How To: Read Compressor Maps

[EEEEeeee36]

I'm not sure how a VGT would help solve low rpm surge unless they come with an extremely wide map. It would be interesting to take a look at the new turbo porche's VGT compressor maps.

John

What do you mean you are not sure how it would control surge? If you know how a VGT works, there are vanes that close down at lower engine speeds; the increased pressure raises the shaft speed thus spinning the compressor faster. At higher engine speeds the vanes open up allowing for more air to pass through with less resistance. So to answer your question, yes the map would be MUCH wider because the turbine is controlling the shaft speed based on the rpm/exhaust inlet pressure. In a traditional turbo the shaft speed would increase in a linear fashion until the blow-off level was reached; a VGT increases shaft speed based on exhaust inlet pressure so it can increase boost at lower engine speeds. Like I said before, VGT turbos have traditionally been used on turbo deisel because of the need for the exhaust side to be higher pressure than the inlet side to use the EGR; however they are becoming more and more common on gasoline cars because of their wide range of effeciency and no need for a wastegate - they keep optimal boost at the widest range of rpms. One example is the Porsche 997 911 Turbo - it now uses a VGT so that it has a high boost at very low rpm and it winds out all the way to high rpm without needing bypass/blow-off regulation.

Food for thought:

"November 16 of this year marked the day a century ago that Dr. Alfred Buchi received the first patent for an exhaust gas turbocharger. Porsche will be celebrating the turbo's 100-year anniversary a little late when it introduces the next 911 Turbo sometime next year with Variable Turbine Geometry. This technology allows the angle of the compressor's turbine blades to continually adjust. While some diesel engines have enjoyed this technology since the Nineties, the higher exhaust gas temperatures created by gasoline engines necessitated the creation of new heat-resistant materials to handle the hotness. Porsche and Borg Warner Turbo Systems were able to overcome the heat issue and have developed a VTG turbo system that will be incorporated into the next 911 Turbo. The VTG turbo will allow Porsche's flat-six to mimic a twin-turbo setup with a much broader torque curve and more flexible powerband than a standard single turbo could provide on its own. Power ratings for the new VTG turbo engine haven't been released and probably won't be until the new 911 Turbo surfaces sometime next year."

Autoblog, 11/27/2005

[NTSOS]

What do you mean you are not sure how it would control surge?

Interesting.....please define compressor surge.

John

[EEEEeeee36]

Interesting.....please define compressor surge.

John

Already have; Post # 86 of this thread.

Typically with a VGT the turbine is paired with a smaller compressor because the turbine is controlling the boost, not a blow off valve. This discussion has gone off track anyways; as Jacob mentioned earlier surge is usually not an issue because most turbines are not paired with compressors that are too large, whether it is a VGT or a Standard Turbo. The reason you do not want to choose a turbo that falls in surge area of a compressor map is because that turbine is probably too big for your engine and it won't run within it's effeciency range; it will take too long to spool up. So really, surge was never really your original question; using a big turbo on a smaller engine was really the question. Again, this is where a VGT turbo could help you. Why do you think Porsche spent all this time with Borg Warner/3K to develop one to work in gasoline engines? You could implement a whole bunch of other controls to do the same thing, but there are VGT turbos out there that will do it for you, or you could just run a larger dual BB turbo and you'd have the same (or maybe better) performance. BB turbos take sooooo little force/air to spin them...

[Def]

VGT's don't really solve the problem of surge though. You have a set pressure ratio and massflow for say 3k RPM on an engine. If that happens to fall on the left of the surge line and your turbine/compressor/engine combination allows it to actually get there, then it will surge regardless of the type of turbine throttling you use.

It's a question of the compressor actually being able to compress the amount of air the engine can ingest at that RPM. Too small an engine and it is not moving enough air at that PR and the compressor ceases to be able to accelerate the air. You hear this loud "metallic turkey" sound, which is the compressor blades stalling and the flow collapsing and re-establishing itself very quickly. Same sound as when you have a stiff BOV spring, since your massflow is very low and the turbo is still spinning, so it falls into the surge zone.

Like I said many posts ago, surge is not really an issue in 99% of turbo vehicles because you typically have to pair a large compressor with a very tight and restrictive turbine. Essentially like putting a 600HP compressor with a 200HP turbine on a little 1.6L engine and even then I doubt you'd really have much of an issue with surge. It's just hard to spin the turbo up early enough to a surge area when you have a little engine in the first place. A large engine can spin it up fine, but then you are moving much more air so the turbo won't surge. Hence why it isn't much of a problem.

Plus once cars start surging on accel, they typically have such large turbos that they accelerate so quickly that they increase engine speed right through the surge area which they could reach if say brake boosted at a low RPM or something like that.

