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Old 12-07-2007, 10:40 PM   #21
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Stepping up in size either won't hurt you or will help you. Stepping down is basically a bad thing. My own setup is a 2.5" dp to a 3" open exhaust, and it got me ridiculous amounts of power and spoolup, but then again I was full stock exhaust before that. :-)

I've heard enough stuff saying 2.5" or 2.75 is fine, and I'm sure it's reasonable. The reason I went with 3" was because I'd read a number of tests which showed that for a given muffler volume, a bigger pipe was quieter.

Two suggestions:
1) get a "glasspack" or "bullet" style muffler - they won't quite it down any, but if you put it around where your cat or stock resonator was, you'll cut out that shitty raspy "honda sound".
2) Use a muffler with offset feeds - i.e. the inlet on the left and the outlet on the right. This will allow a very simple, direct swap of a muffler with bends in it. My car isn't too loud, but many a time I wish I COULD go to that muffler if I wanted. It's not very restrictive, but it's much quieter. Since I used the straight through I'm stuck with open exhaust or an inneficient, baffle-type muffler.

Pics of instal/fab at:
http://abefm.smugmug.com/gallery/1074852#49923395

(WOW! My car looks good clean. That's a trip)

P.S. Resistance to flow goes as the 4th power or radius. Take 6 years of physics classes, and you'll have to prove it. :-) But that means 3" has ~1/2 the resistance over 2.5", and 2.5 is 52% better than 2.25

This is why dual pipes are a joke. TWO whole exhaust systems, for TWICE the weight, only flows twice as much. Going from 2.5" to 3" gives you the same gain, but weighs only 1.2 times as much (linear verses 4th power relationship, you win at a rate of r^3)
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Old 12-07-2007, 10:48 PM   #22
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here's how i explained it the other day on another thread:

Quote:
Originally Posted by random quotes by me in a large uncomprehendable paragraph
may be true, however, the 2.5" system has a cross sectional area of 3.91761 square inches; a 3" is 7.069 Sq"...80% larger.

while it certain may be more than you need, there's no way it will hurt you, and I'd suspect you'd see better spool/ouput all other things being equal.

fwiw, my 3" exhaust with a small magnaflow muffler is much quiter than the same engine with a header, 2.25" exhaust with an additional resonator and larger muffler.

get a magnaflow or simliar. as nice as the price tag of a summit racing muffler looks, the internal baffles kill flow. I'd expect a loss of spool in the 400-500RPM range in that muffler alone in the same size vs. the magnaflow straight through.

pressure drop is key. The restrictions of the tailpipe are somewhere around: the cat (60%), the muffler (30%), and the tubes (10%).

High flow cats and zero restriction mufflers are key.

Measure the turbine exhaust housing outlet for diameter. That should be the first 10-15 inches of the downpipe. From there, expand it 1/2 inch, run another 10-15 inches and expand another 1/2 inch. My exhaust starts at the turbo at 2.5" and ends at 3", stepped to 2.75" in between. The difference between that system and my previous 2.25" catback (same downpipe) is almost 1000RPM faster spool and tons more topend....

Just like your intake manifold tapers in to increase the velocity, the exhaust should taper out. A 3" DP on a turbine with a 2" exit will probably cause turbulence and impede flow as the exhaust gases get lost in the mix filling up a large void.

the general rule is simply: Back pressure in an exhaust system is evil.

the difference in 2.5 piping and 3.0" is trivial, however the cross-directional area is 80% larger....

Exactly that was kinda the point, I didn't elaborate, the velocity is dependent of the cross-directional area. The steady increase in flow area causes a steady decrease in velocity.

The goal is flow; you want to keep the exhaust gases moving. A rapid decrease in temperature or rapid expansion and turbulence should cause the gases to slow. Starting with a matching port will keep the temps high and exhaust moving, as the exhaust cools the second it leaves the cylinder. This is also why we don't tend to port the exhaust on the head, but we do port the intake. Again, to keep the exhaust moving. It's really all a compromise. Throwing the 3" pipe on a 2" turbine outlet will cause both turbulence and a rapid decrease in EGTs, slowing the flow. But if you keep the exhaust moving out of the turbo with a matching port size the back pressure should drop more, since the area behind the turbo wants to keep moving rapidly into the stepped exhaust.

you may be right, i may be wrong. I don't see anything wrong with the idea. I'm no engineer or expert, but I do know the difference from my 2.25" to my 3" catback was substantial.

either way will work, without back to back testing, its hard to say which one is best, the difference could easily be none. I'll comprmise and say as long as you do 2.5" mandrel you should be good. how 'bout that?
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Old 12-07-2007, 11:13 PM   #23
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Not to rain on your parade....

