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Discussion Starter #1
Hey guys, I’m wondering if there is any company making a rear underbody diffuser for our third gen Mazda3? Like the ones showing below for ND MX-5, but they don’t have anything for Mazda3 on their website...



The reason I am looking for it is because I’ve the Borla axle back and the Borla doesn’t fit flush with the rear underbody like the OE exhaust which also acts like a diffuser.

Thanks in advance!




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2015 Mazda3 Sport GT 2.5L MT
- Enkei TS10 18x8.5/50 Gold
- Borla S-Type Axle Back
- Eibach Pro-Kit
- CorkSport Rear Sway Bar
- CorkSport Rear Motor Mount
- CorkSport Short Shift Plate
- CorkSport Shifter Bushings
- CorkSport Front Strut Bar
 

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Do you have pictures of your borla axle back and how it doesn't sit flush? I'm thinking about getting the borla and I didn't know there was some fitment issues with it.
 

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2018 Mazda 3 GT called Coffee.
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I think he's referring to the fact that the Borla exhaust has a smaller muffler that feeds to each exhaust tip vs. the OEM exhaust, which has one big muffler and two exhaust tips that feed out of it.

The OEM rear aero kit addresses this a little bit, but it's probably not what you are looking for.
CK
 

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Discussion Starter #4
Do you have pictures of your borla axle back and how it doesn't sit flush? I'm thinking about getting the borla and I didn't know there was some fitment issues with it.

I think there’s some misunderstanding here, there no fitment issue at all. I was trying to say that the muffler is way smaller than the OE exhaust so there’s might be some aerodynamic problems so I was wondering if there is any underbody diffusers available for this gen Mazda3.

I’m really sorry for making you misunderstood.

Here’s the Borla exhaust, fits nicely and sounds amazing! Go for it!





Sent from my Washing Machine using Tapatalk
___________

2015 Mazda3 Sport GT 2.5L MT
- Enkei TS10 18x8.5/50 Gold
- Borla S-Type Axle Back
- Eibach Pro-Kit
- CorkSport Rear Sway Bar
- CorkSport Rear Motor Mount
- CorkSport Short Shift Plate
- CorkSport Shifter Bushings
- CorkSport Front Strut Bar
 

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134 Posts
Discussion Starter #5
I think he's referring to the fact that the Borla exhaust has a smaller muffler that feeds to each exhaust tip vs. the OEM exhaust, which has one big muffler and two exhaust tips that feed out of it.

The OEM rear aero kit addresses this a little bit, but it's probably not what you are looking for.
CK
Thanks for your reply! However the OEM rear diffuser doesnt cover the muffler, its just like a extension of the rear bumper.

What Im looking for is something like the ones showing in the picture, which could cover the rear underbody, from rear axle to rear bumper.

Thanks again for your suggestions! :)
 

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@am0s.x, I know what you mean, I also have the Borla Axle Back Exhaust and it would be interesting to know if the aerodynamics of the car and air flow direction actually changed with the new muffler installed.

Taking into consideration that the stock muffler is bigger and its built under "lines" possibly re-direct the air flow to go under the rear bumper this creates IMHO the expected and most optimal aerodynamic flow that engineers aspired to when checking the drag coefficient and other factors. I found a video of the aerodynamics of the Mazda 6 which uses the same muffler as the Mazda 3 (Check at 0.40secs):


So, now with the Borla exhaust installed the logical assumption could be that the air flow is being pushed inside the bumper (into) and getting trapped, since there is a bigger gap due to the fact that the new mufflers are smaller than OEM and by not having any lines to re-direct the airflow optimally, thus creating some minimal resistance as the air in the underbody it's not going outside freely and smoothly, but all of this could be BS as this is a very vague assumption of course.

As per my search into this subject I did not find any specific "under rear diffuser" for our cars, the only logical choice which I found was to buy a rear bumper diffuser as per this thread http://mazda3revolution.com/forums/2014-2017-mazda-3-skyactiv-appearance-interior/123770-rear-bumper-diffuser-3.html#post1614705

Also checking some modern cars as for example the new M5 they use a similar rear bumper diffuser but it would be safe to assume that these cars as well have an underbody diffuser to have the most optimal aerodynamics.

I'm checking whether or not to invest on a rear diffuser from TheSpeedLine.
 

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Discussion Starter #7
@am0s.x, I know what you mean, I also have the Borla Axle Back Exhaust and it would be interesting to know if the aerodynamics of the car and air flow direction actually changed with the new muffler installed.

Taking into consideration that the stock muffler is bigger and its built under "lines" possibly re-direct the air flow to go under the rear bumper this creates IMHO the expected and most optimal aerodynamic flow that engineers aspired to when checking the drag coefficient and other factors. I found a video of the aerodynamics of the Mazda 6 which uses the same muffler as the Mazda 3 (Check at 0.40secs):

https://www.youtube.com/watch?v=oHpklO0cfC8

So, now with the Borla exhaust installed the logical assumption could be that the air flow is being pushed inside the bumper (into) and getting trapped, since there is a bigger gap due to the fact that the new mufflers are smaller than OEM and by not having any lines to re-direct the airflow optimally, thus creating some minimal resistance as the air in the underbody it's not going outside freely and smoothly, but all of this could be BS as this is a very vague assumption of course.

As per my search into this subject I did not find any specific "under rear diffuser" for our cars, the only logical choice which I found was to buy a rear bumper diffuser as per this thread http://mazda3revolution.com/forums/2014-2017-mazda-3-skyactiv-appearance-interior/123770-rear-bumper-diffuser-3.html#post1614705

Also checking some modern cars as for example the new M5 they use a similar rear bumper diffuser but it would be safe to assume that these cars as well have an underbody diffuser to have the most optimal aerodynamics.

I'm checking whether or not to invest on a rear diffuser from TheSpeedLine.
Hey David, thanks so much for your detailed information! I've also done a lot of searches and wasn't able to find any rear underbody diffusers for this third gen mazda3s.

Hopefully there would be some options after the speed model come out in the future.

Thanks again for your input and I do really appreciate your great help!

BTW, if you like the TheSpeedLine style diffuser, you may want to try search on Aliexpress, there are a lot more options and cheaper prices.:)

https://www.aliexpress.com/wholesale?spm=2114.search0104.8.2.2ca59376t6dAXi&initiative_id=QRW_20180118173919&SearchText=axela+diffuser
 

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