[MrWhite1015]

found this while looking for some info... dont know if anyone has posted it before but very well explained and detailed article about turbo exhaust systems and some N/A info as well.... on a side note they mention 2.5in exhausts on 300-350hp turbo cars... i know we have 2, 2.5in pipes going all the way back but not sure how that correlates... do you get the same flow as a 5in exhaust (probably not) or is it the same as a 3in exhaust... i know AMS is working on something like this right now so maybe they can chime in and give their .02 on the flow characteristics of exhausts like AE and other two pipe systems out now.



so now here is the write up i found:

The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on 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

o 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.

[enrita]

Oh my god this is sooo good info! Basically than it is a must to have a 3inch Catless DPs in our N54!

[MrWhite1015]

Originally Posted by enrita

Oh my god this is sooo good info! Basically than it is a must to have a 3inch Catless DPs in our N54!

see i think its different for us since we have two of everything... thats kinda why i want someone from ams to chime in since they are working on a single 3in exhaust

[OpenFlash]

lol.. Jay is a good friend of mine. He write that tech info at my request several years ago to clear up some misunderstandings about turbo exhaust design. Definitely worth re-reading.

Cheers,
shiv

[MrWhite1015]

Originally Posted by shiv@vishnu

lol.. Jay is a good friend of mine. He write that tech info at my request several years ago to clear up some misunderstandings about turbo exhaust design. Definitely worth re-reading.

Cheers,
shiv

so what are your thoughts on the options currently available for this car? mostly that of 2, 2.5 inch pipes compared to 1 3in pipe?

[OpenFlash]

Originally Posted by MrWhite1015

so what are your thoughts on the options currently available for this car? mostly that of 2, 2.5 inch pipes compared to 1 3in pipe?

Better to have two 2.5" pipes than on 3" pipe. Not just in terms of cross section but also in terms of not merging the two turbine exhaust flows downstream (which causes backpressure).

Shiv

[MrWhite1015]

Originally Posted by shiv@vishnu

Better to have two 2.5" pipes than on 3" pipe. Not just in terms of cross section but also in terms of not merging the two turbine exhaust flows downstream (which causes backpressure).

Shiv

thanks bro... on another note ...why would a well known turbo tuning/development company like AMS propose such an idea and bother testing it if this is already a known fact ?

[OpenFlash]

Originally Posted by MrWhite1015

thanks bro... on another note ...why would a well known turbo tuning/development company like AMS propose such an idea and bother testing it if this is already a known fact ?

As with everything, you have to juggle your compromises a bit. Having a single large diameter merge all the way downstream where the exhaust gases are slower and more dense may not result in any significant back-pressure. But it will probably be less expensive to manufacture and lighter. Just requires testing which I'm sure they do.

[scollins]

Doing the math, it looks like a divorced 2.5" system has roughly the same cross-sectional area as a single 3.5" pipe.

Area = Pi * r².

Divorced 2.5" system:
Area = 3.141592 * (2.5/2)² = 4.9087375 sq in.
Total Area = 2 * 4.9087375 = 9.817475

Solving for r in a single system:
r = √(Area/3.141592) = 1.767767 in
Diameter = 2*r = 3.5355"

I just don't know if it works out that way in real life. I didn't study fluid dynamics too much in college....

[MrWhite1015]

Originally Posted by scollins

Doing the math, it looks like a divorced 2.5" system has roughly the same cross-sectional area as a single 3.5" pipe.

Area = Pi * r².

Divorced 2.5" system:
Area = 3.141592 * (2.5/2)² = 4.9087375 sq in.
Total Area = 2 * 4.9087375 = 9.817475

Solving for r in a single system:
r = √(Area/3.141592) = 1.767767 in
Diameter = 2*r = 3.5355"

I just don't know if it works out that way in real life. I didn't study fluid dynamics too much in college....

yes yes .. your math checks out ... brings back some memories of high school math... i dont think that, that is really whats in question though ... i think its more about the fluid dynamics of two smaller pipes vs one larger pipe (that merges the exhaust gasses of two turbos)... the math for that...i have no idea

[BOOST3D]

good info

[WCM3]

Originally Posted by MrWhite1015

thanks bro... on another note ...why would a well known turbo tuning/development company like AMS propose such an idea and bother testing it if this is already a known fact ?

