Midrange power loss ---> downpipe

[WRXVII]

I find it quite interesting that downpipe is causing power loss in the midrange around 4500-5500rpm from a dyno done by XS Engineering. This is the exact same thing one of the professional driver mentioned about the use of aftermarket exhaust on WRX --> Best Motoring Impreza vs. EVO DVD

Why is there a power loss in the midrange from the use of downpipe? I do understand that peak power does increase, but I want to understand why it causes power loss in the midrange....

Also noticed that there is a peak power loss from the use of silencer (~5hp). Could the use of silencer increase the low -> high-end power putting the peak power aside?

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[edkwon]

i'm guessing its the same kind of midrange powerloss associated with a free flowing exhaust. Reducing exhuaswt manifold backpressure with a larger downpipe reduces torque.

Ed

[jlevy]

This is kind of strange. I've always heard that turbo cars don't really respone to exhaust changes after the turbo the same as n/a cars do. Bigger is pretty much always better.

A boost spike may explain it, but I wouldn't think it would be that consistent. After putting on my TXS stealthback system (otherwise stock car), I logged the boost curve and noticed a sine wave pattern. The boost shot up to ~14.5 by 3000 rpm then as the factory boost controller tried to maintain constant boost, it resulted in the wavering boost.

-my 0.02

[Bolster]

Maybe you're having the wastegate sucked open. That will hurt the power curve.

[el~sharko]

Originally posted by jlevy
This is kind of strange. I've always heard that turbo cars don't really respone to exhaust changes after the turbo the same as n/a cars do.


-my 0.02

wow, I've "always" heard the opposite. Now I'm realizing that some backpressure is important even on turbo cars(mainly for drivability).

[jlevy]

Since the turbine operates via a pressure ratio, the lower the backpressure the better. In some cases people have used back pressure to control boost creep, but that's really just a bandaid method.

Here is a quote from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on www.impreza.net regarding exhaust design and exhaust theory:

“Howdy,

This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems.

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.

A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.

If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.

Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.

Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.

Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.

Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.”

"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.

There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.

As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.”

"Here's a worked example (simplified) of how larger exhausts help turbo cars:

Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

So here, the turbine contributed 19.6 psig of backpressure to the total.

Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).

So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would. As for output temperatures, I'm not sure I understand the question. Are you referring to compressor outlet temperatures?

The advantage to the bellmouth setup from the wg's perspective is that it allows a less torturous path for the bypassed gases to escape. This makes it more effective in bypassing gases for a given pressure differential and wg valve position. Think of it as improving the VE of the wastegate. If you have a very compromised wg discharge routing, under some conditions the wg may not be able bypass enough flow to control boost, even when wide open. So the gases go through the turbine instead of the wg, and boost creeps up.

The downside to a bellmouth is that the wg flow still dumps right into the turbine discharge. A divider wall would be beneficial here. And, as mentioned earlier, if you go too big on the bellmouth and the turbine discharge flow sees a rapid area change (regardless of whether the wg flow is being introduced there or not), you will incur a backpressure penalty right at the site of the step. This is why you want gradual area changes in your exhaust."

-JL

[ride5000]

kinda makes you think about the 2.5" vs. 3" exhaust diameter debate, doesn't it?

think about it for a minute--of COURSE the piping after the turbo matters... there's still pulses of gas pressure traveling along the pipe. if there wasn't we wouldn't need mufflers at all and we'd still get complete silence.

different exhaust systems will be "tuned" to resonate at different frequencies, and therefore different rpms. therefore at certain rpms you will have more of a scavenging effect (perhaps even to the point of an effective vacuum @ the turbo outlet), and at others you will have increased backpressure. these rpm ranges will vary from mfg. to mfg. as well as different overall diameters from the same mfg.

i don't buy all of what the above post is getting at.. there are no absolutes in life. "bigger is better" is waaaay too easy. this may cause some people to question how on earth i have the balls to challenge a "turbosystems engineer."

to them i would say, "try it yourself, like i did..." take a stock WRX, crawl underneath and pull off the oem axle-back. take it for a spin. notice the increased breathing at high RPMs, ***AND*** the diminished torque at low RPMs.

rub your chin and wonder why.

ken

[jlevy]

I have a TXS stealth back and did not notice any loss of low end torque. Quite the opposite, the car was much more driveable off-boost. It required less throttle to go the same speed and I found I no longer had to turn off the A/C every time I left a light.

The point is not whether the exhaust pulses make it through the turbo, but that making boost earlier in the rev range far outweighs any increase in torque you would see by tuning the pipe diameter for exhause pulses.

think about it for a minute--of COURSE the piping after the turbo matters... there's still pulses of gas pressure traveling along the pipe. if there wasn't we wouldn't need mufflers at all and we'd still get complete silence.

