Help -- need exhaust advice!

[John Miles]

Hi, Tom and others --

What would you guys recommend as a good aftermarket exhaust system for an '03? We just did a back-pressure check on my car, in an attempt to discover why I'm getting only 168 RWHP instead of the expected ~190, and it turns out that it's as high as 5 PSI in my car.

This has undoubtedly been the cause of my relatively-bad gas mileage ever since the car was new, and it's definitely hurting performance now that the blower is present. I imagine I have a collapsed catalyst element or some type of manufacturing defect.

Goal is to install a free-flowing exhaust system that will take maximum advantage of the supercharger without making the car sound like some kid's riced-up Civic. I definitely prefer a quieter exhaust, all other things being equal. Have always been satisfied with the OEM exhaust, although who knows how it would sound if it weren't obstructed.

Current and baseline dyno results on my car look like this:
http://www.thegleam.com/ke5fx/dyno.jpg

My understanding is that about 120 RWHP is the norm for a stock '03, while I was only able to get to 110 by opening the air box up. Without doing that, I saw 105-106 stock.

Thoughts? Price isn't a big issue in this case, but OEM-style appearance and sound would be desirable.

[Tom @ Fast Forward]

Wow. That's a lot of back pressure. That will kill WHP in a hurry. I'll let the others tell you their favorite mid-pipe. I simply run an RB mid-pipe for off road only. ;)

[Serpico]

Here's my view on the different sounds from the combinations I've tried.

1. Stock header + stock midpipe + stock muffler = sewing machine

2. Stock header + roadster sport midpipe + stock muffler = sewing machine on steroids

3. Stock header + RS midpipe + RS muffler = loud & nasty at WOT!!!!!
Nice deep rumble on decelleration .

#3 is my favorite :)

It might be too loud for the older guys, but IMHO a hotrod should sound like a hotrod and I want small animals and children to hear me coming!

[chuckerants]

John, just as a reference, I have a baseline dyno of a 2003 Shinsen with a RB intake that dynoed at 111HP/105 Torque so you're not far off.

But, since you added a supercharger, your "flow" is probably impeded by a clogged CAT.

[Scuba Steve]

...3. Stock header + RS midpipe + RS muffler = loud & nasty at WOT!!!!!
Nice deep rumble on decelleration .

#3 is my favorite :)

It might be too loud for the older guys,...

Wait a minute :tongue: OK, last year I did post that combination was too loud, but I was used to my nice quiet turbo system. I have to admit, the sound of that combination with the SC is growing on me and I even took out the baffle :biggrin:

[Digitac]

I went through the same steps as Serpico on my '01, but with slightly different opinions

1. stock midpipe and stock muffler = sewing machine, nice and quiet

2. roadster sport midpipe and stock muffler = I had a terrible raspy, tincan rattle when cold (less so when warm), sounded like a civic so I went to:

3. roadstersport midpipe + roadstersport 3 muffler = nice sound, but loud, even with the glass-packed baffle in. I can hear the exhaust when cruising. Not a drone per se, but I couldn't hear the stock setup in those conditions. While I'd like to have the stock sound, I didn't want to pay $750+ for the oem cat

I'm not disappointed with the RS3 setup, and I'd love it if it weren't a daily driver and I didn't do semi-long trips in it. I've resigned myself to the fact that a free-flowing exhaust with a supercharger is going to be louder than stock, and I'm not giving up the supercharger!

Jonathan

[chuckerants]

From the motor to muffler, I had the OEM header, straight pipe, a resonator, and an Ansa dual exhaust.

That was way too loud so I replaced the straight pipe with another resonator and now it sounds loud at WOT. :)

[Serpico]

I have to admit that I'm one of the older guys. If you call mid 40's old....

If it were my daily driver and required lots of freeway driving, I'd probably pull the RS muffler and put the stocker back on.

But since my morning commute is only 50 paces and my car is purely for fun , I'm keeping it the way it is.

[John Miles]

So, if I understand you guys correctly (Jonathan in particular), most aftermarket exhaust options do not come with their own catalytic converters? Since I'm very likely to have to replace my cat, I want to make sure that the system I get includes its own cat.

[Wayne-n-Fla]

So, if I understand you guys correctly (Jonathan in particular), most aftermarket exhaust options do not come with their own catalytic converters? Since I'm very likely to have to replace my cat, I want to make sure that the system I get includes its own cat.
My FM 2.25" far a 2000 came with a cat or something , replaced the OME system from the header to the muffler, check the pointy board classifieds, there are mid pipes and AM mufflers there , wher I got mine

[Scuba Steve]

So, if I understand you guys correctly (Jonathan in particular), most aftermarket exhaust options do not come with their own catalytic converters? Since I'm very likely to have to replace my cat, I want to make sure that the system I get includes its own cat.

If you buy the mid-pipe, they come with a catalytic converter. The RS mid-pipe and muffler will provide you with everything you need.

Serpico - 52 here, but the Miata miles stay low because my preference is to bike to work in the summer rather than drive.