Area = pi * r^2 = pi * (d/2)^2 = pi * d^2 / 2^2 = pi/4 * d^2
pi/4 ~ .785

2.5": 4.91 sqin (pretty sure you forgot to square 1.25)
3.0": 7.07 sqin

a3.0/a2.5 = 1.44, or 40% bigger.

The easier way to do it: Area is proportional to the square of the rad (or the dia), so just do (3/2.5)^2 = 1.2^2 = 1.44

The interesting thing is, it's a bit more complicated. The center of the pipe moves the quickest, the outer edge there's no flow at all (otherwise how would soot build up?). So there ends up being some horrid quadruple integral, but the basic answer is resistance is ~r^4, meaning it's not 44% better (as I would certainly guess it being proportional to the area) but 100% better.



Wow! That's really interesting about the magnaflow. I think I have dynaflow on mine, but it's the same basic thing. Baffleless. It was on sale doncha know. I've heard cats come in to varieties: Low flow and anti-polution (OEM models) and low flow and polluting. Basically that the high flow ones are nowhere near as good at scrubbing but still somewhat restrictive. Anyway, I'd recommend a big one, though it'll be pricey. If you want a 2.5" my broke roomie has a NIB one he's trying to get rid of so he can pay me rent. :-)

Also, you want to make your tapers slow. Some exhaust shops will flare a pipe, gaining an inch in diameter over 1.5" of lenth, that's better than a step but just barely, a bad increase will hurt you more than just sticking 2.5 the whole way back. I got a set of "reducers" for V-8 headers, and it was a 6" slope to go up half an inch.

Lastly, total degrees of turns hurt you, so do everything you can to keep things straight. Slow bends, never sharp, and as few as possible. Don't cut a pipe at an angle then weld it to another to make a tight bend. Go around whatever's in the way, or just give up there, go straight up and put on a smoke stack. It'll be the awesomest Miata ever. Maybe paint it like Optamis Prime.

I'm getting off track and should run along.
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Old 12-08-2007, 01:15 AM   #24
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maybe i did the math for the 2.25" pipe. whateves. it's still not 40% more expensive
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Old 12-08-2007, 10:10 PM   #25
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To answer a few peoples question.

At one point I had a 2.5 full exhaust.. mounted a 3" cat-back on my 20G DSM and it spooled 400 rpm fast but there was a huge improvement in topend.

Then swapped to a full 3' and I was getting full boost 18psi by 3500 instead of 4000-4200.

BTW Even going from a 2.5 to a 3 will be better then just 2.5 all the way. The reason is that going from the 2.5 to a 3 will decrease back-pressure down the line. With the lack of back-pressure will cause the exhaust to increase in speed and therefore help spool the turbo and allow better top end.

The only problem is the increase in exhaust speed will increase the change of boost creep, which was a big problem in the DSM world with the 16g and 3" exhaust, which is why some DSMer with 16g still have a 2.5 dp
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Old 12-08-2007, 10:51 PM   #26
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to all those that think a 3" exhaust will make no gains over a 2.5" exhaust when both are coupled to a 2.5" downpipe.

let's make it simple for you, and **** that stupid garden hose example because it's all wet.

let's say you have a 2.5" downpipe.

and with said 2.5" downpipe you have a 2.5" exhaust.

if you pull the exhaust off and just run the downpipe is the car gonna spool faster and make more power?
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Old 12-08-2007, 10:55 PM   #27
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Quote:
Originally Posted by Braineack View Post
I officially rendered my MBC useless. Too much flow.
don't even think of stealing the AbsurdFlow© brand name.
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Old 12-09-2007, 01:23 PM   #28
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Ok guys, I just placed my order at summit racing.
I got:
- 18" Magnaflow straight through muffler. Offset inlet, center outlet. (3") ($80) and its stainless.
- 4 foot of 3" alumized piping
- 3 3" 180 degree bends
-1 45 degree bend
- Header reducer from 2.5" to 3" on a 6" long transition
- A Magnaflow stainless 3" tip

And all this cost $260 shipped.

