Turbo Compressor Maps - Effect of Ported Shrouds and "The Tornado"

A little while ago, I was involved in a discussion about surging with the TA-49 turbo at low-boost, part throttle conditions. Some of us are having problems with compressor surge at around 8-10 psi (pressure ratios in the 1.5 to 1.7 range) or so. Several ideas to fix this problem were discussed. Two of them were using a ported shroud and using a "Tornado" in the inlet pipe. Both were reported as being effective at stopping the surging, but I was wondering, what effects do these devices have on the efficiency of the turbo?

Lo-and-behold, an article appeared in the April 3, 2012 edition of "Automotive Engineering" magazine, publised by the Society of Automotive Engineers (yes, I am a member). The title of the article is "The Quest for Better Turbocharger Compressors". How convenient.

The gist of the article is that modern turbocharger designers face a problem - modern engines (especially ones that use a high amount of EGR to improve emissions and fuel economy) require a turbocharger with a very wide compressor operating map. For those who understand compressor maps, Figure 1 below shows it graphically. Under moderate acceleration, the turbo is required to supply a high pressure ratio with low mass flow (kind of like our TA-49's at about 9psi of boost, right?). This point is "off the map", and surging of the compressor (the infamous "sneezing") will occur.



I included Figure 2 just because is shows a nice cutaway of a modern turbo compressor. Notice that it has a ported shroud, which is called "case treatment" or "CT" in the remainder of the article.


Now here is an important point - the article shows how "CT" (what we know as a ported shroud) affects the compressor map. Figure 3 shows, for a given test turbo compressor, the width of the compressor map at given boost pressures with and without "CT". The "range" is the difference between the maximum air flow and minimum air flow (lowest air flow before compressor surge occurs) divided by the maximum airflow. A higher number means that the turbo has a wider compressor map, as in more range of air flows that it can handle. The "Pressure Ratio" is just that - the boost pressure plus atmospheric pressure divided by atmospheric pressure. What this graph shows is that regardless of the pressure ratio, adding "CT" (ported shroud) to the turbo compressor results in a wider compressor map.




Now for the most important point of the article. The question remains - how do "CT" (ported shrouds) and "Tornados" affect the turbo compressor's performance? The answers are in Figure 4 below. Figure 4 is essentially a turbo compressor map, but with the efficiency "island" information placed on a seperate graph. First, an explaination. "VIGV" means "Variable Inlet Guide Vanes". Imagine a "Tornado" device placed just in front of the turbo compressor, and the angle of the "Tornado's" blades can be changed on-the-fly via commands from the ECU. In simple terms, that's what VIGV's are - they are devices the "pre-swirl" the air entering the turbocharger. The primary difference between VIGV's and the "Tornado" is that the angle of the blades in the "Tornado" are fixed, while the angle of the blades in a VIGV device can be changed.

If you look at the bottom graph in Figure 4 and compare the black line (a conventional turbo compressor) to the purple line (add "CT", or ported shroud), you can see that the "CT" moves the surge line significantly to the left. This means that the compressor is less likely to surge when operated under low air flow conditions. You can also see that away from the surge line, the "CT" line and conventional line sit pretty much on top of each other, which implies that the "CT" (ported shroud) does not significantly alter the air flow performance of the compressor away from the surge line. Now, looking at the yellow, orange, and red lines, these represent the case where VIGV's are added to the system in addition to the "CT". The yellow line can be thought of adding a "Tornado" with the blades angled at 60-degrees, the orange with blades at 70-degrees, and the red with blades at 80-degrees. Increasing the incoming swirl of the air moves the surge line even farther to the left, allowing the compressor to operate with even lower air flow before the compressor surges. However, as air flow increases, you can see that the VIGV's (our "Tornados") really kill the air flow performance of the compressor. The maximum possible air flow is only about 55 to 65% (depending on blade angle) of the conventional compressor. This makes sense, since the VIGV's act as large restrictions at higher air flow.

If you look at the top graph in Figure 4 and compare the black line (conventional compressor) to the purple line ("CT", or ported shroud, added), there is very little difference. This shows that adding a ported shroud has minimal effect on compressor efficiency. In practical terms, this means that the amount of turbine power needed to drive the compressor does not change significantly by adding a ported shroud to the compressor. If you compare the black lines to the orange lines (VIGV's, or a "Tornado", with the blades set to 70-degrees), you can see that the compressor efficiency at a given air flow is much lower with the VIGV's present. In practical terms, this means that a lot more turbine power will be needed to drive the compressor, which means higher exhaust back pressure, which means less engine performance, particularly at high air flows (i.e. full throttle).




