Small loud Exhaust vs Large quiet Exhaust (boost threshold better or worst) help
 

Car Spec: 1995 1.8l BP 0.02" oversized bore standard compression pistons
MS2PnP Base spark map, modified fuel map, currently running wastegate spring pressure 11.5psi at the manifold.

Here is a visual diagram of my exhaust setup. However, for some reason the text got ruined. But the break down is as follows:

Ebay CastIron Log Manifold T25 flange
GT2871 / GT2860 replica turbo journal bearing

S13 style 5 bolt flange on turbo housing to a Flyin' Miata cast elbow downpipe 2.5 inch. Flyin' Miata 2.5" Stainless downpipe to a custom welded 2" flexpipe. The flexpipe is used for a cat delete pipe. From there it runs along the original rusty stock pipe to the stock mid pipe resonator. Then stock pipe all the way back and exits out the rear using a autozone flex pipe to help evacuate the fumes from underneath the trunk space.
https://cimg1.ibsrv.net/gimg/www.mia...01e4378d50.png
There are some areas in the stock cat back pipe where it looks as if the inner/outer diameter is even less than 2". Like maybeee 1.625-1.75 or so... But overall it's a free flowing setup. No Cat, No Muffler, just the oem size resonator.

That's the setup.


Here's the setup I am thinking about doing:

The already existing Flyin'Miata 2.5" stainless downpipe to a -> 2.5" flexpipe to -> 2.5" pipe to a glass pack resonator (same diamter) -> more pipe of same diameter to a 2.5" inlet/outlet Thrust Turbo Muffler.

So pretty much I would like to re-do it all in 2 1/2 inch diameter plus add a better length & diameter aftermarket resonator, and muffler at the end. This is to try and reduce sound level all the way down to as stock as possible. It'd be cool to hear more intake noises than exhaust. I'm getting older and if there's a way to keep the boost threshold the same (or improve?) while lowering the exhaust sound level then I'll be extremely satisfied...

What do you guys think? Do you think going from my current setup to a full 2.5" pipe diameter but also adding a muffler would hurt my boost threshold or do you think it will improve it?

This is the design of the Thrust turbo muffler. It might help some of you more knowledgeable folk determine if it's ganna be an issue vs my current setup...
https://cimg0.ibsrv.net/gimg/www.mia...508e843f31.png

Reason I would pick this is only due to low cost...The muffler looks like a real spool killer but maybe you guys could help me with what you think about all of this because I don't have as much knowledge or experience. Also I plan on welding it myself, so that's why it's been on my mind as an option to do to the car before going to get it dyno tuned.

PS I apologize I over complicated this far more than what was needed. I am tired and it was a long stressful day at work today. >,<

It's basically FM 2.5" downpipe + cat delete + 1.75 or less stock pipe straight back with stock resonator

VS

FM 2.5" downpipe + full 2.5" exhaust with a new resonator and 2.5" in/out muffler at the end..

Which do you think will have the better sound level and spool up time. Do you suspect they will be equals on the boost threshold aspect by any chance? Or will one flow better than the other lol

Thanks

Dude... search the site. All the answers you could ever want.

Spoon fed: 3” has a demonstrated advantage over 2.5”. 1.75” pays a serious penalty. Magnaflow 12589 FTW.

I know.... I have been severely contemplating on the 3", My only dilemma is the "Thrust Turbo Muffler" is like 25 bucks where I'm at. And it only has 2.5" inlet/outlet. Lol.... The Magnaflow would be nice but it's like over 100$. I'll definitely look a lot more into that..... It's just I'm looking to keep the cost way down but I'll do some more thinking.

I've even read that the 2.5" downpipe doesn't restrict the 3" catback so long as the downpipe is of good quality and flow. But on the other hand a 3" downpipe of good quality is going to be even better. It is on my mind to go with the 3" exhaust while keeping my FM downpipe. Bottom line I'm pretty happy with the performance of my current setup. It's mostly I'm wondering if I can lower the noise level and keep the performance, or will one or the other be sacrificed with my options.

As for answers, I'm not sure I know anywhere that says whether a straight pipe resonated 1.75-2" exhaust is defeated in boost thresholds by a 2.5" muffled and resonated. Or does it o.O

Also wondering which would be the quieter one out of the two. If I were to make a educated guess... I'd say the volume will be reduced by about 1/3rd tops.

Summary:

Stock small diameter piping sucks

2.5" piping is good, with a big magnaflow muffler, will be fairly quiet and flow fine.

3" piping with a huge magnaflow 12589 is king. Great flow, good deep and quiet tone. This is the official MT approved setup and what I'm running.

This is the answer.

I have the 2.5" version of the 12589 built in a 2.5" exhaust. One day I will dyno the 2.5" verse the 3" to have a modern up to date comparison.

are you seriously saying $100 extra for a quality muffler that flows well and sounds nice and quiet is a lot of money??

obviously any straight pipe will be louder than having a muffler. and no, it will not hurt the performance of your churbo.

no one can tell you how your $25 ------ Thrust muffler will sound or perform, i doubt any one is running this thing here.

Resonators cancel out high pitches.

Old box style (for lack of a better name) oem manufacturer end mufflers are there to cancel out deep / low pitched sound. With the drawback of being more restrictive than typical free flowing glass pack mufflers.

