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.

that seems like a lot of pressure...

do the people on miata.net say thats enough :lol:

(zack morris's phone)

Haha well played.



Jason: was this measure before cat and after turbo?

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.

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?

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?

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.

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....

Hmmm, I woulda thunk that 4 psi would've gained you like 3%.

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.

Why just a fraction?

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.

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.

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

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.

hmm from that link I would gather a bigger turbine housing should have less pressure buildup compared to a smaller one.

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.

I wonder if a velocity stack on the tailpipe would result in a measurable difference.

Prob not as much as removing the cat. :p

I'm calling you out on this one :) All the info you posted is correct except this^
While naturally aspirated motors have difficulty exhausting every last bit, with the correct valve overlap fi motors can easily begin to exhaust part of the intake charge. A lot of the time turboed and supercharged motors actually lower exhaust valve temps and combustion temps. In most miata applications there probably won't be very much of this because of relatively low boost and poor manifold/collector design but I thought it may be worth noting...

http://www.eng-tips.com/viewthread.c...137022&page=63

...

I like back pressure.

During a massage that is!

I re-measured at 15 psi / 270 hp (dynapack). I got a disappointing 9-10 psi. Previously I got 5 psi at 220 hp/10 psi (dynapack).
I have a custom 2.5" exhaust tystem.
I also plotted it vs power (looking at my dyno plot), and from 4000 RPM to power peak, it fits a quadratic curve pretty well (RPM^2).

Suspicious, I then looked at the pressure drop across my test pipe with a louvered resonator. 40% was across it! I'd have never thought it would be worse than my metal cat. I'll re install my metal cat and re-test. When I had this bolt-in test-pipe made I knew a louvered resonator wasn't as good as a perforated straight-through resonator. However it never occurred to me that it might have more backpressure than a metal cat. Louver resonators lose about 1/2" diameter in the center flow diameter if you look through them. (comparison image below)

Another disappointment was that my custom BEGI divorced downpipe showed ~1.5 psi, comparing just behind the turbine outlet, to just before the outlet flange before the cat / test pipe.

Ian's old FM exhaust, with 260 whp, shows 5 psi at the bottom of the downpipe.

More later.

https://www.miataturbo.net/attachmen...ine=1323042657

Awesome data, thanks. I may check mine when I runs again so us AF guys can have some data. That's if I'm not on the yacht with VJ, Ross, Ron, and Christian, us AF guys are a pretty big deal.

Do you have pics of the adapter you made to measure?

I have a 1/8 NPT hole for EGT at the turbine inlet which I use for TIP (turbine inlet pressure), and one at the top of my wastegate. I use a 1/8" NPT compression fitting with a ferrule and 2 ft of 1/8" copper tubing. Its' to isolate the heat from the vac hose, which leads to a boost gauge. For TIP I use a 0-60 psi gauge from the hardware store (a 0-30 might be better).

How in the hell can a louvered resonator cause so much back pressure? Thats really depressing because that is what I have in my miata and I liked the sound of it so much I put one in my GTX exhaust as well.

Bummer.

more "like legends in your own mind"

and Rummy called them unk-unk's

Perhaps there are louvered resonators out there where the inside diamter (inside of the louvers) is equal to the i.d. of the piping. Most resonators have the i.d. of the louver bits smaller than the piping i.d. So the exhaust needs to constrict down. This would be where the restriction comes from.

Now of course, with an increased i.d. there's less room for quieting packing material...