly with completely messed up variables, but so far I have been doing good MPG's despite the upsized and stickier tires. On a slight uphill stretch of the I-210 I'm still able to maintain a 65mph cruise speed at about 40mpg on the computer readout. Again, the new PSS tires are known to give 1-2mpg worse average than the previous RE760's, so no overall MPG difference means that the muffler is doing its job and doing it well.

2. The ECU took a long time to readjust to the new muffler. Throttle response and fuel economy were very bad for about 2 hours, scared the crap out of me at first. Car was running pretty rich as far as I could tell, but it has readjusted fairly well at this point. I need to reset the ECU and see if we can do better tomorrow.

3. Cold start idle is LOUD! I completely forgot that the Skyactiv engine idles at almost 2k for the first minute, in order to warm itself up faster. The high-idle exhaust volume borders on obnoxious as it tends to resonate around the garage I live in. VERY BAD.

4. Once the car is warm though, idle drops to 600rpm (lower than stock...???) and is extremely quiet and subtle. This I like. The volume is kept pretty subtle for most gentle driving activities, really. Around the city, I don't crest 2.5k rpms and so you can tell the car has an exhaust, but it doesn't seem to be bothering anyone else. Dom comments that it's "still quiet", aka good for me. If the gas is floored though, it gets pretty loud pretty quick.

5. No highway drone. Win.




I'll have to see where is a safe place for me to take a video/soundclip of the exhaust at idle, gentle driving, revved and floored. Stay tuned for video. :)
 
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Here is the video, a little bit quieter than reality. The last two segments have accurate volume, thank you roommate's iPhone 4S lol.

 
Nice write-up.

The sound from the end of the vid was pretty good.

On your point #3 (cold starts), when I installed my CS SRI I noticed the start-up was a big louder, so just curious on how much so with the exhaust.

Finally, on ths subject of CS exhaust, perhaps the design was meant for the old 2.0 (Non Skyactiv) and the 60mm gave room for modders for the 2.0 should they decide to go FI?

Maybe they can develop an Axleback for the Skyactiv?
 
On your point #3 (cold starts), when I installed my CS SRI I noticed the start-up was a big louder, so just curious on how much so with the exhaust.

Finally, on ths subject of CS exhaust, perhaps the design was meant for the old 2.0 (Non Skyactiv) and the 60mm gave room for modders for the 2.0 should they decide to go FI?

Maybe they can develop an Axleback for the Skyactiv?
The cold start high idle is essentially the car holding the throttle partially open, and that's where the exhaust gets a little embarrassing/troublesome in the mornings. The intake mods don't make much difference outside the car on startup, but the exhaust does.

Considering that the market for FI is so small, I wouldn't have even bothered sizing the piping to compromise between NA and turbo. I'm satisfied with my 50mm piping, as of today I'm getting worse fuel mileage because the alignment (toe angles) is way off, but once I have the ride heights fixed, the car re-aligned and the ECU reset, I think I should be able to keep a 30mpg average (city + highway) fuel economy even on 255/35/18 Super Sports. That'd be proof enough to me that the SRI + exhaust combo works.
 
The cold start high idle is essentially the car holding the throttle partially open, and that's where the exhaust gets a little embarrassing/troublesome in the mornings. The intake mods don't make much difference outside the car on startup, but the exhaust does.

Considering that the market for FI is so small, I wouldn't have even bothered sizing the piping to compromise between NA and turbo. I'm satisfied with my 50mm piping, as of today I'm getting worse fuel mileage because the alignment (toe angles) is way off, but once I have the ride heights fixed, the car re-aligned and the ECU reset, I think I should be able to keep a 30mpg average (city + highway) fuel economy even on 255/35/18 Super Sports. That'd be proof enough to me that the SRI + exhaust combo works.

Any updates on the MPG? I have a 13 stock 6spd, going to invest in the SRI and really considering this exhaust as it sounds just nice enough :)
 
If I try real hard, I've been able to eke out 32-33mpg in mixed driving (avg speed of 30mph, probably correlates to 60/40 hwy/city mix). That's on new 285-wide Direzza ZII's too so it's at least good to know the car can be coaxed to perform -- otherwise, I've moved to Baltimore now and it is difficult to crest 20mpg with all the crap you have to dodge.
 
Have you noticed any power gains from this mod?
I have been looking at upgrading my exhaust, but the only ones I can find still have the 360 bend at the resonator.
Also, how much did this end up costing you? I dont' have any hook-ups so I would be paying full price.
 

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