[Bm3R]

how much will stage 2 cost for a 98 m3?

[Def]

eleventy billion dollars! muahahaha

[NTSOS]

Variable geometry induction systems = increased mass flow @ lower engine speed as opposed to fixed geometry, large volume, short runner manifolds not know for stellar low speed efficiency = low speed surge on compressor/turbine combos previously considered immume from low speed/reverse flow conditions = 1%!. Never mind!

[kpolito99]

Not sure if everyone is familiar with this site, but some of these free calculators are pretty cool.

http://www.turbofast.com.au/javacalc.html

It would be interesting to compare how well the estimated solutions for turbo size and compressor efficiency compare to real world tests.

[aris214]

does anyone know a rough estimate of airflow for an M50 2.5L at around 8psi at the various engine speeds? i just want to map the airflow onto a compressor map and see how it would look, just like on the first page of this thread. i dont know how to do those calculations. thanks

[EEEEeeee36]

does anyone know a rough estimate of airflow for an M50 2.5L at around 8psi at the various engine speeds? i just want to map the airflow onto a compressor map and see how it would look, just like on the first page of this thread. i dont know how to do those calculations. thanks

You can use the turbo calculator that Jacob (Def) made earlier in this thread. What you will do is change your VE (volumetric effeciency) going up throughout the powerband; then just plot the numbers. Your engine's VE changes throughout the powerband based on many things...Especially with VANOS changing the intake cam timing as much as 12 degrees, who knows what the engine VE is at any given point.

One good way of approximating it however is to follow the engine's torque curve. I have a dyno of a stock 328 (will be fairly close to a 325) so you can see it. It will probably start out around 85-90% VE, then in the mid-range get to be around 95%, then it will fall off from there. A 325i would be very similar, but because of the intake runner velocity being different on a 325i the torque curve would just be shifted a little higher into the powerband. I would probably assume it is shifted about 500-700 rpm higher than the 328i graph (based on my own personal experience of installing the M50 manifold on my 328i and seeing the peak power/torque move about 700rpm higher).

See if that works. Here is the graph (it used to be a stock 328i vs supercharger graph; I just deleted the irrelevant lines):

[Def]

Keep in mind that the VE will change once you fit a certain compressor/turbine combination on it and have a far different intake/exhaust pressure differential vs. RPM. Like I've said many times on this thread - it gets really complicated really quickly, and there are so many variables involved that you're better off just taking a "best middle of the road guess" at the engine's VE across a wide range of RPM then seeing what turbo fits your application "pretty well" and going from there. There is just too many variables to guess at once you change so many things about the engine, so trying to "approximate" them is only going to likely lead to horribly skewed "results" which make you start looking in the wrong direction for a solution to a problem which likely doesn't exist.

[EEEEeeee36]

Keep in mind that the VE will change once you fit a certain compressor/turbine combination on it and have a far different intake/exhaust pressure differential vs. RPM. Like I've said many times on this thread - it gets really complicated really quickly, and there are so many variables involved that you're better off just taking a "best middle of the road guess" at the engine's VE across a wide range of RPM then seeing what turbo fits your application "pretty well" and going from there. There is just too many variables to guess at once you change so many things about the engine, so trying to "approximate" them is only going to likely lead to horribly skewed "results" which make you start looking in the wrong direction for a solution to a problem which likely doesn't exist.

Jacob man, thanks again for bringing in the "Reality Check" hammer.

[325icintn]

I don't know if this is appropriate for this thread. If not, mod delete...

I am choosing a compressor for my m50

Attached are the compressor map for a TO4E - 54 trim which I will use with a .63 A/R stage III turbine; 3" inlet 2" outlet; running 2" to IC and 2.5" to TB

I have also attached airflow charts at 10 and 7.5 psi.

Comments wanted ....

[///36M]

Plus once cars start surging on accel, they typically have such large turbos that they accelerate so quickly that they increase engine speed right through the surge area which they could reach if say brake boosted at a low RPM or something like that.

So you are sayin that if my line from the turbo calculator goes outside (to the left) of the surge line before it hits 3000 rpm, it doesnt really matter b/c at that point it is not boosted anyway (or has little to no boost) and there wont be any surge on the turbo? We only have to worry if our 3000 rpm point is outside the surge line?

This question is also in context to running a gt35r on a s50 3.0, around 12-14 psi. It seems to me if i run at this lower psi then alot of ppl are running (ex: 18-19 psi), I am seriously close to the surge line. Is this a positive or a negative?

A problem I have run into concerning already boosted cars around here is most are either running a t03/04 60-1 hybrid, aka a tt stage 2 and maxing out around 420 whp, or gt35r's pushing mid 500+ whp. (By MOST PPL, i am refering to most ppl in the 400-600 hp range) I am looking for something in the middle, 450-500 whp, lets say for arguements sake.

