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Old 07-14-2011, 05:04 PM   #1
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Default I measured exhaust backpressure

I made an adapter to fit in an O2 sensor hole, with about 2 feet of 1/8" copper pipe. Several more feet of rubber vacuum hose and a boost gauge.

I had a bolt that fit in the O2 sensor bung. I drilled it through with a 1/8 bit, then drilled a blind hole about 3/8" deep with the bit size appropriate for 1/8 NPT thread. I tapped it, then screwed in a compression fitting for the 1/8" copper pipe.

Results:

Miata with metal cat, 2.5" custom exhaust, resonator and magnaflow muffler:
~ 5 - 5.5 psi at 10 psi of boost at redline.

BMW 540i (V8) stock exhaust with 4 cats, measured upstream of the first cat: 4-4.5 psi, either side.
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Old 07-14-2011, 05:06 PM   #2
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that seems like a lot of pressure...
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Old 07-14-2011, 05:16 PM   #3
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do the people on miata.net say thats enough :lol:
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Old 07-14-2011, 05:35 PM   #4
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(zack morris's phone)

Haha well played.



Jason: was this measure before cat and after turbo?

Last edited by Braineack; 07-14-2011 at 05:36 PM.
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Old 07-14-2011, 05:55 PM   #5
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Yes after turbo before cat.

Before turbo (aka turbine inlet pressure), with the GT2554 it was about 15-20 psi from 3500-6000 RPM, then it spiked to 27 from 6500-7000, at 10 psi boost. With the GT2560 it was like 15-22 psi from 3500 to redline. I have a video of it somewhere.
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Old 07-14-2011, 06:05 PM   #6
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So wait, are people on m.net saying you need backpressure?
and if so, for what purpose? engine output or protecting the turbine oil seal?
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Old 07-14-2011, 06:07 PM   #7
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I must admit, that's rather higher than I would have expected. I cringe at what the difference might be with a ceramic cat installed.

I assume that this was an unbaffled, straight-through muffler, and not one of the XL series, yes?
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Old 07-14-2011, 07:44 PM   #8
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Correct. Biggest straight through you-can-roll-a-golf-ball-throuh-it Magnaflow muffler that would fit (22" x 5x9" ?)

At 6 psi of boost with the stock exhaust I was seeing > 8 psi.

The ceramic Caround "hi flow" cat I used to have, was about 1 psi higher.

Behind the cat is 1-1.5 psi. So any cat is really most of the backpressure.

Have a look at how factory cars do:

http://autospeed.com/cms/title_Frank...5/article.html
http://autospeed.com/cms/title_Press...0/article.html

Commodore, R32 Skyline, 90s WRX ... all > 6 psi.

OE's must think 6 psi is a good compromise (cost, quietness, etc).
BMW must think < 5 psi on the 540i is fine. I wonder how today's highly tuned factory cars do (e.g. S2000, M cars, EVO, etc).


BTW I wonder why exhaust vendors show dyno results and not backpressure. The latter is far more repeatable.
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Old 07-15-2011, 12:10 AM   #9
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At one point, I was running a 3" exhaust, with a chambered muffler. I was measuring 9psi pre-cat back pressure at redline and 5psi post cat.

At the dyno, I disconnected the muffler, and gained no measurable power. The AFR didn't change either, indicating that this level of back pressure, does not affect power. Could affect spool though....
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Old 07-15-2011, 03:49 AM   #10
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Hmmm, I woulda thunk that 4 psi would've gained you like 3%.
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Old 07-15-2011, 04:37 PM   #11
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Quick calc.

Pushing 1000 cfm (est exhaust flow at 200 hp) with 4 psi of backpressure takes 17 hp.

This is not to say you'll gain 17 hp going from 4 psi to 0 backpressure, you'll gain just a fraction of that - maybe 5 hp.
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Old 07-15-2011, 05:35 PM   #12
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Quote:
This is not to say you'll gain 17 hp going from 4 psi to 0 backpressure, you'll gain just a fraction of that - maybe 5 hp.
Why just a fraction?
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Old 07-15-2011, 08:43 PM   #13
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Quote:
Originally Posted by JasonC SBB View Post
Quick calc.