[MarkB]

As Tom has posted numerous times, there is no hp benefit to an aftermarket exhaust at our boost level. The clogged cat is the only problem. I will be going to a dyno to tune and to check if my cat is clogged whenever my PC-Pro arrives.... :/

[Digitac]

Sorry, I should clarify. The GoodWinRacing mid-pipe DOES have a cat, as does (as far as I know) the FM Midpipe. The ones without are usually called Test Pipes or something. My delema was that I had a plugged cat and was faced with either ~$750 for a new cat from Mazda (that would have to be welded into the existing midpipe) or $300 for a whole midpipe with a high-flow cat. I figured the Mazda part couldn't be two and a half times better than the GoodWin one, I went with the midpipe and installed it myself. I only replaced the muffler to get rid of the annoying exhaust sound I had (which could have been unique to my car, or even more likely: the installer, me!).

Jonathan

[John Miles]

Cool... then it sounds like this (http://www.good-win-racing.com/Mazda-Performance-Part/11-1001b.html) is what I would need, if I wanted to avoid buying another OEM midpipe.

Only thing is, I need to find out if the lack of CARB approval is going to be a problem at emissions-check time here in Washington State. I don't know if we require/inspect for CARB compliance, or not.

So, (Tom et al.), you're pretty sure that pairing something like this midpipe with a Borla dual exhaust would not result in any HP gains? Assuming the cat is the problem in my case, would it be a good idea to run this midpipe with the stock muffler?

Edit: ScubaSteve, is this (http://www.good-win-racing.com/Mazda-Performance-Part/60-1331.html) the "RS midpipe/muffler" that you and others are referring to?

[Tom @ Fast Forward]

John,

That mid-pipe is a good one. Brian sells top quality parts. It will make a substantial gain in WHP from the plugged cat you have now.

Our dyno guy replaces plugged cats with a high flow one that he gets from the autoparts store. He simply cuts out the old and welds in the new.

[Scuba Steve]

Edit: ScubaSteve, is this (http://www.good-win-racing.com/Mazda-Performance-Part/60-1331.html) the "RS midpipe/muffler" that you and others are referring to?

You are correct. It is a very nice setup.

[John Miles]

You are correct. It is a very nice setup.

Thanks; it's on order for installation next week. Really appreciate all the help with this! I'll post updated numbers when the results are in.

[jwalton]

Wow, the timing on this thread is amazing. Scary really. I was just about to start a new thread on my exhaust adventures when I spied this. Here's my story.

I have an '01 with Goodwin's midpipe and a Racing Beat single tip out back. I had to replace the midpipe due to a clogged cat (Sound familiar anyone, anyone? Bueller? Bueller?) which I discovered on the dyno. Thanks Tom.

When I put that thing on, holy cow did it get loud. Really drony, all the time, and crazy loud on acceleration. It sounded great while blitzing through a canyon, but totally unlivable on a day-to-day basis. As a 39 year old, it was getting really old getting revved on by every 18 year old in a Civic just because I decided to shift gears.

I literally just go back from my local muffler shop with what I consider to be a great solution, though it did take some <ahem> convincing to squeeze it in. The tech suggested replacing the resonator in the midpipe (which is pretty much worthless) with a much larger one from Magnaflow. We chose this one:

http://www.magnaflow.com/02product/shopexd.asp?zone=main&id=441

It's a 4"x9" oval with the inlet and outlet both offset to the same side. The 2.5" tubing diameter is an exact fit for the midpipe.

Be forewarned. We had to stress relieve and dent in the lower corners of the can for clearance, and it's still tight but doesn't hit anything. I'll try and get some pics for anyone who's interested, but it will fit. The sound is just perfect for me. The original normally aspirated sound of the Racing Beat cat-back is still there and at the same volume. You can really hear the SC whine too now.

Hope that helps.

[John Miles]

jwalton, what were your peak WHP figures before and after the cat replacement, out of curiosity? Did you do a back-pressure test? I'm curious to hear what a normal back pressure value would be for these cars.

Interesting re: the noise from the RB muffler. I was under the impression that those were actually among the quieter aftermarket options. I am not averse to making noise, necessarily (used to enjoy setting off car alarms with the Tubi in my 308GTB) but those sorts of antics just seem out of place in a Miata.

[chuckerants]

I didn't do a back pressure test, but when Tom and I were at the dyno, we showed 15 HP improvement after we disconnected the Cat.

[jwalton]

Tom was actually at the dyno that day, I wasn't. Long story covered elsewhere around here. Anyway, they ran the car with and without the exhaust and showed close to 10hp delta across the board. I'm not sure what that translates to in backpressure, but there should only be a couple of hp so we knew that the cat was clogged. The RB was fine normally aspirated, I really liked it, but I think that we just changed too many characteristics of the motor at the same time that it set up some bad resonances. 96 shortblock, '01 head, an MP62, blah, blah, blah. It needed some muffling help real bad.

308??? <sigh> I always loved the looks of those cars. That's what I get for growing up with Magnum.

[99mx5]

I have a RB 4-1 header, RB resonated midpipe and RB single-tip PowerPulse muffler.

The car sounded just right with the RB header and muffler with a stock midpipe. The car got a bit loud when I installed the midpipe, so I reinstalled the stock muffler. It quieted the car down a bit but its not much different than having the RB muffler, so I may reinstall it.

[Scuba Steve]

John - If you are still checking this thread, I finally inserted the optional glass packed baffle today. It toook all of two minutes because the bolt is slightly larger and I had to drill (enlarge) the existing hole.

The sound is now perfect, it sounds much like the RS exhaust prior to mid-pipe and SC. Crusing is only slightly louder than stock, yet it has a full growl when hitting the throttle. An added benefit is on hard throttle, the SC whine is now louder than the exhaust. :yes: