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Exhaust diameter 2.5" vs 3"... Noticeable difference?

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Exhaust diameter 2.5" vs 3"... Noticeable difference?

 
Old 04-17-2018, 12:09 PM
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Default Exhaust diameter 2.5" vs 3"... Noticeable difference?

Finally going to order my turbo kit. I currently have a 2.5" cat back exhaust going into an ANSA silverline dual outlet exhaust. This exhaust came on the car when I purchased it a few years ago, but I really enjoy the sound. I'd like to keep it and save some cost on the turbo kit, however I realize that a 3" exhaust would be better. My question though, is how much better? Does anyone have any concrete stats comparing the 2 sizes?
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Old 04-17-2018, 12:41 PM
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On a 200hp setup its mostly transient response which no one will have data for, no.

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Old 04-17-2018, 01:11 PM
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Tests have been done. You want the 3". Tests are 10+ years old so you will have to do a ton of searching to find the data if you don't believe us.
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Old 04-17-2018, 01:25 PM
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Sound will change, so don't keep an exhaust based upon sound. Adding a turbo is like adding a HUGE muffler all by itself.
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Old 04-17-2018, 02:08 PM
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You can keep the small exhaust for now if youre trying to save money. If its just a street car it will still be a load of fun even if it isnt optimized to the max.

Maybe in a couple years youll throw a full 3" at it with the MTnet approved muffler and see what you were missing.
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Old 04-17-2018, 02:37 PM
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Originally Posted by miataa View Post
Finally going to order my turbo kit. I currently have a 2.5" cat back exhaust going into an ANSA silverline dual outlet exhaust. This exhaust came on the car when I purchased it a few years ago, but I really enjoy the sound.
how loud is it now?
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Old 04-17-2018, 03:20 PM
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If Flyin' Miatas numbers are to be trusted, on a 266rwhp 1.8 they got 16hp and boost 200rpm earlier with a 3" exhaust vs a 2.5".
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Old 04-17-2018, 03:54 PM
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Originally Posted by jstck View Post
If Flyin' Miatas numbers are to be trusted, on a 266rwhp 1.8 they got 16hp and boost 200rpm earlier with a 3" exhaust vs a 2.5".
Thanks for the info! I feel that's pretty significant. Really appreciate it!

Originally Posted by shuiend View Post
Tests have been done. You want the 3". Tests are 10+ years old so you will have to do a ton of searching to find the data if you don't believe us.
Haha, I always trust all of you miata guru's, that's why I asked here.

Originally Posted by Braineack View Post
how loud is it now?
It's definitely louder than stock by a few decibels, but has a really nice deep tone at a low pitch. I would like to say it's not obnoxiously loud, but I've had some neighbors complain about the rumble from my morning warm-up enrichment cycles lol.
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Old 04-17-2018, 04:10 PM
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Bottom line:
if already have, keep 2.5 until you get to it's limit.
if buying/building/upgrading, 3" no question
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Old 04-17-2018, 07:44 PM
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FMs big bore exhausts are not straight through.

Those tests are not testing what you and I think of as a 'turbo exhaust'.

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Old 04-17-2018, 07:56 PM
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Old 04-17-2018, 09:18 PM
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Lots of good info proves 3”>2.5”. Where things got fuzzy for me was length and muffler volume. With the smaller 2.5” size I was able to get 2 smallish mufflers in-line and terminate just before the diff. With the 3”size I would not have enough muffler volume in the same location. I would have to use a large oval in the back and terminate near the bumper.

With my 300 WHP 9 psi Rotrex It was hard to imagine the larger size helping out. If anything it may have hurt midrange torque where I needed it the most. Someday I’ll pull the system and dyno it wide open just to see how much it’s holding back HP.

Just another perspective to consider for SC cars.
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Old 04-17-2018, 09:40 PM
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If anything it may have hurt midrange torque
what makes you say this?
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Old 04-17-2018, 10:09 PM
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@18psi....SC car uses exhaust scavenging and velocity like an NA. On the merican LS3 builds I’ve seen the 2.5” midrange was stronger than the 3”. Given the weak sauce midrange of the Rotrex I figure it needs all the help it can get.
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Old 04-17-2018, 10:12 PM
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Yeah I've heard that many times before. But have yet to see a dyno plot to confirm it, which is why I question it.
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Old 04-17-2018, 10:18 PM
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@18 psi, sorry I don’t have controlled A to B comparison. Don’t mean to pollute the MT forum with info I can’t back up. Judge the merits of my statements on my past accomplishments and take it for what it’s worth.
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Old 04-17-2018, 10:24 PM
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Originally Posted by k24madness View Post
Lots of good info proves 3”>2.5”. Where things got fuzzy for me was length and muffler volume. With the smaller 2.5” size I was able to get 2 smallish mufflers in-line and terminate just before the diff. With the 3”size I would not have enough muffler volume in the same location. I would have to use a large oval in the back and terminate near the bumper.

With my 300 WHP 9 psi Rotrex It was hard to imagine the larger size helping out. If anything it may have hurt midrange torque where I needed it the most. Someday I’ll pull the system and dyno it wide open just to see how much it’s holding back HP.

Just another perspective to consider for SC cars.
300hp at 9psi?? That's gotta be a record and one hell of a supercharger, no turbo can do 300hp at 9psi. Very curious how you accomplished this.
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Old 04-17-2018, 10:55 PM
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@Borka, 11:1 CR, custom cams, ported head & E85.

With the proper EFR turbo it would be pretty close at the same PSI and fuel. Nothing magical really.
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Old 04-17-2018, 11:20 PM
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Originally Posted by k24madness View Post
@18 psi, sorry I don’t have controlled A to B comparison. Don’t mean to pollute the MT forum with info I can’t back up. Judge the merits of my statements on my past accomplishments and take it for what it’s worth.
No worries. It's more just out of curiosity

Originally Posted by borka View Post
300hp at 9psi?? That's gotta be a record and one hell of a supercharger, no turbo can do 300hp at 9psi. Very curious how you accomplished this.
incorrect.
there are several that can, but no real reason to do that
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Old 04-19-2018, 02:06 PM
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Originally Posted by borka View Post
no turbo can do 300hp at 9psi.
Completely false.
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