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Old 09-01-2017, 11:31 AM   #1  
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Default Rotrex + RB Header Exhaust Sizing

Have NB with C38-61 and a racing beat 4 to 1 header and need to do rest of exhaust. 9.5:1 CR. Given Racing Beat's 2.4 in tube after collector, does it make any sense to go 3 in for the rest? Should I be looking at redoing the header and/or collector later?
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Old 09-01-2017, 01:15 PM   #2  
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Do both for science.
edit: 2.5" and 3"
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Old 09-01-2017, 01:39 PM   #3  
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On my long term testing list are larger exhausts with the Rotrex. What little information I've found online suggests bigger is better. In for the science.
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Old 09-01-2017, 02:39 PM   #4  
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peak power number would indicate flow, which would indicate exhaust sizing needed
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Old 09-01-2017, 04:48 PM   #5  
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I would bet money you'll find gains on a 3 inch. Time and time again people have been proving there is gains to be had with a larger (within reason) exhaust system.
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Old 09-03-2017, 01:24 PM   #6  
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I feel confident you'll find more top end power with the 3" over the 2.5" all things being equal. Where it gets murky is pipe length. With the 2.5" I was able to get enough muffler volume squeezed in before the diff to have it terminate there. With the 3" I would need to run a larger can behind the diff. If you're planning to exit at the back of the car anyway then that's not a big deal. I plan to add rear diffuser so getting exhaust out before diff was important to me.

I would love to see a good test done. Theory suggests the 2.5" will have better midrange. Who knows until tested though. FWIW I got 297whp out of my Rotrex using 2.5" exhaust.

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Old 09-05-2017, 05:14 PM   #7  
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Everything I have ever seen on exhaust size is about flow. Too large of a pipe and you loose flow. Too small and you restrict flow. A smaller pipe can help evacuate the cylinder on the exhaust stroke. The exhaust will have momentum and essentially vacuum out the exhaust if properly sized.

What is big enough?
Well what are your power goals?
What is realistic with your setup?

If you can produce 500HP then you need a larger exhaust than if you can produce 250HP.

Below are 2 links that will help you determine size for your car. Note that there are a tone of exhaust size calculators out there. Most put you in the ballpark, but will differ a bit. So take them with a gain of salt, and find one you like.
How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos
Exhaust Size Calculator

Comparisons (not on miatas)
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racing beat, rotrex

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