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post #31 of 245 (permalink) Old 09-19-2013, 06:13 PM
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Not sure if things changed between generations but the 2.0L should get a 2.0" exhaust, yes. Upsizing post-cat to 2.25" does reduce the "pipe friction" for the exhaust gas so no matter what, flow is improved.

I don't think it's worth trying to upsize on a stock-block/head MZR 2.0. Keep in mind the MZR engines only go to ~7k RPM so we're not even in the regime yet where the built K20A boys start to complain about needing 2.25" or 2.5" piping to avoid choking the engine. 10k RPM's + aggressive cams + ITB's is a much higher flowing setup than ours, so if the biggest built K20's are using 2.5" I doubt that you can make use of it here, yet.
You're making several generalizations that are just plain misinformed. TYPICALLY, increasing pipe diameter will decrease flow restriction and CAN potentially increase power so long as exhaust velocity is maintained to induce a strong scavenging effect.

Its just a more complicated set of variables than you're making it out to be.


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post #32 of 245 (permalink) Old 09-19-2013, 09:49 PM Thread Starter
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there is also the argument that you need back pressure. Honestly I probably won't get an exhaust, I feel like headers are where you get the most gains anyway.


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post #33 of 245 (permalink) Old 09-19-2013, 10:43 PM
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there is also the argument that you need back pressure. Honestly I probably won't get an exhaust, I feel like headers are where you get the most gains anyway.
Exhausts requiring backpressure is a myth as old as cars, and it's been disproven.


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post #34 of 245 (permalink) Old 09-20-2013, 12:24 AM
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Exhausts requiring backpressure is a myth as old as cars, and it's been disproven.
I think both sides on this argument might be going to extremes. My personal experience with and faith in the engineering prowess of such companies as Borla and Corksport tells me that sticking to a 2" pipe might not be the best idea. But at the same time, a post I read awhile ago from someone who really seemed to know his stuff also makes me think that back pressure is indeed a real factor to be taken into account and that going with say, a 3" pipe or larger on our NA engines might not be a good idea. Here's the post by daygoVR6 from another Mazda forum:


"ok i gotta reply to this cause its really got me shaking my head...idk where people get their info from...I worked in the R&D department of JBA Headers and was behind many many projects..I also help tune a 2000+HP Turbo mustang drag car and have worked with multiple race teams building engines and cars designed to do one thing, put people back on their trailers crying like a bitch....so before anyone slams me keep that in mind cause Im not going to waste time trying to argue

High flow cats do not do much for small displacement, small hp engines/cars and are typically a race only modification - the exhaust will only get louder - exhaust companies/aftermarket companies like to market a product like a high flow cat for street cars simply do one thing..make more money...high flow cats are great for boosted applications that are making 300+hp to help reduce spool time by reducing back pressure, or high HP naturally aspirated engine applications that are on the street - typically the 400+ hp and up crowd

Sorry our small hp engines will not benefit much if at all - literally a 2-4 hp gain at most is what you will see and it will only be apparent on a dyno...if you are looking to gain 2-4 HP with a $200+ part ...go change your oil to full synthetic in the crankcase and trans...there is your 2-4 HP gain, and it will cost less initially...

A high flow cat will also reduce your back pressure too much in many cases on the smaller 2.0L engines and you will actually loose your bottom end torque VS a simple cat back exhaust system (ever heard of too much of a good thing)...meaning...you will be slower in your 0-60/70MPH accel. time ......you will gain some umph on the top end - so there is some gain to be had and this is not all negative -> but the added seat of the pants feel you wont get until you are around 75MPH and up - how many of us here actually track their car or get to where they are going 75+ on a drag strip...maybe the last 300 feet on the 1320 with these cars...and the gains for the seat of the pants feel is minimal at best. And keep in mind you are going to be street driving and not always under wide open throttle where the gains will be most beneficial i.e. like on a track

Changing a catalytic converter to something other than the factory part in ANY STATE is highly illegal and can incure some hefty hefty fines, yes a high flow cat is potentially passible visually, cause not every tech out there is that smart...but a lot of shops wont touch the car unless it comes in with no plates and on a trailer and looks to be a "non-op" race car or show car...and you wont be able to drive your car away from the shop either...it will have to go back on the trailer, its a lot of hassle for a simple part, which is why you will see "race pipes" as a "track only" part, good luck getting around inspections as well if you live n a state that may not have a smog requirement but inspection requirements...and no state in the US allows you to remove at cat unless the vehicle is a non-op race car, show car, or a farm vehicle. Your local laws may not mention it but it is a federal law.

You can not believe me or try to argue what Ive stated, but simply put this is pretty general info ...go google whats out there, talk to guys who are non-turbo/nitrous/supercharged with small C.I. engines and see whats out there, even the honda car tuners these days have done away with running hi-flow cats and have gone to other methods of tuning and finding HP for the street"

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post #35 of 245 (permalink) Old 09-20-2013, 03:47 PM Thread Starter
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It's not really a straight forward answer. The way I see it though, an exhaust is what 600 bucks to gain less than 10 hp? Not worth it to me, I'd rather invest that money in headers (which are also slightly cheaper, what like 450 to 500?) where I'm likely to see larger gains.


