Tutorial:how to read a turbo compressor map
 

http://www.clubcalibra.com/engine-tu...gcifs9a7bsn0b2 there my 1st post in helping people on here.
i hope this helps you as much as it has helped me understand it!!!

pretty good tutorial, I had no idea what I was looking at when I was looking at compressor maps

Not included are the engine operating lines at various RPMs. It's always a a diagonal line going from the lower left where the 0 pressure ratio / 0 flow point would be, going up and to the right. As RPM increases the line leans more to the right. At a given RPM, the points on the line represent the engine flow at different amounts of boost. Anyone wanna draw up an exampe for a miata at 4000 and at 6000 RPM? The intersection on the 1.0 pr horizontal line would be the NA hp output of the engine.

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Yeah, i have never been able to figure out how to use compressor maps. Know about max power and PSI, but when putting in calculations to the maps online, i always get plots that seem way off. The ones where you put in injector/brake fuel consumption/ect.

His avatar: https://www.miataturbo.net/showpost....postcount=2016

Compressor maps I have no problem with. How the hell do you interpret a turbine map?

http://www.turbobygarrett.com/turbob...0-15turb_e.jpg

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YES!!! Voodoo magic I tell you... And OP I have an Excel document to do compressor maps. :loser:

You get 9.5-10.5 hp per lb/min, so it's only showing hp per PSI.

Of course, it also shows where efficency peaks, as it's a limit function, so you want to get a psi before the line levels out.


Please post it up!

So what you're saying is that it's the way to determine the choking point on the turbine side of things?

Yes. So on that turbo, i wouldn't want to go above 29psi as i'd expect nothing but hot air above that.

The lb/min corrected airflow doesn't make sense to me, i'll need to look more into that.

The above, and typically published Garrett turbine "map" is merely the asymptote of the flow curves, at peak efficiency, of each of the constant speed lines.

A real, complete map is attached. The green lines are the flow lines for different, constant shaft RPM values. The red lines are the efficiency lines, at different values of shaft RPM. Constant shaft power lines would be hyperboles or 1/x lines. The published "maps" are the asymptote of the green lines.

You will see that the turbine gets more efficient at higher RPMs. This is why a GT2860 would have a lower boost threshold than a GT2871, despite the fact that the compressor efficiencies are similar, at say, 5 psi and 3500 RPM, and they have the same turbine - because the GT2860 compressor is spinning faster at that condition. The turbine will develop more shaft power at that flow condition when matched to the GT2860 - voila, it will accelerate the compressor faster.

Given a real turbine map and a compressor map, one would use some software to iteratively solve for the spoolup / boost vs RPM curve of a motor / turbine / compressor combo.

I hear the reason they don't publish the complete maps is it makes for much easier reverse engineering. If true, I think this reason is a wee bit lame.

The problems I have are:

1: That map shows what we're assuming to be the choke line at about 18 lbs/min, regardless of pressure ratio. The compressor map for the turbo I took that map from shows excellent flow all the way out to 35-40 lbs/min.

2: Pressure ratio where? Compressor pressure ratio? Ratio across both sides of the turbine?

3: What does "corrected" mean? What is uncorrected gas turbine flow? How do you correct it?

Found the same picture Jason found, and here's a good thread with an explanation. http://www.turbominis.co.uk/forums/i...=vt&tid=182558

Was trying to sum it up, but i'll just link the source.

EDIT: Will post some answers here to make it so everyone doesn't need to read:

Q: Pressure ratio where? Compressor pressure ratio? Ratio across both sides of the turbine?
A: "the pressure ratio is across the turbine"

Q: What does "corrected" mean? What is uncorrected gas turbine flow? How do you correct it?
A: "the gas flow is corrected to temperature and exhaust manifold pressure"

This is a more detailed explanation of the efficency i was talking about:


"As we increase the mass flow to the turbine, at first it can "absorb" the increase almost completely. It does this by spinning faster. But as you continue to increase the mass flow rate, you eventually hit a point where the turbine starts to pose a restriction to the flow. Increasing the mass flow past this point you will now see a definite restriction from the turbine, and the pressure drop across the turbine will increase. Looking at the level part of each curve, you can see that you get to a point where you cannot force the turbine to flow any more, no matter how much you increase the pressure ratio across the turbine. For example, the curve climbs pretty far from a 1.5PR to a 2 PR, but after 2 it's almost level up to a PR of 3.

It just so happens that this restriction is a good thing, because unless the turbine is restricting flow, it cannot extract any power from the gas to power the compressor.

Said another way, once the curve starts to level out, the turbo is fully spooled-- it has reached it's maximum efficiency, or ability to get work from the available gas energy. Also note that you'll sometimes see the right hand side labelled as a secondary Y axis showing "efficiency". It just so happens that efficiency increases with a larger housing."

So at that point where the turbine side is maxed out(the curve leveling off), it's also the same point where the compressor side starts to build positive pressure?

No, the curve leveling off is when you stop getting more power from increasing to higher boost. The compressor should be making positive pressure long before that.