So my logic seems to tell me that running something like a gt35r at a low effeciency is, (lol) inefficient, and will cost me spool up time and money. As an economist i hate being inefficient, and would like to create my power goal at the top end of the turbo's efficiency... Am I on the right track here? My logic goes on to tell me that this will create faster spool up and as long as I get the right turbo, should keep my high rpm HP up where I need it.

I guess this went OT, if you want me to edit I will be more than happy. If generalizations are wrong i apologize, this is what I have learned so far. I have a long way to go.

[Def]

I'm saying that you typically don't hit those points on the street. I doubt a 3L S50 is going to hit 14 psi before 3k RPM. It will be accelerating quick enough that if it's borderline on the surge line when doing the compressor map, then it'll usually get the RPMs up fast enough to avoid surging in real usage.

A GT35R at ~500rwhp is a good level. You don't want to "max" out a turbo when first choosing one, as then you're left with no upgrade room and you typically run into more heat problems the harder you work your compressor towards the "end" of its useful range.

[e36'n]

I have a question regarding using a turbo with a small turbine size with a larger compressor which shows to be efficient on the 2.5l. The turbo is question is a Holset HX30 which has a decently large compressor but the turbine is only 6cm2 (although it looks fairly decently sized).

Will this turbo be to small for my application? I'm attaching pics of it and the compressor map.

hx30 compressor map.jpg

hx30.JPG

[Def]

Well how much power are you looking for? That's the big determiner on if a turbo is too small.

[Def]

PS - I'd berate the engineers at Holset for using those units. Total pressure is *COMPLETELY* different than absolute pressure(which is what they are actually referring to on the PR axis).

[Def]

Looking at the corrected mass flow, without correcting the massflow in my spreadsheet I'd say you could run somewhere in the 20-22 psi range as a safe maximum and would probably be somewhere around 450ish rwhp(maybe higher, depends on how the engine breathes).

No idea on the turbine since the area measurement doesn't give too much info. Based on what I've seen on other Holsets, it'd get to a choked condition before flowing ~50 lbs/min, so you'd likely need a bigger housing to max the compressor out.

[e36'n]

PS - I'd berate the engineers at Holset for using those units. Total pressure is *COMPLETELY* different than absolute pressure(which is what they are actually referring to on the PR axis).

Can you explain that a bit more? So Holset uses total pressure and Garrett uses absolute? What's the difference and how do you convert it?

Looking at the corrected mass flow, without correcting the massflow in my spreadsheet I'd say you could run somewhere in the 20-22 psi range as a safe maximum and would probably be somewhere around 450ish rwhp(maybe higher, depends on how the engine breathes).

No idea on the turbine since the area measurement doesn't give too much info. Based on what I've seen on other Holsets, it'd get to a choked condition before flowing ~50 lbs/min, so you'd likely need a bigger housing to max the compressor out.

Thanks for looking at it. The holset website claims that is can provide .35 kg/s or 46.3 lb/min of airflow. So really this turbo should be fine up to at least 15-18psi.

[e36'n]

Jacob I've got another question for you.

With regards to how bigger turbo's flow more air and therefore provide more power at the same psi, how can one go about figuring this out.

Example:

The Holset HX35 is capable of flowing 0.46 kg/s or 60.9 lb/min at a 3.0:1 pressure ratio, whereas the HX30 can only flow 35 kg/s or 46.3 lb/min.

I understand the principle that a bigger turbo can force more air per psi, but how do you actually calculate it to figure out that turbo "A" will provide 20% more power than turbo "B" at the same psi level. Thanks again.

[Def]

Jacob I've got another question for you.

With regards to how bigger turbo's flow more air and therefore provide more power at the same psi, how can one go about figuring this out.

Example:

The Holset HX35 is capable of flowing 0.46 kg/s or 60.9 lb/min at a 3.0:1 pressure ratio, whereas the HX30 can only flow 35 kg/s or 46.3 lb/min.

I understand the principle that a bigger turbo can force more air per psi, but how do you actually calculate it to figure out that turbo "A" will provide 20% more power than turbo "B" at the same psi level. Thanks again.

I don't know why people think this - it's absolute NEVER true when talking about the compressor. The compressor provides the massflow that the engine needs at the outlet pressure which is regulated by the wastegate. The efficiency of the compressor, amount of compression, and inlet temperature will tell you how hot the exit temp is. Note I never mentioned how "big" the turbo was in that example.

