Selecting a turbo/reading compressor maps

[revolutionz_s13]

Ok. I have spent hours reading on this. I am looking at a Garrett GT25/40R turbo for my 95 KA24DE motor. I cannot figure out how to read compressor maps for the life of me using what little info I did find. I think I need the flow rate of the KA in order to make the maps more relevant. Although I am not new to the world of turbos (mostly experience with bolt-on kits), I am new to how to match a turbo up to a car based on the flow and efficiency which is seen on the compressor map. I am trying to put together a kit for my car, and want the best possible combination of parts/fastest spool time. My goal in the end is to run anywhere from 350-450 rwhp. Here is a link to the compressor map for that turbo.
http://www.atpturbo.com/root/maps/gt2540r.htm
Here is the link to the turbine map. ( what do I need to know about this map?)
http://www.atpturbo.com/root/maps/gt28rsturbine.htm

Also, from what I gathered, a lower A/R will produce a quicker spool-up, but a higher A/R will give more top end power, but more lag. Is this correct. If so, I would be looking at the .64 A/R for this turbo. It is now 3:30am, and I'm not sure if the post quite makes sense, but I am at a loss after about 4 hours of research on this. It is killing me. Any help or other information that I might need would be appreciated.

-Nate

[caall99]

i wish i could help but i haven't quite figured out the whole matching game either. once this thread gets some good answers i think it should be put in the FAQ because i am sure there are alot of people interested in understanding turbo maps.

[theicecreamdan]

I'm not sure on the maps, but you've got the exhaust size part right.

[revolutionz_s13]

OK. Orion on Nico showed me a site that explained compressor maps very easy. I have drawn up a spreadsheet that lists all of the corrected airflow numbers for a KA24DE. If you want the spreadsheet to plot the points on a map, PM me or email me @[email protected] If you want to learn how to read a compressor map, go to this site: http://www.turbosaturns.net

[420sx]

im actually surprised thats not covered on www.howstuffworks.com that was the first place i looked:) im always being confused on turbo specs beyond sizes and etc.

[revolutionz_s13]

I also found another great site last night, with all the info on how to plot compressor maps, plus just all kinds of info about turbo systems. Here is the link:
http://www.turbobygarrett.com/turbobygarrett/tech_center/tech_center.html .

This site helped me alot. I have now figured out how to plot points on a compressor map, and figure out when I start spooling, etc. There is alot of good info on there. If you have any questions, I will answer them best I can, but I am still learning also.
-Nate

[Uisce]

Choosing the right turbo requires that you know what you want from the car. Are you looking for peak power, or are you looking for response and general performance? As with anything else, there is no free lunch. If you shoot for 500hp on a 2.4L engine, you will sacrifice driveability and boost response. If you want an engine thats fun to drive and highly responsive, you will sacrifice peak power.

If you want response AND 500hp, buy a V8.

In my case I have a 2.4L inline 4 (KA24DE) and I want to put a turbo on it (I already have, actually, but whatever). But what is the best turbo for what I want?

I'm a road racer and a drifter. To be competitive, I need at least 300hp, but I also need good boost response for throttle (drift angle) modulation.

I also need to know what my useful rpm range is (say 3000 to 6500), and I need to know how much boost I plan to run. How much boost you intent to have is the primary determining factor in how the engine is built, so this is a big budget concern. With good fuel and timing tuning, and upgraded pistons and head gasket, I expect to be able to run 14-15psi without significant risk to the engine.

Now that we know some basic parameters, we can begin turbo selection. The easiest way to do this is to see what other people are using for the same application, and copy them. Thats fine, but don't go around claiming to be any sort of expert. If you actually want to understand whats going on, you'll need to be able to read compressor maps.

A compressor map is just a graph. It has a y axis and an x axis, and a series of concentric circles are plotted on it, along with several curved lines.

The Y axis (vertical) is PR, or Pressure Ratio. PR is determined by dividing compressor outlet pressure by compressor inlet pressure. Its a rather simple calculation, but there are a few factors that need to be accounted for.

First, lets assume that we are at sea level, so atmospheric pressure is a theoretical 14.7psi. If you live at a very high elevation, you'll need to know what the average atmospheric pressure in your area is. So turbo inlet pressure should be 14.7, right? Well, only if you run drag-style with a turbo sticking out of your hood and no filter. Since dirt is part of racing, I would advise against that. Lets assume that you are losing about 1psi of pressure between the air filter and the turbo, so your inlet pressure is really 13.7psia.

This is a good time to explain the difference between psia and psig. Psia stands for pounds per square inch atmospheric. Its the ambient standard pressure. Psig is gauge pressure, or the difference between the pressure measured and the atmospheric pressure. This may seem like a silly distinction, but its important to know because total pressure at the turbo outlet is psig + psia.

So lets say your compressor outlet pressure is 17psig. You can now calculate PR by adding inlet psia (13.7) to the outlet psig (17) and then dividing by inlet psia. (13.7 + 17) / 13.7 = 2.24. Your pressure ratio can be estimated, for out purposes, as 2.2.

But wait, why is my outlet pressure 17psig, when I said earlier that I want 14-15psi of boost pressure int he manifold? Because you are running an intercooler. Right? You had better be! The larger the intercooler, the more pressure you lose from intercooler inlet to intercooler discharge. For our purposes here, we'll say 2-3psi of pressure drop, which gives us 14-15psi at the manifold.

So now we want to take our compressor map and draw a horizontal line across it, starting at the "2.2" mark on the Y axis.