EDIT: Curve leveling off on the turbine side is when you stop making more power from higher boost on the turbine side. You still need to take in account the compressor side, which is why some vehicles can run higher PSI than others on a given turbo and still make more power then they would at a lower PSI.

That above source also showed up on a Diesel Truck forum... http://www.dieseltruckresource.com/d...-t168543.html?

If you read the thread i linked to, that's a link in the thread.

Found it on the linked thread... Just found it by Googling and figured I would post the 'straight from the horse's mouth' thread.

The reason that i didn't post that up is that it's discussed in the thread i linked that some of the information on there might be wrong. Would recommend reading the first linked thread before getting to that one.

Ok, now I've got a half-dozen pages that I need to read through and absorb. On the plus side, it appears I'm not the only person who has asked these exact same questions...

good info, i like geeking out on the numbers/theory side of things and I have always wondered about those damn turbine maps, but never looked into it.

So much misinformation...

Garrett's published turbine maps are of little use for matching, as JasonC mentioned. His attachment is a "real" map.

I was going to post a painfully in-depth explanation of how to use a proper turbine map for matching purposes. However, without actually doing the exercise of working through the numbers it would only totally lose everyone. And without proper maps, you can't do a match anyway.

Suffice it to say that matching a turbine to a compressor requires a flow, speed and power balance. You first determine the compressor operating conditions, and then use a turbine map to determine whether it is suited to drive the compressor under those conditions. It is a highly iterative process. You have to make assumptions. You have to know the limitations of the map (steady flow conditions, measurement uncertainty, etc)

A higher flowing turbine requires lower expansion ratio, which is why an engine thusly equipped makes more power at a given boost level (less backpressure; higher VE).

Blah blah blah

Nagase and Joe:

(Nagase the passage you quoted about maxing out the turbine is incorrect)

The turbine side CAN flow more than the turbine "map" shows because that is only the flow through the turbine, the rest of the exhaust gases can flow through the wastegate. Even if the turbine is in the "maxed out" or level portion of the line, it's not really maxed out - if you move to the right along the line, even though flow is flat, pressure can rise, and the turbine *will* develop more shaft power for the compressor, and more exhaust flows through the wastegate.

The actual shaft power that the turbine produces is proportional to the efficiency times the exhaust mass flow rate times the pressure drop. To figure out the exact power developed, IIRC, requires one to look up the enthalpy of the exhaust gas (I'm an EE and my thermodynamics is a fuzzzy memory now).

The shaft power required by the compressor is likewise proportional to the air mass flow rate times the pressure gain, divided by the compressor efficiency.

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With a given turbine/compressor/engine combo, there is a point where, as boost is turned up, the engine starts to show a torque curve that drops off more sharply at the peak power area (i.e.the % gains are smaller per add'l psi of boost near peak power vs lower in the RPM range). If the compressor isn't approaching the choke region, it is the *engine* that actually "chokes" at the top-end as the turbine requires much more backpressure to generate boost, because it reduces engine volumetric efficiency. You will see this as sharply increasing turbine backpressure at the topend (I saw this with my GT2554 at >10 psi). At this point I think anything that improves topend breathing will "uncork" some hp, and it will be good for another few psi. This can be done with a more hi-rpm tuned intake mani, a larger a/r, or possibly retarded cam timing or duration.

(see discussion between me and Savington about intake manifolds and turbines. https://www.miataturbo.net/showpost....&postcount=127
The question is, which mod doesn't give up so much low-end)


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Joe, the "corrected flow" is just the flow referred to standard atmo conditions. It's a convenient way to quote flow numbers, or to plot flow vs pressure, because otherwise the plot will change according to temperature (and you'd have to plot a family of curves).

^yup.

The escalating turbine inlet pressure you see with small turbos running a lot of boost makes VVT veeery nice to have. You minimize the reversion of the high pressure exhaust gas into the combustion chambers by dialing out some valve overlap (retarding the intake cam). You still have the pumping loss associated with pushing that high pressure exhaust gas out of the chamber, but at least some of the VE implications of the high exhaust pressure can be ameliorated.

My VVT tuning at 10 psi on my GT2560 wants it to go to full retard at about 6000 RPM IIRC. Are you suggesting that if I turn up the boost, it's gonna want full retard at lower and lower RPM?

Also, I'm playing with the idea of purposely retarding my intake cam 1 tooth, for further max retard. The VVT range is 47* (crank degrees), and in the midrange my motor doesn't like anything more than about 22* advance from max retard, so retarding it another tooth will not prevent me from reaching optimum advance. I'm thinking the engine may like more retard than the standard maximum at higher boost... and perhaps during cruise, extra retard may reduce pumping losses... are there any major downsides to miniscule overlap or even underlap at cruise, idle, and maybe high boost/high RPM?

Never go full retard.

Seriously, you'll just have to play with the vvt on a dyno. At full load/revs there will be a point of diminishing/negative returns with more and more intake retard, since you're moving the IVC point to later in the cycle. How much retard is best will depend on intake & exh pressure, and how aggressive your cams are. Idle usually works well with minimal overlap. OEMs use a good bit of overlap at cruise for EGR.

I'd move the intake cam one tooth and then experiment.