The reason why really "big" turbos make more power at a lower manifold pressure is that they are typically saddled with large turbine wheels and housings. This gives a much later spool time, but they flow more at the same pressure ratio across the turbine. So you end up with a situation where the compressor might even not be as efficient as a smaller turbo, but that small increase in heat is more than offset by the fact that you might have as much as 5-10 psi less pressure in the exhaust manifold. That ultimately means you're going to have less exhaust residual after the exhaust stroke, and make more power.

Lots of internet guys seem fascinated with sticking honking big turbos on their cars and running low boost to be "safe" - and they talk about how cool it is that their peak HP is more than if they had used a smaller turbo. It has nothing to do with the compressor, which in many cases is being even LESS efficient than a smaller and properly sized one for the power required, but it has everything to do with how it now takes them an extra 1000-2000 RPM to spool their big turbo up when breathing through a big turbine housing and spinning up a lazy big turbine wheel.


The fact that the larger turbos "flow more air" on the compressor map is largely a function of the inducer of the compressor wheel reaching a choked condition(speed of air has gone sonic), where you just can't flow more air no matter how fast you spin the turbo. This is why the speed lines drop off sharply when reaching the max a turbo can flow(i.e. it's taking an exponentially larger RPM increase of the compressor wheel to eek out that last little bit of flow and get the inlet to a truly choked condition).

[Def]

Can you explain that a bit more? So Holset uses total pressure and Garrett uses absolute? What's the difference and how do you convert it?

Total pressure is the pressure when you stop/stagnate a streamline in a flow. Absolute pressure is the static pressure(i.e. pressure of just the air molecules perpendicular to a streamline) referenced to a total vacuum.

As an example of each, as sea level there is approximately 14.7 psi absolute when you are just sitting there. Go up in altitude around a mile up or so and it might be as low as 12 psia(can't remember the exact number).

Now we're back at sea level, and you stick your hand out of a car moving 60 mph. Notice how the air pushes your hand back? For simplicity sake's, let's still assume you have a static pressure of 14.7 psia on the back of your hand(it's slightly less, but we'll ignore this), but your hand is being pressed back? Why is this? Because you've stopped/slowed a streamline of the flow, which gives you an idea of the total pressure of the flow. This total pressure might be something like 15.7 psi. If your hand is 40 square inches of area, the delta P of 1 psi gives you 40 lbs of force pressing your hand by back stopping/slowing this flow(relative to the car).

http://en.wikipedia.org/wiki/Total_pressure

http://en.wikipedia.org/wiki/Static_pressure


Explain it a little further.

Thanks for looking at it. The holset website claims that is can provide .35 kg/s or 46.3 lb/min of airflow. So really this turbo should be fine up to at least 15-18psi.

Yea, it should be fine for that IMO.

[e36'n]

I don't know why people think this - it's absolute NEVER true when talking about the compressor. The compressor provides the massflow that the engine needs at the outlet pressure which is regulated by the wastegate. The efficiency of the compressor, amount of compression, and inlet temperature will tell you how hot the exit temp is. Note I never mentioned how "big" the turbo was in that example.

The reason why really "big" turbos make more power at a lower manifold pressure is that they are typically saddled with large turbine wheels and housings. This gives a much later spool time, but they flow more at the same pressure ratio across the turbine. So you end up with a situation where the compressor might even not be as efficient as a smaller turbo, but that small increase in heat is more than offset by the fact that you might have as much as 5-10 psi less pressure in the exhaust manifold. That ultimately means you're going to have less exhaust residual after the exhaust stroke, and make more power.

Lots of internet guys seem fascinated with sticking honking big turbos on their cars and running low boost to be "safe" - and they talk about how cool it is that their peak HP is more than if they had used a smaller turbo. It has nothing to do with the compressor, which in many cases is being even LESS efficient than a smaller and properly sized one for the power required, but it has everything to do with how it now takes them an extra 1000-2000 RPM to spool their big turbo up when breathing through a big turbine housing and spinning up a lazy big turbine wheel.


The fact that the larger turbos "flow more air" on the compressor map is largely a function of the inducer of the compressor wheel reaching a choked condition(speed of air has gone sonic), where you just can't flow more air no matter how fast you spin the turbo. This is why the speed lines drop off sharply when reaching the max a turbo can flow(i.e. it's taking an exponentially larger RPM increase of the compressor wheel to eek out that last little bit of flow and get the inlet to a truly choked condition).

Hmmm...so why is it then that a GT35r will spool almost as fast as say a GT30r but make more power at the same boost level?

[Def]

They don't spool almost as fast... It might only be a few hundred RPM difference, but it will have a lower turbine inlet pressure(35R).

Plus if you're talking about on an S50/S52, I've already shown that a GT30R is a horrible choice for those engines at the beginning of this very thread. It has an inefficient compressor towards redline on both engines.