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Old 06-21-2016, 03:09 AM   #21
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Originally Posted by patsmx5 View Post
You're getting more patient with newbs, I'm impressed!

OP- the idea that a 3" exhaust connected to a 2.5" turbine outlet is flawed. You're right that 2.5 is smaller. But if you run a 2.5" pipe all the way back, and measure the backpressure at the outlet of the turbine, and then do the same test with a 3" pipe all the way back, the 3" pipe will show less backpressure on the turbine. Less backpressure will mean a higher mass flow rate at a given boost level, thus more power and better spool. Sure going from a 3" to 8" exhaust won't make much difference for the reason you mentioned, but going from 2.5" to 3" is well-proven to be worth it.
I'm going to put a 3" tomorrow and I'll hit the dyno asap. Paying for 3 pulls to put this one behind us will be money well spent
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Old 06-21-2016, 03:13 AM   #22
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I'm going to put a 3" tomorrow and I'll hit the dyno asap. Paying for 3 pulls to put this one behind us will be money well spent
Good, it's good to test things and see what works and what doesn't. I can tell you this has been tested many, many times. Both on miatas, as well as many other platforms. In fact I'm almost certain there's a thread discussing this exact topic (2.5 vs 3" exhaust on a turbo miata) on this site with dyno graphs already, but more data is always welcomed.
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Old 06-21-2016, 03:16 AM   #23
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I have seen it, but the numbers are skewed as one is a fresh build still in break in, the other is 5000 miles later
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Old 06-21-2016, 03:18 AM   #24
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And as an added bonus 18psi gets to watch me pay to prove him right....Right?
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Old 06-21-2016, 03:20 AM   #25
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And as an added bonus 18psi gets to watch me pay to prove him right....Right?
Yeah, we all get to watch. Like I said, if you don't believe or just want to see for yourself, test away. If you really want to be a scientist, datalog the runs, make sure the boost pressure/AFRs are equal between the runs. It's going to lean out if the tune isn't changed when you go from 2.5 to 3" exahust.
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Old 06-21-2016, 04:15 AM   #26
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And as an added bonus 18psi gets to watch me pay to prove him right....Right?
I really got to see this. FYI I am leaving my 2.5 exhaust intentionally just to slow down blowing my engine just a touch longer when I get my track speed kit. Will be going to 3" like a smart person as soon as I build a motor and find a 6 speed.
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Old 06-21-2016, 08:53 AM   #27
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No guise, k you needs backpressur to build more power just op be op and profitfrom lolz.
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Old 06-21-2016, 09:08 AM   #28
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And as an added bonus 18psi gets to watch me pay to prove him right....Right?
I always enjoy watching people discover the truth. It must be the humanitarian in me
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Old 06-21-2016, 11:58 AM   #29
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Originally Posted by Pretending2koolz View Post
I'm going to put a 3" tomorrow and I'll hit the dyno asap. Paying for 3 pulls to put this one behind us will be money well spent
It's been behind us for a long time. Feel free to spend your money putting it behind you, though.
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Old 06-21-2016, 08:38 PM   #30
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Originally Posted by Girz0r View Post
No guise, k you needs backpressur to build more power just op be op and profitfrom lolz.
This guy gets it!
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Old 06-21-2016, 10:32 PM   #31
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This was a duplicate

Last edited by Pretending2koolz; 06-21-2016 at 10:34 PM. Reason: Dup
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Old 06-21-2016, 10:33 PM   #32
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Since we are on the pipe size topic, does the ic plumbing also need to be 3" vs 2.5"
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Old 06-21-2016, 10:40 PM   #33
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No
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Old 06-22-2016, 12:29 AM   #34
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Originally Posted by Pretending2koolz View Post
Since we are on the pipe size topic, does the ic plumbing also need to be 3" vs 2.5"
nope. 2" will easily take you all the way to 250, and 2.5 will basically support whatever your little heart desires

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Old 06-22-2016, 01:11 AM   #35
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Will an 84.5mm bored block survive 14psi long enough to break in and dyno?
What? That's totally safe. Everyone and their mother runs 84mm and I am pretty sure emilio sells 85.5mm pistons for NA use.
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Old 06-22-2016, 08:58 AM   #36
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Originally Posted by 18psi View Post
nope. 2" will easily take you all the way to 250, and 2.5 will basically support whatever your little heart desires

Isn't the rule of thumb 2" hotside and 2.5" on cold of the intercooler? Or 2.5" all the way through?
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Old 06-22-2016, 09:10 AM   #37
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Most do 2.5 all the way cause it's cheaper when hacking up one of those ebay kits to make fmic piping. But yeah msm/begi/fm hotside piping is 2-2.25
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Old 06-22-2016, 09:35 AM   #38
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Originally Posted by Girz0r View Post
Isn't the rule of thumb 2" hotside and 2.5" on cold of the intercooler? Or 2.5" all the way through?
I recommend 2" hot side and 2.5" cold side, just because 2" makes it easier to clear everything on the hot side.
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Old 06-22-2016, 09:41 AM   #39
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Yup, that's good advice. fitting 2.5 on an ac/ps car via traditional routing is near impossible
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Old 06-22-2016, 09:43 AM   #40
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8" all the pipes.
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