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Rotrex + RB Header Exhaust Sizing

Old 09-01-2017, 12:31 PM
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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, 02:15 PM
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Do both for science.
edit: 2.5" and 3"
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Old 09-01-2017, 02:39 PM
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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, 03:39 PM
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peak power number would indicate flow, which would indicate exhaust sizing needed
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Old 09-01-2017, 05:48 PM
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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, 02:24 PM
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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.

Last edited by k24madness; 09-07-2017 at 11:27 AM.
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Old 09-05-2017, 06:14 PM
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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)
http://www.hotrod.com/articles/ccrp-...-inch-exhaust/
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Old 09-18-2018, 02:12 AM
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Back from the dead! It’s going to happen. I broke my 2.5 inch exhaust so I’m going to build a 3 inch setup. Dyno will happen. Probably only relevant to the 300+ hp rotrex cars out there. Like K24, I put down 312hp with a 2.5 inch exhaust.
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Old 09-18-2018, 09:55 PM
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I look forward to the results. Savington wants me to do the same on my car. You got a great before dyno so this will be a clear A to B test.

I am betting you’ll see less than 10 whp from the change. If it’s 20+ I will be shocked.
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Old 09-18-2018, 10:44 PM
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I read an article where Perrin tried exhausts on a mini r53 and they saw 2-3% over a 2.5 inch but this was a 1.6 and not a lot of power so we might see more than that.
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Old 10-10-2018, 12:54 AM
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3 inch makes more power. The base run (2.5 inch) was with timing pulled. Normally it made 312/235 on the 2.5 inch exhaust
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Old 10-10-2018, 12:55 AM
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Also an interesting note... it increased boost
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Old 10-10-2018, 01:13 AM
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Nice 11hp bump! I was close I guessed less than 10hp.

How much louder is it? Did you change the layout at all?
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Old 10-10-2018, 01:29 AM
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Layout is the same but I used a bigger muffler. Still ungodly loud


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Old 10-10-2018, 01:33 AM
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New boost plot
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Old 10-10-2018, 01:55 AM
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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.
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Old 10-10-2018, 02:16 AM
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Not sure if there would be anything gained on my setup by moving the expansion section 4-6 inches back. Next exhaust project will be a custom header. First I want to do some spying at runoffs later this year for inspiration.
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Old 10-14-2018, 02:32 AM
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Originally Posted by Twibs415 View Post
Also an interesting note... it increased boost
That seems odd. If you are reducing the backpressure by going to a larger diameter exhaust, I would expect your boost to go down.
Was anything else changed?
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Old 10-14-2018, 04:34 AM
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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.
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Old 10-16-2018, 12:33 PM
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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.
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