Rotrex + RB Header Exhaust Sizing
09-01-2017 | 12:31 PM
#1
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Newb
Joined: Jun 2015
Posts: 8
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From: San Diego
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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09-03-2017 | 02:24 PM
#6
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.
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.
Last edited by k24madness; 09-07-2017 at 11:27 AM.
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09-05-2017 | 06:14 PM
#7
Newb
Joined: Jan 2013
Posts: 18
Total Cats: 1
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)
http://www.hotrod.com/articles/ccrp-...-inch-exhaust/
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)
http://www.hotrod.com/articles/ccrp-...-inch-exhaust/
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10-10-2018 | 01:55 AM
#16
ʎpunq qoq
Joined: Jan 2014
Posts: 604
Total Cats: 201
From: Western Australia
Great info... thanks for the dynos.
I run a 3" straight through exhaust on a N/A engine and it works... but it's damn loud. The key is to use run a 2.5 - 3" expander from the back of the headers pretty much in line with the shift selector on the gearbox.
I run a 3" straight through exhaust on a N/A engine and it works... but it's damn loud. The key is to use run a 2.5 - 3" expander from the back of the headers pretty much in line with the shift selector on the gearbox.
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10-14-2018 | 04:34 AM
#19
Nope nothing else was changed. I would agree with your statement usually but with a supercharger I think, And this is just theory.
Forget what you know about a turbo setup. With a rotrex making let’s say 10 psi we could assume that there is some backpressure pushing back into the cylinders during overlap for the sake of arguement let’s say 2psi bleed back into the cylinder so your supercharger is actually pushing 12psi but your gauge will read 10 psi. Cam selection plays a big roll in this from my understanding.
So, with the bigger exhaust we can assume I lost back pressure and boost went “up” but is actually the same it was all along. The only way to measure this is if I had a few pressure sensors rigged up in the old exhaust and in the runners of the header.
Forget what you know about a turbo setup. With a rotrex making let’s say 10 psi we could assume that there is some backpressure pushing back into the cylinders during overlap for the sake of arguement let’s say 2psi bleed back into the cylinder so your supercharger is actually pushing 12psi but your gauge will read 10 psi. Cam selection plays a big roll in this from my understanding.
So, with the bigger exhaust we can assume I lost back pressure and boost went “up” but is actually the same it was all along. The only way to measure this is if I had a few pressure sensors rigged up in the old exhaust and in the runners of the header.
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10-16-2018 | 12:33 PM
#20
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Joined: Feb 2017
Posts: 201
Total Cats: -16
From: Apple Valley, California
BRP MP62 setup here,
When I cut my exhaust off (the idiot who owned the car before me cut ALL the flanges off so I had to chop it when i changed my diff/trans) I lost 1.psi mid range and 1.5 up top, but it feels like the car has more 'oomph' now.
When I cut my exhaust off (the idiot who owned the car before me cut ALL the flanges off so I had to chop it when i changed my diff/trans) I lost 1.psi mid range and 1.5 up top, but it feels like the car has more 'oomph' now.
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