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. (detect)
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. (detect)
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