The X axis (horizontal) on a compressor map is air flow. As rough estimate, you can equate air flow in lbs/min to horsepower, so to push 300hp, plan on consuming about 30 lb/min of air. (some read in CFM, you can convert to lbs/min by multiplying by 0.076, which is the standard density of air in lbs/cubic foot at sea level).

The air flow a turbo pushes (which is what the engine consumes) can be calculated also. I'll spare the math and calculations here, but if you know your engine displacement, PR, and then estimate intake air temp and volumetric efficiency, you can use a handy excel spreadsheet to figure it out.

For my 2.4L engine with a few theoretical assumptions about VE and the like, I have the following air flow at 14psig:

RPM LBS/MIN
3000 16.22
3500 18.92
4000 21.62
4500 24.33
5000 27.03
5500 29.73
6000 32.44
6500 35.14
7000 37.84

Now you want to mark each of these positions along the X axis and draw a vertical line through each one across the compressor map, and label each by rpm. At each point where the vertical lines intersect the horizontal line you drew through the 2.2 PR mark, make a big dot. These dots represent airflow at a given RPM for your engine (holding PR constant).

So thats great and all, but what the hell good is all this?

Lets look at the funny rings and lines plotted on the compressor map. Each of the rings represents an efficiency zone, typically referred to as an "island." The islands are usually labeled with a percentage. That percentage indicates the compressor's adiabatic efficiency within that island. I'm not going to explain adiabatic systems here. If you want to know about it, get a thermodynamics book like I did. In any case, the island int he middle is the highest efficiency that wheel can run. Thats the sweet spot for that particular turbo.

On the far left of the funny shape is a line called the surge line. Compressor surge is an unstable condition in which the turbo is unable to meet the flow demands for a given pressure (it is too far outside of its efficency range to effecively compress the air). You will run into compressor surge when you try to operate the turbo in the area to the left of the surge line. Similarly, the far right line of a compressor map is known as the choke line. You want to keep your rpm/flow dots between the surge and choke lines (on the islands). If your dots fall outside of the surge line, the turbo is too big, if they fall outside of the choke line, the turbo is too small for the boost you want to run on your engine.

The horizontal curved lines across the islands are also labeled with numbers. These numbers represent wheel speed. Yes, thats right, your turbo can easily spin as fast as 150,000rpm. Think about that next time you decide to wait to change your oil.

So now is the fun part. You print your comrpessor maps, you grab your ruler and a pencil, and you draw your horizontal line at 2.2, and your vertical lines by RPM, and make the dots. The point of the game is to get as many dots as possible into the high efficiency islands. Its good to find a map where the last few dots are all within the center island, and none of the dots are more than a few islands out.

When you find a compressor map that fits these criteria, then you have found your compressor wheel, congratulations.

Lets move on to the rest of the turbo. More specifically, what the hell is an A/R, and why do people seem to throw that term around as if they understand it? Most people cannot actually define A/R, much less understand the benefits or drawbacks to various A/Rs.

A/R is the area of the nozzle (inlet for compressor) divided by the radius of the turbo centrline to the centriod of the scroll at that area. Basically it describes the relationship between the size of the inside of the scroll (the snail-shell part) with the radius from the center of the turbo to the middle of the scroll tube.

Ok, its hard to describe without a drawing, and I can't draw. Buy a book of you are that interested (I suggest Turbochargers by Hugh MacInnes, and if you still want to know more after you read that, buy Maximum Boost by Corky Bell, that should keep you busy for a while...).

If you are lost, lets just say that a higher A/R (Area/Radius) means a bigger housing and move on.

Compressor housing is usually fairly constant for a given compressor wheel. You can change comrpessor housing A/R, but all in all it doesn't make much of a difference.

The turbine (exhaust) housing is a different story. Your engine is pumping a given volume of air at a given rpm. The exhaust volume must pass thru the turbine housing. If you push a mass of air thru a small tube, you get high velocity, but low total flow. If you push it thru a large tube, you get a lot of flow and low velocity. The same is true for turbine housings. A small turbine housing will give great spool, but will limit the peak power capability (because it has limited flow at high rpm). A large turbine housing will give great top-end flow and power, but will now provide much by way of throttle/boost response. For a road race/drift car, you want something in the middle to give you decent top end power without losing too much response. For a T25/T28 based turbo, I would suggest an A/R in the .80 range, ideally.

Finally, lets look at wheel trim. Trim is another turbo term that is often tossed around carelessly, but few people really take the time to understand it. Trim is a very simple thing.

You take the square of the larger diameter of the wheel (called the major diameter) and you divide it by the square of the smaller diameter (called the minor diamter), them multiply by 100.

For a compressor wheel the minor diameter is the inducer side (air goes in) and the major diameter is the exducer side. Its exactly the other way around for turbines. Larger wheel trim means more flow, if nothing else changes.

So lets do a quick review:

1) Figure out what you want from the turbo
2) Figure out what boost you want to run
3) Figure out your airflow/rpm at that boost
4) Get some compressor maps
5) Draw your lines and plot your dots
6) Find a map where your dots are in the best efficiency islands
7) Pick a turbine housing

And away you go.

[stoanhart]

:bowdown: :bowdown: :bowdown:

[caall99]

i too bow down! :bowdown: :rawk:

[revolutionz_s13]

Holy hell, I think you just wrote your first book! :eek3: Good info tho! I think for my application, I am going to choose the Garrett GT3076R, which can be found at http://www.turbobygarrett.com. It will start to spool around 3500 rpm, and I will easily be able to hit the 400-450hp I want. I think that would probably work best for me, as I can't find the maps for anything that spools any faster and gives me about 400whp. If you got any ideas, let me know

[hellion240sx]

did some searching and came across this thread and once i get back i will check out these sites to see if i can get this (hopefully finally!)