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Hey guys,

Let me preface my question by stating that I have spent several long nights scrubbing forums to better my personal knowledge and find answers, though it seems there is not much hard info on the latest generation skyactiv engines, especially the 2.5L, in reference to modifications... also, I have ridden motorcycles exclusively for the past 10 years, therefore this is a completely new world of knowledge for myself.

The question goes, even though a CAI or SRI and an axle back exhaust retain all of the standard sensors that the ECU bases adjustments off of, would the ECU be capable of adequately adjusting AF mapping after swapping oem for these parts? I understand that major changes to an engine are going require an aftermarket dyno tested tune, but I cannot seem to find a definitive answer to whether these two rather insignificant modifications will require a tune as well, or if the ECU is capable of properly adjusting for these changes.

For insight, I will be going with an axle back exhaust which retains the oem exhaust inner diameter all the way through, as I care more about efficiency than freeing up power. I will be looking for an intake with oem tubing diameter as well, as to allow the MAF sensor to measure correctly. My end goal with this 2018 2.5L is to retain a quiet yet better sounding, well mannered car, while also removing factory performance hinderings.

If you know of threads which will answer these questions that I may have missed somewhere, feel free to just leave a link. Any insight or experience is appreciated.
 

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Hey guys,


The question goes, even though a CAI or SRI and an axle back exhaust retain all of the standard sensors that the ECU bases adjustments off of, would the ECU be capable of adequately adjusting AF mapping after swapping oem for these parts? I understand that major changes to an engine are going require an aftermarket dyno tested tune, but I cannot seem to find a definitive answer to whether these two rather insignificant modifications will require a tune as well, or if the ECU is capable of properly adjusting for these changes.
The ECU will have no problems adjusting to these minor changes. They aren't really changes except for the hardware anyhow. You won't see much if any real performance gains.

For insight, I will be going with an axle back exhaust which retains the oem exhaust inner diameter all the way through, as I care more about efficiency than freeing up power. I will be looking for an intake with oem tubing diameter as well, as to allow the MAF sensor to measure correctly. My end goal with this 2018 2.5L is to retain a quiet yet better sounding, well mannered car, while also removing factory performance hinderings. If you know of threads which will answer these questions that I may have missed somewhere, feel free to just leave a link. Any insight or experience is appreciated.

I think you will find that the Mazda OEM parts are about as efficient as any aftermarket parts. The factory intake works very well and keeps intake temps under control as well if not better than aftermarket CAIs and SRIs. Most SRIs do nothing but suck up hot underhood air.
The OEM exhaust is tuned for the SkyActiv motor and its high compression ratio. It is designed to provide maximum scavenging of exhaust gas so as to minimize knock. Aftermarket parts might not work as well and some have proven to actually reduce power. The sound might be different but again, minimal if any real performance gains. As for quiet, most available aftermarket exhausts are not by any means.
There are many many threads and posts on these subjects, and a couple stickies at the top of this subform too.
 

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The ECU will have no problems adjusting to these minor changes. They aren't really changes except for the hardware anyhow. You won't see much if any real performance gains.




I think you will find that the Mazda OEM parts are about as efficient as any aftermarket parts. The factory intake works very well and keeps intake temps under control as well if not better than aftermarket CAIs and SRIs. Most SRIs do nothing but suck up hot underhood air.
The OEM exhaust is tuned for the SkyActiv motor and its high compression ratio. It is designed to provide maximum scavenging of exhaust gas so as to minimize knock. Aftermarket parts might not work as well and some have proven to actually reduce power. The sound might be different but again, minimal if any real performance gains. As for quiet, most available aftermarket exhausts are not by any means.
There are many many threads and posts on these subjects, and a couple stickies at the top of this subform too.
I have done a few more nights worth research, and I believe that with your insight, and with what I would like this car to be for me, I am going to do as you recommended and keep the stock intake system. I am going to replace the oem filter with a higher flow reusable AEM dryflow filter, or a higher quality equivalent, if I find one. In reference to exhaust, I believe the Racing Beat axle back is going to be perfect for my preferences. It seems to be the only aftermarket exhaust for the 2.5 that retains very low decibels yet improves the sound notably. Also, it retains the oem two inch piping ID, which will keep the exhaust gas velocity up and help with mpg's and proper back pressure, as I read on a custom exhaust build thread. Exactly what I am looking for.

