[OpenFlash]

Hi guys,
Here's a good post from a senior engineer at Garret Turbochargers from a few years ago. It was posted on a Subaru forum to clear up some of the misconceptions about exhaust scavenging, tube sizing, etc,. that some people had. It's a good read:

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. "


END OF PART 1

[OpenFlash]

PART 2

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.

Jay

[Carlito12]

great info ! thx !

it appears 3" separate pipes is the best way to go for performance...

[droptop335]

Given the above, would the "golf tee" mod that has been described in other threads contribute to reducing backpressure, or is that just too far down the exhaust system to make any difference?

[Shawn_speed]

God Vishnu:

Any good threats here or otherwise that would shed light on how the design and implementation of N54's exhaust system compares to the guidlines articulated above?

Shawn
Phoenix

[Terry335]

Originally Posted by shiv@vishnu

Hi guys,
Here's a good post from a senior engineer at Garret Turbochargers from a few years ago. It was posted on a Subaru forum to clear up some of the misconceptions about exhaust scavenging, tube sizing, etc,. that some people had. It's a good read:

I don't think they're connecting the dots. Better be more overt.

[Hegemony]

Terry,

Why don't you just come out and summarize that the article basically states that tapering down the downpipes will increase backpressure and thus make a cat-delete downpipe mod less efficient than it could be if you were to move that tapering further down the exhaust or eliminate it by maintaining a consistent pipe diameter?

Stated otherwise: If you're going to delete your primary cats, you'd be better served to run 3" pipes all the way back. Especially if you're going to push the car to 500whp+.

-Hegemony

[CASHBY]

Good Read.

[OpenFlash]

Originally Posted by Carlito12

great info ! thx !

it appears 3" separate pipes is the best way to go for performance...

In the case of the BMW, 3" is overkill since there are two exhaust systems. In other words, if we are talking about a 400hp car, each exhaust system only needs to support 200hp. Keeping things separate is, however, a good idea. The idea of using an X-pipe for post-turbo scavenging effects is flawed. There are no discrete exhaust pulses in the exhaust system since the turbine wheel "chops" up the exhaust flow. If there are no discrete exhaust pulses (separated by vacuum pulses), exhaust gas scavenging never comes into play. All an X-pipe will do is increase backpressure which will reduce power.

Shiv

[Revlis]

Thanks Shiv, very interesting and completely counter to what I had "Thought" regarding Turbo exhaust.

Interesting.

[droptop335]

Originally Posted by Hegemony

Terry,

Why don't you just come out and summarize that the article basically states that tapering down the downpipes will increase backpressure and thus make a cat-delete downpipe mod less efficient than it could be if you were to move that tapering further down the exhaust or eliminate it by maintaining a consistent pipe diameter?

Stated otherwise: If you're going to delete your primary cats, you'd be better served to run 3" pipes all the way back. Especially if you're going to push the car to 500bhp+.

-Hegemony

Why would anyone use a cat delete downpipe that has any tapering at all? If you're going to go to the trouble of removing the cat, you might as well put in a full flow replacement anyway.

[Hegemony]

Shiv,

I agree that it's overkill now. I've always heard that the guideline is a .5" per 100whp. Thus 2.5" pipes on a true dual system should carry you up to 500whp.

Generally speaking, would you agree with this logic?

-Hegemony

Originally Posted by droptop335

Why would anyone use a cat delete downpipe that has any tapering at all? If you're going to go to the trouble of removing the cat, you might as well put in a full flow replacement anyway.

CEA,

Did Riss Racing not feel that this was an issue due to the 2.5" size of the exit flange?

[e.n335]

Thus we have a dual pipe exhaust, 2.5" should be more than enough. 250hp each pipe. Is it feasible that an exhaust shop replaces the primary OEM cat's with 200 cpi metal cat's in the OEM downpipes ? How much would this be ? Could be attractive ...

[CASHBY]

Originally Posted by droptop335

Why would anyone use a cat delete downpipe that has any tapering at all? If you're going to go to the trouble of removing the cat, you might as well put in a full flow replacement anyway.

