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Aspiring Mad Scientist
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Discussion Starter #1 (Edited)
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:



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:



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.



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.



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.



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.



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. :wink:
 
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Silver Surfer
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well this is informative.. even though i dont have a skyactiv... none the less good job and keep us posted :D
also good luck ;P


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DIY PRO
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can't wait to hear how you make out. make sure you take photos while up on the lift and any issue that you run in to. I've been looking for something in the same mindset you have thought out here and worked threw.
why did you go offset instead of strait?
Like the 12215
or a bit smaller in width but a bit longer like the12635 in size
I am very very interested in how you make out and I WANT SOUND CLIPS, please.
 

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Video with side washed audio please... :yes:

I see so many exhaust note videos where they plant the camera right where the exhaust well.....exhausts. "Listen to my setup! Vrrrrroommmmcrrrrrackkkptfffffsssshhhhhkeaaaawwwwmmm...*then spits water vapor in camera lens like a poorly produced porno* "

Heads up on possible outcome.

Guy did this with a magnaflow 12266 muffler while keeping stock cat and resonator.


Here is the same guy, same car but decided to delete the resonator because it wasn't loud enough.



In other words, you might end up removing the resonator if you want a deeper and louder sound.

Here is my favorite one so far. Muffler delete , straight pipe with angled cut exhaust tip. Sorry, I love posting this cause I love this sound so much. Might get drone while driving though.

 

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Discussion Starter #6
can't wait to hear how you make out. make sure you take photos while up on the lift and any issue that you run in to. I've been looking for something in the same mindset you have thought out here and worked threw.
why did you go offset instead of strait?
Like the 12215
or a bit smaller in width but a bit longer like the12635 in size
I am very very interested in how you make out and I WANT SOUND CLIPS, please.
I went offset instead of straight because the space back there is very tight, and you do need to make a lateral transition between the axle-back flange and the muffler tip. Anything longer than 11" will hit the car's structural metal, and the 2" internal core size of the muffler is IMO a necessary point to keep the entire length of exhaust at 50mm and smooth out/speed up the airflow. I'll take plenty of pictures I promise, Chris at Castro Motorsport took a quick look and didn't see anything else that could get in the way but there's a lot of finger-crossing to be done.

And yes, I'll have the camera held at head-level because I agree, putting it at the pipe level doesn't give you any sense of overall tone or volume. But I still care less about the sound and more about the gas I can save. :stuart:
 

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Racing Beat is coming with a skyactiv specific exhaust closer to spring, we'll see how they approach this in their design

My only problem is cutting the stock pipe. I just don't want to do it, but I am intrigued on how yours is going to turn out, can't wait to see it
 

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I bet it comes out nice. Thanks for trying instead of asking what if


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I am interested in the tips. Would like something that comes out more and fills out the bumper cut out of my ms3.

Name of exhaust shop?
 

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I'm thinking something close to this Design, I was thinking Of just Deleting the Muffler however i need a backup plan in case its to loud. So thank you!
 

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Discussion Starter #13
It is pretty intuitive but I found a lot of people don't think about it, just like "stiffer springs = better handling".

It's also non-trivial to fit the muffler in there, which is what I'm actually excited about. The straight pipe design is of course very obvious.

90210- I mentioned it like 3 times already lol...
 

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I recall from a technical article that the length of the piping determines the frequency the exhaust resonates. Which determines the RPM it will drone. Basically this means that the shorter the piping is, the lower RPM it will drone. Too short will put it really close to the typical highway cruise RPM or even into your acceleration range. If you're like me and short shift around town this is unlikely but whenever you rev it up for a highway on-ramp or other event, you'll hear it drone. For example, I had a Mustang that dumped before the rear wheels and it would drone at 2k RPM. It was loud and buzzy around town but on the highway while revving closer to 3k RPM, it sounded really good.

The moral of this story is that shorter isn't always bad but sometimes, it is. :shuriken: It's more important that the flow velocity is better at low RPM than how long or short the piping is. If you don't care about sound at all, you will once it starts droning.
 

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Discussion Starter #15
That's definitely right, thanks for your reminder. The Corksport SRI freed up some sound and so already the car drones at 3k rpm, point #1. :blush 1: Droning is caused by standing waves in an open pipe as far as I remember, and the stock exhaust piping effectively ends at the entrance of the stock baffle muffler. There should be no droning stock because the baffles should be designed to kill standing waves in the piping.

The new system has continuous piping all the way to the tip, so really the tubular path length has increased, but this time there's no baffle system and so as you said, at some point the standing waves will collect and I'm going to drone. I thought that longer piping = lower rpm drone though, but I don't remember. It's also modifiable by adding a resonator of course, so if I do drone I was planning to stick an even longer resonator into the stock midpipe and pushing the drone range somewhere else. I'm hoping that I don't run into this problem, but if I do, that would be phase II of this project and I'd properly tune the sound if I'm forced to switch resonators anyways. :yes:
 
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Drone can be especially bad in hatchbacks and wagons, so it might be a bit tougher to get rid of. If you run into droning, rather than change resonators (which can be hit-or-miss) you can install an adjustable helmholtz resonator to find a good spot. 5th gen camaro and pontiac G8 guys have a company that specializes in j pipe exhausts to cancel the drone.

That's a dead end on the adjustable piece:



Here's a post I found with more info, but I can't find any examples on Solo's website:

PCW - Performance Car Web - View Single Post - J-Pipes and Helmholtz Chambers

Just a cool idas that you can probably make yourself for cheap.
 

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Discussion Starter #18
Looks like I was able to avoid the drone. :wink:

To recount my experience at Castro, the construction was simple in theory, the execution was very professional and high-quality, but damn if the fitment of the system 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.


Piece of junk.


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.


Turns out, we ended up cutting all the way to the rear subframe. Figures.


Inlet pipe on the Magnaflow was then flared to accept (via slip-fit) the piping from the picture above.


Now the first real test fit.


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.


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.


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.


Tack weld in place:


And inspect what can only be described as a close call.


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.


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.


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.


With the hard part finished, we mocked up the end tip fitment and tacked that in place. Starting to take shape.


Then prepare to TIG weld the stainless steel together to form the final product.


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.


Master Chris at work. Very, very nice welds.




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.


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.


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.






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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You have the best written and most well documented posts I've seen on any forum. Kudos to you Color0. :thumbup1 1:

About the loud high idle in the morning when the car is cold. If the skyactiv is anything like the the MS3 just depress the gas pedal about 10% while you start the car. Then let off the gas pedal when the engine fires up. It will prevent the car from going into a really high idle. Normal high idle is 1800+ on the MS3 depending on how cold it is outside. Depressing the pedal while starting lets the high idle only go to 1200 regardless of the ambient temps, which is a lot quieter.

I'm sure my neighbors were happy when I learned that trick. The skyactiv is similar to the MS3 in that we both have direct injection and high pressure fuel pumps so it should work.
 
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