I looked up the Magnaflow 12589. It's much more designed to only cancel out high pitch noise. Not the deep pitched sound levels. That's where the Thrust turbo muffler for 30$ came into my plan. I'm aiming for a quiet exhaust sound. Something where if you floor the pedal you hear the engine intake noise coming from under the hood. And not the exhaust. Idk if you ever floored a stock honda civic + aftermarket intake but there is zero exhaust noise. Even at idle, you practically hear no exhaust from inside the car. (Aside from interior sound insulation) You really only hear intake/engine noise. I'd kinda like that in a Miata. Really interested what it'd be like with a turbo Miata. That's what I have going on in my mind.

This is why I'm including the information below.

Thrust muffler (old box style muffler) reduces deep low pitches
https://cimg2.ibsrv.net/gimg/www.mia...e516b58b31.png


Glass pack muffler reduces high pitches (I also have the turbo for that)
https://cimg0.ibsrv.net/gimg/www.mia...9b5f4ba137.jpg


Magnaflow 12589: It's designed like a high flowing glass pack muffler... I just don't see how it's going to reduce the low pitch that is actually the part I consider loud right now. lol
https://cimg1.ibsrv.net/gimg/www.mia...1abc75c708.png

Lars the shoeless shoe maker. lol

making 3" exhausts all day every day and not rocking one.

I shoulda just asked what's the best way to quiet down my exhaust to as silent as freaking possible while keeping spool at the bare minimum it currently provides without a loss. Sorry for the confusion.

Let me be clear though, the current spool up isn't horrible. It does reach target 11.5psi by around 4,400rpm in 4th gear. I can upload a datalog, to add in the extra psi/rpm build up data. That's not too bad for a medium-large journal bearing ebay replica turbo, small diameter exhaust running a Megasquirtpnp.com base spark tune. So it must be somewhat free flowing. I've always imagined maybe there's a siphon effect happening with it going from a quality full 2.5" downpipe to the smaller test pipe size. Higher velocity out the last half. I'm just trying to find out from someone if a larger diameter exhaust (while extensively muffled) can compete.

Don't get me wrong, the magnaflow setup sounds perfect. /me drools. And a 3" would be best.... I'm still on the fense, I'd rather just keep the boost threshold I currently have with this setup but being able to quiet down the deep tone down to stock or very close.

The high tone is alrdy nonexistent due to the Churbo & oem resonator. It's just the lack of the end muffler (square box style) that creates the high level of the low pitch sounds coming from the exhaust. That's why my ideal solution was to increase the overall diameter (to keep the spool?) and add the cheap box style at the end.... Thinking that would be the best solution that I could think of.

But wanting to try and ask for some verification. I'm just restless and it was tough to word it the way I was trying to think

i was about to say that, first you ask about straight pipes, then turbo mufflers? then you say you want stock quietness.

the big magnaflow is not stock quiet, its not loud, but it does have a deep and pleasant sound.

if you seek a silent exhaust, ill let others chime in, as i cant answer that.

If you want as silent as possible. just use a stock exhaust. If you want performance AND a quiet exhaust. Use the magnaflow 3inch setup. It is worth its price.

30 dollar muffler will probably just explode.

Get a job.

Best answer. Why even bother with a turbo if you just want to choke it with Ding Dong exhaust? If you're penny pinching simply by the cost of the muffler, you're probably in the wrong game.

What's the concern with how it sounds on the outside? It's going to be different in cabin. You just have to decide what you value more. Performance or pennies.

Well I own a 2.5" and 3" version of basically the same exhaust. 2.5" has 2 resonators in it and lives on the naturally aspirated VVT car. The 3" goes on the boosted car. I will one day just build up a full 2.5" down pipe just to do a comparison of spool and power on the dyno.

https://cimg7.ibsrv.net/gimg/www.mia...db7c62a99a.jpg

My 3" exhaust with Magnaflow spools a T3/T4 churbo more than 1000rpm faster than that, on a 1.6L motor. See orange plot. Your spool sucks.

https://cimg2.ibsrv.net/gimg/www.mia...a4bc1e9547.png

I helped put one of those thrush mufflers on my brother's turbo Plymouth Acclaim. It made the car very quiet. It would probably do most of what you want it to do.

It's $25, try it out. But know that you will absolutely be giving up some performance to a Magnaflow straight-through design.

"Turbo" mufflers are terrible for turbos.

Thank you man Reason why I picked the muffler is not only the cost, but it's the similar design of those types of mufflers that help reduce out the loudness of deep/low sounds by A LOT. I also used the Thrush muffler before in the past (before the car was boosted, and I still own it but it's the 2.0" version) and it is a known unit to me that does quiet the car down.

I'm looking to see if there's anyone with some data, engineering background perhaps or just personal experience who knows how much those mufflers (which reduce deep pitch sound) are spool killers in actuality. The thrush is a simple design and it does work well for the sound. I am just not sure how much it will hurt the spool, even if i increased the diameter of the exhaust pipe by another inch

The vast majority of helpful car enthusiasts out there just have experience on their own with being able to get the sound level to similar results that are desirable. That low deep pitch gurgle. Pretty much the lower the better for most people. I suppose it's going to be VERY hard to find anyone with knowledge/experience that isn't an engineer/expert or something to help me with this one.

Like you said, I'll probably be best just welding the old 2.0" one that I already have to my current system, seeing how much it actually effects the data. And if it isn't too bad then it should help by redoing everything in 2.5. I had already done a bit of research to get to this point. It's just a lot of extra work and time so it was still worth asking for me to save some time and effort!

If cost is a question, doing it multiple times is not the answer.