Does that mean jet engines are silent?

-my 0.02

[wrx320]

Wow, So if I go outside and take the muffler off a stock turbocharged wrx I am an engineer?

[WRXcellerate]

Main Entry: idiocy
Pronunciation: 'i-dE-&-sE
Function: noun
Inflected Form(s): plural -cies
Date: circa 1529
1 : extreme mental retardation commonly due to incomplete or abnormal development of the brain
2 : something notably stupid or foolish

[ride5000]

you guys want to think you know it all, be my guest! i've got nothing to prove to anyone but myself.

keep on thinking in black and white terms and we'll see who's the idiot in the end; clearly it is not obvious to you at this point, but there's more to this issue than your apparently limited minds can take into account. i know how much i don't know, which is what sets us apart.

have fun!
ken

[ebeck]

I know how much I don't know. I wondered about the stealth back and the stock or STI muffler. I really like quiet. I had a loud droning exhaust I would stick a knife in my ear. No, wait a minute, my wife would stick a knife in my ear!

Seriously, Hows the stralts bcak and stock tip doing? Seems the muffler is not that bad. Again, I don't know. I just read all the posts about how a cat back does nothing. Real power is in the turbo back. Doesn't that mean up, down, mid and stock muffler should work fine and also be quiet? May be a few horses sacrificed but quiet. No?

[Jon [in CT]]

Originally posted by WRXVII
I find it quite interesting that downpipe is causing power loss in the midrange around 4500-5500rpm from a dyno done by XS Engineering. This is the exact same thing one of the professional driver mentioned about the use of aftermarket exhaust on WRX --> Best Motoring Impreza vs. EVO DVD

Why is there a power loss in the midrange from the use of downpipe? I do understand that peak power does increase, but I want to understand why it causes power loss in the midrange....

Also noticed that there is a peak power loss from the use of silencer (~5hp). Could the use of silencer increase the low -> high-end power putting the peak power aside?

So where are these XS Engineering dyno graphs of a WRX? Surely you didn't drive your WRX from Poughkeepsie to SoCal to get a dyno graph.

Originally posted by ride5000
you guys want to think you know it all, be my guest! i've got nothing to prove to anyone but myself.

keep on thinking in black and white terms and we'll see who's the idiot in the end; clearly it is not obvious to you at this point, but there's more to this issue than your apparently limited minds can take into account. i know how much i don't know, which is what sets us apart.

have fun!
ken

You have no idea of how much you don't know. For instance, many engineering problems/issues are indeed black and white for anyone experienced in the particluar field.

I, so far, have seen no evidence that a smaller diameter turbo-back exhaust improves low-end torque over a larger diameter exhaust that is otherwise similar in design. I agree with Jay-of-Garrett's views on this issue (originally a series of posts in thread http://forums.nasioc.com/forums/show...r&pagenumber=2), except for his discription of how the exhaust system should be designed upstream from (ahead of) the turbo:

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.
...
As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.

Here Jay is describing solutions to issues that arise when optimising the exhaust for a pulse (twin-scroll) turbo, which attempts to utilize the kinetic energy in the exhaust pulses. The WRX uses a constant pressure turbo, where the only significant factor is the pressure difference/ratio across the turbine.

[ride5000]

Originally posted by Jon [in CT]
... where the only significant factor is the pressure difference/ratio across the turbine.

jon, i completely agree, and that is where my original point stands its ground.

intead of being condescending, why don't you (or anybody else for that matter) apply the information from here to the issue at hand, which is indeed (as i've stated above) the pressure difference across the turbo.

i really don't know why people balk at the fact that it is NOT as simple as putting on the fattest pipe you can afford or fit under the car.

ken

[WRXcellerate]

Your tottaly right there is much more to do than just slap on the biggest exahust you can, BUT, back pressure is your ENEMY post turbo, no matter what anyone tells you the less backpressure you have after the turbo is optimal (easiest way is bigger exahust post turbo)

[Avedis]

I want to know the full details of the test before I'll draw any conclusions. Perhaps the car was tuned to the ragged edge, they added a dp, and dyno'd it... and overboosted/detonated/pulled timing in the midrange. Perhaps they reset the ECU after installing it. I find it doubtful that they did any engine management between the first dyno and the second.

--jeff the skeptic

[Wheeler Bement]

have you guys seen this??

http://www.mattrandolph.com/mattspec.htm

bigger is better, but nothing at all won't get you much