How To: Compressor Mapping

[mkriebs]

I made this thread in an effort to better inform and educate the community on how to pick the right turbo for their cars and their performance needs. This is simply a tool, it does not reflect Performance Autowerks's opinion on anything. These are simply calculation to help you figure out how your engine will perform with certain turbos.


Well, here is the compressor map for our GT2860RS, with a couple of points plotted.




First, I am not going to get into detail on how these numbers are figured. If you would like to know yourself, I have good sources that I can get you, if you are interested.

Now, let me explain how to read this. On the left, there is the Pressure Ratio, and on the bottom is the Flow. All Garret maps that I have seen, read in lb/min, but some other manufacturer's maps read in cfm, or some other numbers I don't fully understand. The plots are found by numerous equations (which I conveniently made a spreadsheet for, so you can play with the figures yourself, if you would like). The lines in the middle are the range of efficiency. The lines that go more up and down are the % of efficiency, and the lines that go more horizontal are RPM's the turbo is spinning at. Just because a plot is outside those lines does not mean that its horrible, it just means that the heat the turbo is producing is not the most efficient for power production, and that you are approaching the redline of the turbo components, thus shortening turbo life. The dotted line on the left is the Surge Threshold, any plots that are left of that, need to be avoided, otherwise surge will result.

So, I plotted just a couple of points... These points would keep the car in the range of the sensors. The first plot is at 3500rpm and 22psi, where this turbo will be at full spool. Though it is out of the range of efficiency, its not too far out to be considered bad (since they can't put all of the figures on the map, they leave these out).

And, the second plot is 18psi at a 7000 rpm redline. This is perfect for the upper end, if you are commanding 18psi. It is right in the range of efficiency.

Now, what do the end numbers result in, you ask? The bottom numbers can be closely related to brake horsepower (crank hp). It has been found that there are approximately 10.5bhp per lb/min of MAF flow. So, one could estimate the (CRANK) power produced at certain plots this way. These power numbers are simply an ESTIMATION. They may seem low, do not let it discourage you from a particular turbo, as tuning plays a very major role in how a turbo spools, and how the car reacts to the air flow.

My idea of how this turbo spools, and how it flows 'til redline is purely speculation. These numbers are NOT set in stone, and they DO NOT reflect much really, to be honest. This is simply a tool to pick out what turbo is best for your engine. Obviously, if you are looking for about 325whp, then this GT28 would be ideal. If you are looking for more, then you would need a bigger turbo.

All of the Garret compressor maps are on their website. Unfortunately, Precision will not provide their compressor maps, so we kind of have to guess at it.

If you guys wonder what I do in my spare time... I spend it being a complete petrolhead nerd. I made a spreadsheet of SEVERAL different plots for the LNF. These can be used to make plots on whatever compressor map you have available. I chose to do one on the GT3071R, mainly because I thought the shape of the efficiency range was pretty well suited to the traits I have experienced with our engine in my dabblings with compressor mapping.

Attached is a .zip file of the spreadsheet of plots. Feel free to use it to your advantage.

Here is the compressor map with the plots on it:


You can see that the boost is able to build rather quickly. Now, one thing I am still learning is whether the engine can keep up with the compressor plotting. If the engine could build the boost that this plot shows, or if it would lag behind. All of my theories have pointed to yes, it would indeed be able to keep up, so far. So, in my theory, a GT3071R on the LNF would be able to build boost very quickly.

The first plot is 3250rpm, and the last is 6000rpm, and the plots are in 250rpm increments. Surprisingly, at 3250rpm, its showing we could push 16psi. Which, seems to be on par with other setups I have observed.

Now, a funny thing I found is that I cannot use the same plots for the GT2876R (slightly bigger than the GT28 from our kit, no worries there), that I used on the GT30. I actually have to build boost a bit slower at first, so I don't cross over the surge line. I have heard that the GT3071R actually spools faster than the GT2876R, and this proves it.

