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Exhaust Pressure Testing

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Agreed. I ordered 3" magnaflow straight-through setup to not have a single part of the system sub-3".
@engineermike really inspired me with this thread. Thank you sir. I love power gains without timing or boost increase. Efficiency.
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I've been eyeballing this thread for a bit.

I have ARH 1-7/8" Catless long tubes, with their bottkeneck eliminator, into a Borla Atak exhaust. Which makes it 3" all the way back to the mufflers, where the weird ATAK section is probably less total area than 3".

Overall, makes my exhaust seem pretty well suited to breathe well once I throw a Whipple on.

Really nice to read the data points, and see areas for improvement.
 

mangosmoothie

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re-reading this thread....

Is the 3hp per psi drop crank hp or wheel hp?

What effect would headers have? If I have 1 7/8 kooks headers into the stock cat back..... am I leaving even more on the table compared to a car with stock headers? Or does a free flowing header with high flow cats make the rest of the system not as important?

I know the headers did something because I had to drop a pulley size to make the same amount of boost.
 
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engineermike

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Is the 3hp per psi drop crank hp or wheel hp?
I don’t think it’s accurate enough to make that distinction.

What effect would headers have? If I have 1 7/8 kooks headers into the stock cat back..... am I leaving even more on the table compared to a car with stock headers? Or does a free flowing header with high flow cats make the rest of the system not as important?
The primary part of the header won’t really affect this. As for the remainder of the system, I found that if any section of the exhaust is 2.5” then it’s typically adding >1 psi backpressure. I tested mine with one 2.5” tip plugged per side, leaving dual 2.5” outlets on a straight through muffler and it added significant backpressure.

I know the headers did something because I had to drop a pulley size to make the same amount of boost.
Rest assured, you gained airflow and power before dropping the pulley. That’s how a centrif works; when you lower its discharge pressure the flow through the compressor increases.
 

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I tested mine with one 2.5” tip plugged per side, leaving dual 2.5” outlets on a straight through muffler and it added significant backpressure.
Have you thought over adding the H-style crossover to your new system since you finished everything ?
 
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engineermike

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Have you thought over adding the H-style crossover to your new system since you finished everything ?
It’s intolerably loud how it is so I definitely am planning on taking steps to quiet it without adding backpressure.

The first and easiest step is to machine some orifices to install in 2 of the 4 2.5” outlets. This not only reduces area, but also sets up a quarter wavelength reflection. My first trial was 1” orifice plates, which reduced the noise level significantly, especially a 125 hz peak I recorded. However, this sacrifices 20% flow area vs a 3” pipe. I installed 1.5” orifice plates, which only gives up about 5% flow area vs 3”, but I don’t know how well it will work as a wave reflection attenuator yet.

If I can’t get satisfactory results with the orifices, the next step is the H pipe. I have most of the needed materials to fab one up already.

If the above still isn’t good enough, I’ll fab up some true quarter wave resonators. These don’t add any backpressure and complement the packed straight-through muffler characteristics well.
 

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The first and easiest step is to machine some orifices to install in 2 of the 4 2.5” outlets.
Magnaflow rear mufflers, correct ?
You’re probably sick of fabbing exhaust parts and you sure don’t need me telling you how a H-crossover will knock down some dB’s.
I’m anxious to learn more as you move forward.
 
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engineermike

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Yes, these are the magnaflow rear mufflers.
 

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I got magnaflow street 3" mufflers like you but single exit each since it's a 17.
My backup plan if this doesn't work is go back to stock catback, resonator and all, and fab up 3" boost referenced cutouts on the after-header midpipes. They supposedly open between 3-6psi (loudvalves version).
 

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Went to the track yesterday and just changed my axle back.

Previous setup 1-3/4" long tubes 3" free all the way to the axle back.
2.5" adapters, stock pipes with GT500 single tip mufflers.

3.25" pulley on E85 @ 2199 DA

1.42 60' , 6.38 1/8@110 , 9.92 1/[email protected]

Now changed axle back for 3" no reductions

Same boost and fuel @1925 DA

1.44 60' , 6.35 1/8@112 , 9.85 1/4@ 141

There were some gains there.

Dropped a pulley to 3.125"

1.40 60' , 6.24 1/8@112 , 9.69 1/4@ 142.31
 
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engineermike

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Nice! The quick and dirty calculator says you were making 834 hp. Increasing exhaust brought it to 867. The pulley brought it to 891.
 
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engineermike

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By the way, I made the magnaflow spool piece (where the exhaust valves go) into hemholtz quarter wave reflectors. This did a lot to bring down noise levels in the 1500-2500 rpm range.

8E376402-FCAB-46E5-9F6F-74A04F6B3277.jpeg
 
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engineermike

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@GregO I just welded in an H pipe a few inches upstream of the stainlessbros resonator/muffler. Man, what a difference in noise level, especially at heavier throttles. Thanks for the suggestion!
 

Zrussian13

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@GregO I just welded in an H pipe a few inches upstream of the stainlessbros resonator/muffler. Man, what a difference in noise level, especially at heavier throttles. Thanks for the suggestion!
Did it affect flow/ restriction?
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