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post #36 of 245 (permalink) Old 09-20-2013, 04:03 PM
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True, though how much the gains any of these car airflow/exhaust systems see in a bubble, by themselves, I'd be interested to know more definitively. A bottleneck anywhere along the line might keep you from realizing the full gains you're supposed to be getting from one component or another. But then again there, you don't want to decrease back pressure by too much either for our small CI NA engines.

Then there's the logistics of a header. Depending on where you live and the laws and the strictness of emissions testing in the area, an aftermarket header could get you in trouble. Especially a catless one, which most are. And the one catted one that I've seen, the gains didn't seem all that stellar. Plus, they're noisy!

An exhaust on the other hand, the sound alone can be worth the price of admission never mind the power gains. But if you're going for a performance exhaust I do believe that it's probably a good idea to also increase your air intake to see most of those gains.

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post #37 of 245 (permalink) Old 10-02-2013, 01:59 PM Thread Starter
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Ok so I'm trying to decide what suspension I want to run for the 2014 season, and this is me trying to document all my thoughts:

I currently have koni yellows and eibach pro-kit (SHIT). I definitely want to switch over to a coilover set-up just to have the spring rate options.

1) custom valving on my current yellows by koni-na. then figure out how to run coilover springs. I talked with koni and they said the OD of my yellow struts are 52mm and the spring perches they sell are 50mm. So that would be problem number 1. I'm probably going to get camber plates to mount the struts, bumpstops and upper spring perch shouldn't be an issue.

2) koni race inserts (8611-1257). two options with these, try and figure out if they will fit in the stock strut housing, or see if I can still order the AWR strut housings. again, probably camber plates and everything else up top should be straight forward (i'm hoping).

3) keep it simple and buy KW/MS coilovers. Only issue with this is that I really like koni shocks and I think I can create a better set-up with the koni route. Pros to this set-up: simplicity and rear spring perch.

The other problem with the first two options is the rear spring perch, I have emailed tri-point about it, not sure if I'll be able to get any (waiting for a reply from them). I guess I would be ok getting oe style springs in the rear, I know swift makes 250# springs that will fit in the back. Not the most ideal spring rate, but I figured it could do for the time being.

I think that covers it. Feel free to add any wisdom or point out a mistake in any of my ideas

Oh I also have been using information on home-made koni coilovers from this website, which is actually an awesome reference for autox-ers

http://farnorthracing.com/autocross/konis.html


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post #38 of 245 (permalink) Old 10-02-2013, 02:02 PM Thread Starter
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I might go buy a caliper to help with measuring, I swear I had one but cannot seem to find it. Then I can get accurate measurements on the strut housings.


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post #39 of 245 (permalink) Old 10-02-2013, 04:09 PM
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I called Ground Control recently and they told me they do have a coil-over conversion kit for Mazda3 struts now. The kit also comes with rear spring hats. They quoted me $450. Springs and camber plates are separate of course.


I have my plan down to two options and I'm in a similar spot as you. 1. Get a custom valve on my existing Bilsteins, hack the spring perch off, and buy the G/C kit + 2.5" Eibach springs + camber plates or 2. Buy some Bilstein PSS9's + Camber plates (Pss9's do not come with them) + 2.5" Eiback springs and ditch the springs that it comes with.

Still shopping around on custom revalves for the Bilstein struts. I think Bilstein may actually have the best price or a very competitive one at least, gonna call today and verify, but I believe all 4 cost around $400-500 last time I asked and since they do make the things it should be done right.

Option 1 which is the custom valve + ground control really has more appeal to me for a few reasons. One is cost, it should be less expensive than the PSS9 route. 2. Say for instance I buy 500 in/lb springs. I can tell Bilstein the spring rate, give them the motion ratio of my suspension and I'll have a nicely matched up set of converted coilovers and rear shocks to my springs. If you take any single or double adjustable coil-over (doesn't really matter the brand, KW, Bilstein ect) and just slap it on the car with the springs of your choice you're really just twisting the knob or knobs until it feels about "right." Not as ideal even on high end shocks that actually have damping adjustments that work as they are supposed to (most don't). In other words you are completely in the dark. Adjustable shocks are really kind of over-rated (according to suspension Guru's like Dennis Grant) and I think the custom valved non-adjustable Bilsteins may be a better setup out of my 2 options. Here is what the Far North Racing Guy (Dennis Grant) says:


"Note that I consider knobs on shocks as ways to get the forces to match on the dyno, or to rough in forces while testing; I never, ever, ever muck with the shocks during competition. I've run hundreds of shocks on my shock dyno and the repeatability of shock adjusters is downright horrible on anything except the highest end shocks (and even these have their quirks). When you have the damping right, it's right - unless you do something to change the natural frequency. Changing the shocks to try and change balance is an act of desperation - or of a Stock class car, which has few other options."

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post #40 of 245 (permalink) Old 10-02-2013, 05:27 PM
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Mmhhmm. I think my KW/MS kit is great but if I could go back and do my suspension again Bilstein B16 PSS9's would get a good long look. Can't beat the price either for a plug-and-play, easy button option.

I was told by a friend that FatCat Motorsports have vast experience rebuilding/revalving Bilsteins PSS's. They are a good reputable outfit.

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