Stock turbine flange is 3 inch... stock exhaust flang and all aftermarket flanges are 2.5 inch.

[CASHBY]

Originally Posted by Hegemony

Did Riss Racing not feel that this was an issue due to the 2.5" size of the exit flange?

Stock turbine flange is 3 inch... stock exhaust flang and all aftermarket flanges are 2.5 inch as I stated above....

On the stock downpipes they go from 3in in DIA to 2 inch in some spot.

We can make the same DIA all the way through, but there are no 3 inch aftermarket exhaust systems and 3 inch flanges wont bolt to 2.5 inch flange.... wel they can but there will be a bit of a lip/wall

[droptop335]

Originally Posted by CEA 3

Stock turbine flange is 3 inch... stock exhaust flang and all aftermarket flanges are 2.5 inch.

That makes sense, but then again, if you're gonna go through the trouble...

However, based on what Shiv said, 2.5" should be sufficient for our engine, even if it's Proceeded...

[CASHBY]

Originally Posted by Hegemony

Terry,

Why don't you just come out and summarize that the article basically states that tapering down the downpipes will increase backpressure and thus make a cat-delete downpipe mod less efficient than it could be if you were to move that tapering further down the exhaust or eliminate it by maintaining a consistent pipe diameter?

Stated otherwise: If you're going to delete your primary cats, you'd be better served to run 3" pipes all the way back. Especially if you're going to push the car to 500whp+.

-Hegemony

Well we obtained 27whp and 49wtq over stock with tappering to 2.5 inch.... so it was more than efficient to remove the cats.. we made a full 3 inch all the way back, but again no one has 3 inch exhausts on our car... and lots of people would like to run stock exhuast...

Also we could always taper back further, but the stock exhaust is one peice and after market exhaust all have flanges further down in all different spots...so it would be imposible to develope a downpipe that tappers further down the exhaust that is going to be a "one size fits all" downpipe.

[CASHBY]

Originally Posted by droptop335

That makes sense, but then again, if you're gonna go through the trouble...

However, based on what Shiv said, 2.5" should be sufficient for our engine, even if it's Proceeded...

+1

[OpenFlash]

Yep.... 2.5" should be more than fine. Just keep piping as straight as possible. Keeping both pipes separated is also optimal. Completely understandable to taper down the pipe at the turbine exit due to packaging constraints. But again, keeping the transition smooth (not stepped) is important.

Shiv

[CASHBY]

Originally Posted by shiv@vishnu

Yep.... 2.5" should be more than fine. Just keep piping as straight as possible. Keeping both pipes separated is also optimal. Completely understandable to taper down the pipe at the turbine exit due to packaging constraints. But again, keeping the transition smooth (not stepped) is important.

Shiv

+1... We slowly taper from 3, to 2.75 to 2.5 to allow it to flow rather then an abrupt 3-2.5. and our bends are really as straight as posible

[RiXst3r]

Originally Posted by shiv@vishnu

In the case of the BMW, 3" is overkill since there are two exhaust systems. In other words, if we are talking about a 400hp car, each exhaust system only needs to support 200hp. Keeping things separate is, however, a good idea. The idea of using an X-pipe for post-turbo scavenging effects is flawed. There are no discrete exhaust pulses in the exhaust system since the turbine wheel "chops" up the exhaust flow. If there are no discrete exhaust pulses (separated by vacuum pulses), exhaust gas scavenging never comes into play. All an X-pipe will do is increase backpressure which will reduce power.

Shiv

so basically, this article proves that AA made a mistake by putting an X pipe in their exhaust?

Then what about thing little article that AA drew up... is it BS?

[OpenFlash]

Originally Posted by RiXst3r

so basically, this article proves that AA made a mistake by putting an X pipe in their exhaust?

The mistake was made when they stated that it was done for exhaust scavenging purposes. That is untrue.

An X-pipe does have merit when it comes to fitment since it basically acts as a support brace. It can also be used to change the tone of the exhaust in naturally aspirated cars through basic pulse tuning. But in terms of performance in a turbo car, an X-pipe does nothing good. It just contributes to exhaust flow turbulence/backpressure.

Shiv