So, in conclusion, the data show:

1. Adding "CT" (ported shroud) to the compressor allows it to run at lower air flow rates without surging across the entire operating range. The effect of the "CT" on the maximum possible air flow and efficiency of the compressor is very small.
2. Adding a fixed set of inlet guide vanes (such as a "Tornado") allows the compressor to run at much lower air flow rates without surging across the entire operating range. However, the fixed inlest guide vanes ("Tornado") will significantly reduce the maximum air flow that the compressor will achieve, and will significantly reduce the compressor efficiency, particularly at high air flow levels (i.e. full throttle).

One caveat: These data are for one particular turbo compressor of unknown type - the results might be different for other turbo compressors. However, I would tend to believe that the general trends would be the same.

I thought that some of the folks on this board might be interested in this - hopefully somebody learned something interesting. Suffice to say, after looking at these data, I will never install a "Tornado" into my inlet pipe.

My car still traps 118+mph with a tornado at 25psi so that's about it for a TE44 so it can't hurt that much. It'd do 120mph if I could get more boost but with the tornado installed, it's still my hot side that is holding me back.

This data holds true for the compressor but doesn't take into account the relationship between the turbine and compressor. With a better hotside, I may be able to see the tornado's effect on compressor efficiency but an engine can't eat more than it poops regardless of compressor efficiency. The tornado does work for surge on the street and I'd just remove it at the track if I thought it slowed me down.

Thanks for posting none the less. Great reading.

My car has trapped 119 mph with a TA49 (worse cover than TE44) at 25 psi boost with 100-octane unleaded and no alky (compared to your unknown fuel with alky). Rest of combo is in my sig. Also, I'm running a stock cam versus your 208/208 cam. So, more mph with a slightly weaker turbo and smaller cam.

Yes, clearly the hot side of the TA49/TE44 holds them back, but at those high air flows, any restriction in the intake tract is going to hurt compressor efficiency and cost some hp. Are you sure your Tornado isn't holding you back a bit?

Have you ever tried running all-out with the Tornado removed? It would be interesting to see back-to-back all-out runs with your combo with and without the Tornado. My gut feeling is that you would pick up a mph or two with a clean inlet tract. Not saying it's going to add 100 hp or anything, but my gut feeling is that it will add something.

For what it's worth...

I have pulled it at the track and mph stays the same. I forgot to reinstall it though and it surges so bad at part throttle that I just no longer take it out. I just use pump gas and alky with conservative timing so you may have mph'd more from timing, a leaner AFR, a lighter car, more efficient converter, who knows; but the hotside is getting in my way long before the tornado. This is due to the fact that my hotside will never flow more than my compressor so the tornado doesn't have a chance to make an effect as the compressor is never at the edge of the map with or without the tornado accept at part throttle operations. The data presented only looks at flow of a compressor and not the whole compressor/turbine combo.

Thanks for posting your results - very interesting. I guess the Tornado doesn't make that much of a difference in your combo - definitely the restrictive nature of the turbine has a lot to do with that. I have never seen a Tornado in person - can you estimate the angle of the blades on the Tornado relative to the air flow direction? The above article used blade angles of 60, 70 , and 80 degrees (pretty extreme angles), so maybe the Tornado has shallower blade angles that don't affect compressor efficiency as badly.

And yes, the mph difference could be due to a lot of things. Your tune (in your sig) is conservative compared to what I run, but I'm still proud of my 119mph runs with a stock cam/engine and 100-octane fuel. I believe that my very straight and unrestrictive intake path helped contribute to this. I only get surging at very specific conditions (moderate acceleration, around 9psi of boost), so it's easy for me to drive around it. For these reasons, I don't see a Tornado in my future .

Thanks for posting your results...

1) Surging with a 49 is the result of terrible air presentation to the compressor wheel.I have an improved efficiency compressor housing,turbine housing and wheels.The butt dyno is really grinning but this is not a scientific conclusion.The turbo has to go on cycle on an exhaust dyno for hard data.I'm an engineer working with the best fabricator/machinist in "turbodom".My stall with a n/l, 3000rpm Art Carr has dropped to about 2780 rpm.The car comes up on boost from a dead wholeshot (no brake torque) in about 10 ft as measured using my driveway as a scale.The car was barking the dot's inflated to 20 psi at 2 psi,however unscientific this is. Power is coming on lower and the turbo is quieter.(Noise is a result of friction).The unit itself is a Ta61 with Garrett .6 & .63 housings.The wheels are a 60-1 compressor and a 69 trim turbine wheel.Experience was previously gained from work on a Te63e. I'm very quiet about this stuff because the baselines for improvement have not been documented and my "gum flap" is worthless. Keep up the good work because there is lots out there.