My first order included an Edelbrock muffler (looks like flowmaster) but you guys recomend to get a Magnaflow so I changed my order.
BTW I'm NOT running a cat, I hope its not too loud. Will it be?
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Old 12-09-2007, 02:05 PM   #29
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I'd still recommend a "bullet" muffler up front, it takes out the raspiness. Otherwise it probably should be fine. Check out Y8t's and mine photo pages to get some ideas on routing.

It'll be louder than normal, but not nothing you can't talk over or super annoying.
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Old 12-09-2007, 02:29 PM   #30
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Quote:
Originally Posted by AbeFM View Post
I'd still recommend a "bullet" muffler up front, it takes out the raspiness. Otherwise it probably should be fine. Check out Y8t's and mine photo pages to get some ideas on routing.

It'll be louder than normal, but not nothing you can't talk over or super annoying.

I agrued the same when I got mine built. For one thing a 4" round muffler will hardly fit with 3" pipes, for second, it's not needed. The one thing I didn't want was a loud annoying exhaust, it's a bit loud, but it's not raspy/annoying/resonate.

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Old 12-09-2007, 02:52 PM   #31
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RASPY!

Not annoyingly so, though. Since I built mine turbo-back, I stopped just before and just after the bullet, and found it made a huge difference. Didn't take out much volume, but it really changed the character. It would have been inconsideratly (and cop-attention grabbingly) loud, but the sound was awesome with just a bullet on mine.

Yours does seems pretty quiet, but it could be I don't have my speakers up loud. :-)
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Old 12-09-2007, 03:04 PM   #32
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Quote:
Originally Posted by 90turboMX5 View Post

- 3 3" 180 degree bends

So, are they doughnuts or just very short straight pipes?

- L
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Old 12-09-2007, 03:12 PM   #33
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Quote:
Originally Posted by AbeFM View Post
RASPY!
I urge you to define this for me:

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Old 12-09-2007, 03:14 PM   #34
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Quote:
Originally Posted by l_bader View Post
So, are they doughnuts or just very short straight pipes?

- L

2 of those are 4.5" radius "U" shaped 180 degree bends.
The other one is a 6" radius.
All the tubing is 3".
So, after all the discussion, should I use a bullet muffler or not. Is it going to fit?
Does it sound raspy without it?
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Old 12-09-2007, 03:14 PM   #35
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man I hate when they blab on and on before they get to the point of the vid.
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Old 12-09-2007, 03:24 PM   #36
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Quote:
Originally Posted by 90turboMX5 View Post
2 of those are 4.5" radius "U" shaped 180 degree bends.
The other one is a 6" radius.
All the tubing is 3".
So, after all the discussion, should I use a bullet muffler or not. Is it going to fit?
Does it sound raspy without it?
Depends who you ask, I suppose. :-) I was impressed at the difference with mine, but I've never take it out to see how just my dynaflow would have handled it without. It's a bit of a pain to put in.


Lord Vader: Fine, ok, your car isn't* raspy. :-) I should record mine, now I'm curious.

*Isn't very raspy.
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Old 12-09-2007, 03:34 PM   #37
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SO what about adding this
http://store.summitracing.com/partde...5&autoview=sku
or should I get a 18" one?
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Old 12-09-2007, 03:37 PM   #38
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When I build my exhaust, it will have as many resonators as possible. They reduce noise without hindering flow, if it's not a baffled one. I'd run the longest one I could that would fit. For me, the quiter the better.
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Old 12-09-2007, 03:45 PM   #39
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Quote:
Originally Posted by patsmx5 View Post
When I build my exhaust, it will have as many resonators as possible. They reduce noise without hindering flow, if it's not a baffled one. I'd run the longest one I could that would fit. For me, the quiter the better.
Ha, that will be one heavy exhaust. Im trying to make it fairly light too. But I'm going to use a sanitary clamp from McMaster to disconnect the muffler part out and save some weight when I'm autocrossing.
Do louvered glasspacks create lot of backpressure?
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Old 12-09-2007, 03:46 PM   #40
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Quote:
Originally Posted by patsmx5 View Post
When I build my exhaust, it will have as many resonators as possible. They reduce noise without hindering flow, if it's not a baffled one. I'd run the longest one I could that would fit. For me, the quiter the better.
Indeed. If you're going to do it, you might as well get the longest one you can. They are a little hard to fit in,

That bend right there was a little snug for me. I found myself wishing I'd gotten a longer one, but I was pretty happy with what I had. I suppose, in theory, the longer the pipe the lower your first mode will be - mine had a nasty resonance at about 1800 rpm, my 'cruising home late trying not to wake up the neighbors' speed. So longer might have helped there.
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