Again, I am still learning my way through this. So, this is not biblical truth, and there could be some errors in it somewhere. But, one of the ways i learn is by teaching, and talking. So, feel free to comment. Ask some questions too, as this will make me see where I need to research more. This is fascinating to me, and I hope I can make learning this easier for other people (because it hasn't been easy for me, lol).

Thanks for watching!
Matt

Here are a couple spreadsheets to help you get started! This first one does the math for you for plotting the points.
Turbo Compressor Math Spreadsheet.zip

This one contains the maximum air flow one could see on an unregulated turbo. The numbers on the left are the boost and PR (Pressure Ratio), on top obviously are the RPMs for the motor, the numbers in the chart are the Corrected Mass Airflow Numbers, which would help us predict the power we could make at a certain RPM. To get the chart to change, all you have to do is use the above spreadhseet, and enter in different engine RPM and the boost as 10psi, then input the CMf (Corrected Mass Airflow) into the 10 psi box on the top of the page (hope that makes sense...).
Cobalt Compressor Maps.zip

[Deathscythe]

Guys, don't be afraid to ask questions. That's the reason this is a sticky thread.

Ask away

[slvrred-]

what seems to confuse me is how you find RPM's and how many pounds of boost there is at those plotted points.......

[IdreamofYenko]

what seems to confuse me is how you find RPM's and how many pounds of boost there is at those plotted points.......

I was wondering that myself. I'm still not sure on what I'm seeing on the maps or all that yet. Need to read those articles Rob keeps throwing around, but that was my biggest question about this.

[PsychoCam]

so since I have the gt2860rs on my 2.2 now. what u are saying is it will be good up to 325 hp? How much boost can I push thro my turbo?

[BackyardTurbo]

You are leaving out some important stuff when factoring how it will perform. VE being one of them. Also, in reality there is a small pressure drop from the boost at the turbo and the boost in the manifold. If you are referencing your boost to the manifold you are prolly slightly over the Pressure Ratio you are calculating for. But we'll leave that out.

Some info:
You need to figure out the engine's volumetric flow, then determine a boost pressure you'd like to try and at what temperature:



Lots of numbers but here's what you see. The 122.05 is the engine displacement in Cubic Inches. To the right of it is the RPM amount, which varies as you can see in each notepad calc. Below the CI is 1728, not sure what that represents lol. Then you are dividing the RPM by 2 since this is a 4 stroke engine and the Intake Valves open once every 2 revolutions.

Now that you have your various engine flow numbers you can input some boost and temp numbers then plot your points. You are using Absolute Pressure, so 22psi is 36.7 psia. Multiply psia * Engine volume * 29 (not sure what the 29 is either) and divide by absolute temperature (again, not sure what the 10.73 represents there).

This gives us a raw engine flow in Lbs/Min. This does not factor in VE. I tried to research what an actual LNF VE is but I will have to take a log tomorrow to get a more accurate number. For now we will use a rough 85%. So you are taking the raw Airflow and multiplying it by .85 to get a close representation of the actual airflow going into the cylinders.

Now you can take all of these numbers and use it to make some lines and dots... LOL. I kid.

So taking the 22psi (36.7psia) you can find the pressure ratio: 22+14.7/14.7 = 2.4965 (or 2.5 for arguement's sake) Draw a line across 2.5.

Then take the max airflow for the engine (the 7k corrected measurement) 38.2415 and draw a line vertically. This is the area the turbo would be operating in. To get really crazy, use the multiple points and you can see exactly where you will be running thru the band!


edit: you can see on the 2860 map its able to spool up quickly and efficiently but will not be able to perform like that in the top end, you will have to lower boost to keep it as efficient as possible. On the 30, you can see trying to force it to spool up that hard and that fast is not desirable but the top end would be quite a bit stronger and more efficient compared to the 2860. This is where picking the right turbo comes into play! Revving higher with more boost will gain even more power with the 30! Play with the numbers and you can see exactly what it will do now!