Hmm.
I wonder what they referenced their angles to?
Was there a longer inlet pipe involved in their tests such as is used on the G/N's?
The Tornado idea was an adaption of an idea I told Doc about years ago.
If you bend the air going into the inlet of the compressor,you are going to create inlet frictional losses....always a tradeoff.
The more the angle of the vane relative to the axis of the compressor wheel,the more the flow loss....kind of obvious IMO.

Don't know about the inlet pipe - not mentioned in the article. Based on the results, it appears that the angles are referenced to the axis of the compressor. So, the 80-degree vanes have a pretty extreme angle to the air flow. This would explain why they were the most effecitve at moving the surge line to the left. I don't know what the angle of the blades on the Tornado are. If they are much shallower than what was tested here (lowest tested VIGV was 60-degrees), then it would make sense that they would have less effect on the compressor efficiency and maximum air flow.

Yes, it should be kind of obvious that the higher the blade angle, the more flow restriction, so the more impact the vanes will have on compressor efficiency and maximum flow. Maybe that's not intuitively obvious to the non-engineering types on this board, I don't know. I put this on here for general interest (at least, I found it interesting, maybe nobody else does) and something to consider for those looking to make these kinds of modifications. Everyone is free to do what they want with their own cars, obviously. Your mileage may vary, batteries not included, terms and conditions apply, etc.

TBH,if they had varied the vanes from say 30 degrees off the shaft axis to parallel at higher wheel speeds,they would have seen a more realistic picture of what those vanes can do for a turbo.
In the case of the Buick,keeping the air straight going into the wheel does help relieve surging at low wheel speeds [boost levels],when it's present.
In all honesty,a bench test is not indicative of what a turbo will do "in situ".
It only shows a relative trend.
Case in point:

A turbo without an inlet pipe will flow more air than one with a pipe.

The pipe is obviously a restriction to flow.
The mass of air in the pipe is also much less than what the turbo sees without a pipe.
That presents an interesting problem.
If the air in the pipe has less mass,it's more prone to swirling.
Ask anyone who's taken the rear screen out of a stock MAF in a stock inlet system.
The readings get all screwed up...correct?
That vortex in the pipe makes the air denser towards the outside wall of the pipe and less dense in the middle.
The problem with swirling air is that when it's travelling in a spiral,it isn't travelling in a straight line so well.
Common sense,no?
This thread is a good idea though.
Glad you brought it up.

I'm curious:

Where did the inlet of the pipe actually start?
How long was it?
Any inlet bell on the end?
The reason I ask is that most guys swear they see better spoolup with a larger inlet pipe than with a smaller one.
Now maybe the diameter doesn't matter as much once the air gets moving [within sensible limits]?

I suspect what may also improve response and power is the removal of the narrow band o2 from the passenger side exhaust manifold if you are converting from mass air to speed density.

I know of two cars that gained improved turbo response (63e) from moving the narrow band to the downpipe.I guess I have to agree about rwhp because of the boost pressure with respect to pressure ratio you stated.

I agree that a pressure drop is not likely, but air tumble is a likelyhood and air tumble will reduce flow crosssectional area into the volute which reduces the amount of air going into the slot (changes gas velocity to pressure to move the turbine wheel). I'm sure the improvement is small but I know both Bruce Plecan and the Radius Kid demonstrated this imperically.My fluid mechanics professor demonstrated laminar (no sensor) vs non laminar (sensor) flow in a flow tank at school.

TTipe - Just to clarify, *all* of the flow through the exhaust is highly turbulent. We're talking about a gaseous mixture moving at high speed, high pressure, and high temperature... very high Reynolds number and thus quite turbulent. Cylindrical pipe flow goes turbulent at Re ~ (o) 1000, and these flow numbers are likely (o) 100,000.

The sensor is likely introducing some slight restrictions in the exhaust flow, but I'm really not sure that it's a first-order effect. Bison's experience shows that it's not significant. It's also probably subject to lots of other factors, so the results would vary from car to car.

mgmshar - Thanks for an interesting read. Makes sense to me that a fixed "tornado" device wouldn't likely result in significant gains across the entire range of operating conditions, but may have benefits under certain applications (part throttle, etc). I appreciate the discussion.