I am not necessarily concerned with power output, as the car isn't normally driven in a manner in which that matters. But, I think these two changes will give me a quicker throttle response, better sound, and possibly better mpg's, which will make me happy. I am only looking for ways to make this cars' drivability and sound better, if possible.
 

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Don't waste your cash on an aftermarket "performance" filter. The OEM paper filter flows more than enough air and filters just as well if not better than those aftermarket filters.:smile2:
 

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I have done a few more nights worth research, and I believe that with your insight, and with what I would like this car to be for me, I am going to do as you recommended and keep the stock intake system. I am going to replace the oem filter with a higher flow reusable AEM dryflow filter, or a higher quality equivalent, if I find one. In reference to exhaust, I believe the Racing Beat axle back is going to be perfect for my preferences. It seems to be the only aftermarket exhaust for the 2.5 that retains very low decibels yet improves the sound notably. Also, it retains the oem two inch piping ID, which will keep the exhaust gas velocity up and help with mpg's and proper back pressure, as I read on a custom exhaust build thread. Exactly what I am looking for.

I am not necessarily concerned with power output, as the car isn't normally driven in a manner in which that matters. But, I think these two changes will give me a quicker throttle response, better sound, and possibly better mpg's, which will make me happy. I am only looking for ways to make this cars' drivability and sound better, if possible.
Sounds like you're on the right track. Unlike other vehicles the 3 has had the exhaust and intake designed for flow and performance. Any improvement becomes minimal. If you're after sound that's a different story :)

This is just a point of interest with regards to exhaust design. You mentioned 'proper backpressure' with regards to the new exhaust you're selecting. You're going to come to the same result here (likely) but I think your physics is a little off. I'm mentioning this not to show off, but that it seems as if you are looking to learn more.

Exhaust backpressure is the enemy and in no way helps performance. It can be defined simply as the resistive force in your exhaust impeding flow. If you consider that, think of the intake. Nobody wants any restriction there, so why the exhaust?

But it is true that increasing the size of the pipe too much will reduce performance. What is going on is due to what is known as exhaust valve scavenging. There's a brief time in the piston cycle when both intake and exhaust valves are both open, right at the end of the exhaust stroke into start of intake stroke. The idea is that the fast exiting hot exhaust gasses create a suction on the intake to help pull in fresh air.

This is of course all dependent on the velocity of the exhaust gas. The faster it flows, the more suction there will be. This is where fluid mechanics kicks in. If you take the flow of a specific amount of a gas (mass flow rate) going through a 2" pipe and change that to say a 3" pipe, the velocity will slow down. The mass flow rate has not changed however. You're still moving the same amount of exhaust, it's just moving at a slower speed.

That is what reduces the suction on the intake. When designing a performance exhaust, eliminating as much backpressure as possible should be the primary concern. This is why mandrel bends are desirable over crush bends. The real trick is to make the exhaust as free flowing as possible without sacrificing gas velocity.

There, probably more than you'd ever care to know :p
 

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Discussion Starter #6
I have done a few more nights worth research, and I believe that with your insight, and with what I would like this car to be for me, I am going to do as you recommended and keep the stock intake system. I am going to replace the oem filter with a higher flow reusable AEM dryflow filter, or a higher quality equivalent, if I find one. In reference to exhaust, I believe the Racing Beat axle back is going to be perfect for my preferences. It seems to be the only aftermarket exhaust for the 2.5 that retains very low decibels yet improves the sound notably. Also, it retains the oem two inch piping ID, which will keep the exhaust gas velocity up and help with mpg's and proper back pressure, as I read on a custom exhaust build thread. Exactly what I am looking for.