Hope this helps

[slvrred-]

^^^^^ now would those lines that you added to the maps be the same (for the boost level and rpm) on every map we would be looking at?

[BackyardTurbo]

The vertical line I made will remain the same unless you start revving over 7K or you increase displacement. The pressure ratio line will move up and down with the amount of boost you are planning to run. You can also move the vertical line right by porting the head or just making the VE better in general, you just have to measure your VE before and after and apply the change!

[slvrred-]

Right so on the GT30 if you run a few more pounds of boost then you'd be more into the efficiency range as with the 28 at 7k your already beond it

[BackyardTurbo]

Here's if you took the 30 to 8K RPM, same boost still



You dont wanna go there with the 2860 lol!! 7k is already too much for it.

[slvrred-]

yea taking the 30r to 8k really put its to work. The 2860 is supposed to be a slight upgrade and its out of range at 7k.......thats kinda sad if you think about it. Can't image if we looked at the stock K04 map at 22psi and 7k......

[IdreamofYenko]

Ok so I'm actually starting to understand the flow charts. I'm not sure how you calculate where the rpms are on the chart tho

[BackyardTurbo]

You figure out the airflow at a certain rpm and then find that point along the bottom, go straight up to your Pressure Ratio and place a dot!

[IdreamofYenko]

You figure out the airflow at a certain rpm and then find that point along the bottom, go straight up to your Pressure Ratio and place a dot!

Ooooh lol I get it now. It all pulls from those equations.

[mkriebs]

Nick the spreadsheet I posted uses the LNF VE coefficient. I just put it as a standard. I should have stated that in the OG post. Also, the pressure drop from the turbo to the intake manifold is already in the compressor mapping spreadsheet. I must state that the LNF is an incredibly efficient motor, and the calculations you posted are correct, thank you for posting them, and answering questions as well! I am admittedly a bit rusty on this.

As for the RPM's and lbs of boost question. I input what lbs of boost i would want to see at that RPM. Obviously if I put in something ridiculous its going to throw off the chart, so use some common sense there. I also should have clarified a bit better. In the second spreadsheet, the red shaded boxes represent the max flow you can push through the motor at a certain RPM, which is thus represented on the second compressor map image by the red dots. This is calculated through the VE, boost, flow, etc. from the other spreadsheet. These spreadsheets are to be used in tandem with the compressor map images. My image is more aimed toward someone with an electronic, or stock boost controller on an LNF. This shows what RPM I should be commanding what boost to keep it within the efficiency of the turbo. It also shows me what psi I should aim for with certain approx power goals.

I think the way nick did his mapping is a bit less confusing, and more concrete in the end. Like I said, I am rusty, and still learning bits of this myself, so bare with me, I also wrote this article damn near 2 years ago!!! But, what his image shows is like the car is set at a certain boost (for the 3071 its static at 21 psi). So, at 21 psi at 3krpm the turbo would be surging, meaning it wouldn't make 21 psi at 3k rpm. This is where my compressor map would come in handy (or the second spreadsheet so you could map your own!). It shows exactly what I COULD run at 3k rpm, which would be right around 10psi.

I hope im not talking in circles and have answered a few questions, haha.

[BackyardTurbo]

Yea, what I did is very basic. Chances are you won't be holding 22psi flat from 3k to 7k. It was just to get the math and how to plot points across easily.

[mkriebs]

Yea, what I did is very basic. Chances are you won't be holding 22psi flat from 3k to 7k. It was just to get the math and how to plot points across easily.

Yes. Mine also can help predict how much power you would make at a certain boost level, at a certain rpm. Say, on the 3071, running 34 psi which we have found is max efficiency (for the most part), you would be spewing out 43 lb/min, which is about 430whp on an LNF (depending on dyno). I would say a safe range would be 400-440whp, though.

[YelloEye]

This is some good information, need to read over it again when I get home and play with those sheets.

[06_ion2]

i dont get it

[IdreamofYenko]

Back when you are calculating the pressure ratio for what is 14.7 a figure?