Pushing 1000 cfm (est exhaust flow at 200 hp) with 4 psi of backpressure takes 17 hp.

This is not to say you'll gain 17 hp going from 4 psi to 0 backpressure, you'll gain just a fraction of that - maybe 5 hp.

hmm. Where is HP coming from, to overcome this backpressure? I would argue, that as long as the exhaust is all leaving the cylinder head, the back pressure it meets on the way, is irrelevant. If the backpressure is significant enough, more exhaust will not exit the cylinder head, and then you will see a drop in VE, and resulting drop in HP.
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Old 07-16-2011, 01:07 AM   #14
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Quote:
Originally Posted by oreo View Post
hmm. Where is HP coming from, to overcome this backpressure?
There are several different factors at play here.

First, the engine, in simplest terms, is a large air pump.

Pumping any fluid, be it air, water or semen, takes work. The more restriction you have to pump against, the more work it takes. Think of trying to blow air through a large straw- pretty easy. Then try to blow the same amount of air (in cubic inches per second) through a tiny little coffee stirrer- harder.


Next, you mentioned "... as long as the exhaust is all leaving the cylinder head..." and that's another part of the equation.

All of the exhaust does not leave the cylinder head; not even on a naturally-aspirated motor. Imagine running the engine with no exhaust manifold at all (just open ports), and for the moment, forget everything you know about valve overlap and things like "scavenging" and the like. We'll also handwave over the concept of pressure differential across the turbine, and just pretend it doesn't exist for the sake of making this easier to comprehend.

In this best-case-scenario, at the top of the exhaust cycle when the exhaust valve closes, there's still a combustion chamber's worth of exhaust gas sitting in the top of the cylinder. That's wasted space that's not going to be filled with fresh air and fuel during the intake cycle.

Now, imagine that the pressure of that chamber's worth of air is higher than intake-manifold pressure. When the intake valve opens, that pressure is going to resist the flow of air into the cylinder, and as the piston travels downwards, that trapped exhaust gas will expand, taking up yet more space that's not going to be filled with fresh air and fuel.

The higher the pressure of the residual spent gas in the chamber, the less fresh air and fuel will be admitted.

And how do you raise the pressure of the residual gas in the chamber? Simple: add exhaust back-pressure.



Finally, the part I handwaved over earlier. How does the turbine work? Well, you have high pressure on one side (from the head) and low pressure on the other (out the tailpipe.) The greater the difference in pressure between these two sides, the more work you can get the turbine to do. If you increase the pressure on the outlet side of the turbine, then you have decreased the pressure differential across the turbine, and it can no longer do as much work turning the shaft to drive the compressor.
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Old 07-16-2011, 07:30 AM   #15
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would love to see the pressure difference in the turbo manifold between same turbo with different turbine housing, small and big one, example .48 ar vs .63 ar turbine housing on a T3
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Old 07-19-2011, 06:19 PM   #16
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http://sportcompactcar.automotive.co...t-4/index.html

Sport Compact car test on ST-4 (which has a pretty free flowing factory setup) shows 7 psi. Magnaflow replacmenet muffler dropped it to 5.3 psi.
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Old 07-19-2011, 06:47 PM   #17
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hmm from that link I would gather a bigger turbine housing should have less pressure buildup compared to a smaller one.
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Old 07-20-2011, 12:38 PM   #18
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That would be true and would explain why bigger a/r's have a stronger topend. Comparing the 2 a/r's turbine maps would show why, if they show that at near-redline hp worth of exhaust flow and at the shaft power the compressor needs, the larger a/r shows higher efficiency, which translates to lower delta P across the turbine.
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Old 07-20-2011, 12:57 PM   #19
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I wonder if a velocity stack on the tailpipe would result in a measurable difference.
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Old 07-20-2011, 04:51 PM   #20
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Prob not as much as removing the cat.
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