I'm sure with all research and development, some companies like to try something that opposes the norm. AMS never said it was their final design, only subject to testing.

Good read.

[scollins]

Originally Posted by MrWhite1015

yes yes .. your math checks out ... brings back some memories of high school math... i dont think that, that is really whats in question though ... i think its more about the fluid dynamics of two smaller pipes vs one larger pipe (that merges the exhaust gasses of two turbos)... the math for that...i have no idea

I was thinking that to have similar flow "capacity" characteristics, a merged system would have to be at least 3.5". But I think as you bring two flows together, they generate turbulence at that point, which reduces velocity (and hence increasing back pressure to some degree.) So it is quite possible that a single pipe system might need 4" to produce the same results as a divorced 2.5" system. But that is just pure speculation on my part...

[Eric@AMS]

we will be releasing this test today. I think that people will be pretty surprised on the outcome I will post a link to the thread in here when I post it

Eric

[Tobias1980]

Great info guys!

Is there any good overview pictures of the stock, complete exhaust system?

[BavarianBullet]

Originally Posted by scollins

Doing the math, it looks like a divorced 2.5" system has roughly the same cross-sectional area as a single 3.5" pipe.

Area = Pi * r².

Divorced 2.5" system:
Area = 3.141592 * (2.5/2)² = 4.9087375 sq in.
Total Area = 2 * 4.9087375 = 9.817475

Solving for r in a single system:
r = √(Area/3.141592) = 1.767767 in
Diameter = 2*r = 3.5355"

I just don't know if it works out that way in real life. I didn't study fluid dynamics too much in college....

Two 2.5" pipes vs one 3.5" pipe will have the same cross section but a bit more surface area so the dual 2.5" will likely flow slightly less. In fact, it does flow less based on this one URL at least (cuz I didn't study CFD either): http://www.pipeflowcalculations.com/airflow/index.htm

1219 cuft/min for 3.5" and 591 cuft/min for 2.5" for a 15ft straight pipe.

I'm interested to see how much of a restriction the turbos themselves are at the high power levels we've seen to date (400+WHP). Based on the claimed dyno results seen thus far from those who bolted on exhaust mods after downpipes, I'd say post-downpipe exhaust gains will continue to be relatively small (~10whp) until bigger turbos get bolted on.

I'd also love to see a wastegate exhaust flow vs duty cycle correlation (WG valve position plus estimated pressure at turbine p1 at a given WG DC). May not be easy to get accurately but it would be very informative.

BB

[GreenPlease]

Don't forget about laminar flow and the associated frictional (pumping) losses of a divorced 2.5" system compared to a single 3" system.

2.5" divorced system
Length (assume) 96"
Diameter: 2.5"
Circumfrance: pi*d= 7.85
Total surface area: 7.85*96*2(divorced)=1507.2 square inches

3" single
Length (assume) 96"
Diamter: 3.0"
Circumfrance: pi*d=9.42
Total surface area: 904.32 square inches

The exhaust flow through the 3" system is exposed to 40% less surface area while only having 28% less volume than the divorced 2.5" system. The translation into frictional losses.... I really don't feel like doing that math but its not insignificant.

I spent a lot of time fiddling with watercooling PCs. From experience 1/2ID tubing is vastly superior to twin 1/4ID. If I recall, flow rates dropped about 40% in later setup. If you have to ask, 1/2ID is a bitch to route inside a case.

[GreenPlease]

While we've got a nice thread going on exhausts, has anyone run an X pipe on a 335i? As I understand it, itallian cars such as Ferrari/Lambroghini run an X pipe to enhance second order vibrations (sound) and that's what gives them that "exotic" sound. I'd be curious to see how our (perfectly balanced second order forces) I6 sounds with an X pipe.

[Eric@AMS]

As promised our comparison test: http://www.e90post.com/forums/showth...88#post3868788