I am not necessarily concerned with power output, as the car isn't normally driven in a manner in which that matters. But, I think these two changes will give me a quicker throttle response, better sound, and possibly better mpg's, which will make me happy. I am only looking for ways to make this cars' drivability and sound better, if possible.
Sounds like you're on the right track. Unlike other vehicles the 3 has had the exhaust and intake designed for flow and performance. Any improvement becomes minimal. If you're after sound that's a different story <img src="http://mazda3revolution.com/forums/images/smilies/smile.gif" border="0" alt="" title="Smiley" class="inlineimg" />

This is just a point of interest with regards to exhaust design. You mentioned 'proper backpressure' with regards to the new exhaust you're selecting. You're going to come to the same result here (likely) but I think your physics is a little off. I'm mentioning this not to show off, but that it seems as if you are looking to learn more.

Exhaust backpressure is the enemy and in no way helps performance. It can be defined simply as the resistive force in your exhaust impeding flow. If you consider that, think of the intake. Nobody wants any restriction there, so why the exhaust?

But it is true that increasing the size of the pipe too much will reduce performance. What is going on is due to what is known as exhaust valve scavenging. There's a brief time in the piston cycle when both intake and exhaust valves are both open, right at the end of the exhaust stroke into start of intake stroke. The idea is that the fast exiting hot exhaust gasses create a suction on the intake to help pull in fresh air.

This is of course all dependent on the velocity of the exhaust gas. The faster it flows, the more suction there will be. This is where fluid mechanics kicks in. If you take the flow of a specific amount of a gas (mass flow rate) going through a 2" pipe and change that to say a 3" pipe, the velocity will slow down. The mass flow rate has not changed however. You're still moving the same amount of exhaust, it's just moving at a slower speed.

That is what reduces the suction on the intake. When designing a performance exhaust, eliminating as much backpressure as possible should be the primary concern. This is why mandrel bends are desirable over crush bends. The real trick is to make the exhaust as free flowing as possible without sacrificing gas velocity.

There, probably more than you'd ever care to know <img src="http://mazda3revolution.com/forums/images/smilies/tongue.gif" border="0" alt="" title="Tongue" class="inlineimg" />
I am always looking to learn more, and I appreciate the insight. Researching the workings of things is a bit of a hobby, as well as simply coming up with ways to make things better, or at least better to me.

I knew what exhaust scavenging was, but I certainly hadn't thought about the exhaust's direct role in that. That's pretty interesting, and obvious, now that you explain it. My main concern with the intake has been to what extent I can improve it without the need for re-tuning, as I am certainly not trying to turn this car into any more of a go cart than it already is. It seems a simple panel filter change will be adequate. The consensus seems to be that the stock intake is very well designed in terms of temperature regulation and flow impedance, so I have decided to simply go with a better flowing panel for now.

The more homework I do, the more it seems you are correct in that this engine is very well engineered from the factory, which is completely backward from any cars or motorcycles I have messed around with in the past. It will be pretty cool to pick apart the skyactiv-x engine, when Mazda finally decides to put it into production, too.

Given that reduced back pressure will in fact help the engine then, I believe the Racing Beat exhaust will still be a great choice in my preference. Probably better flow, same inner diameter, yet better sounding while remaining quiet.
 

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Almost a shame when the intake and exhaust is designed well isn't it? We're used to crap!

In the past the rule of thumb has been to let the ecu figure it out for itself within 10%. So if you have a 120hp car and you add 10, you should be fine.

Problem is that's not set in stone. Every car is a little different. Skyactiv is already very different from most in many ways, so it's hard to say.

I have to say that I was very impressed to see the stock exhaust on my 3 mandrel bent instead of the usual cheap and crappy crush bent.
 

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Almost a shame when the intake and exhaust is designed well isn't it? We're used to crap!

In the past the rule of thumb has been to let the ecu figure it out for itself within 10%. So if you have a 120hp car and you add 10, you should be fine.

Problem is that's not set in stone. Every car is a little different. Skyactiv is already very different from most in many ways, so it's hard to say.

I have to say that I was very impressed to see the stock exhaust on my 3 mandrel bent instead of the usual cheap and crappy crush bent.
It certainly is, in a way. I have always had fun tinkering with my motorcycles in the past, and occasionally a car. I came into this car with the same assumption that it was as any other, expecting shortcuts and shortcomings here and there. But, I am realizing these Mazdas really are well engineered from the start, the more research I do. I cannot complain! For me, the best part about this 3 is the transmission. I have never felt such a well put together, smooth transmission. It is so much fun to drive.
 
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