How to read compressor maps

i figured a thread like this may be helpful to some when choosing a bigger turbo

 

here is an example of a compressor map

http://www.rbracing-rsr.com/turbo/TurboMaps/t3-60.gif

 

the idea behind a compressor map is to give you an estimate of how much pressure you would have to run at a certain amount of airflow to meet a horsepower goal (horsepower being measured at the crank)

 

some basics to start about the maps

Pressure Ratio- given as (Atmospheric pressure+ Manifold Pressure)/(Atmospheric Pressure)

 

basically if you want to run 20 psi at sea level, you pressure ratio will be

 

(14.7+20)/14.7= 2.36:1

 

keep in mind that atmospheric pressure varies by altitude

 

Air Flow- given in either LB/min or cfm.

 

10hp=1 lb/min

150cfm= 100hp

 

Surge Line/Limit- The line farthest to the left, denotes when compressor surge or stalling will occur. basically no air will flow, you want to be to the right of it

 

Choke Line- The line farthest to the right, after this point there is no further flow rate increase possible. Basically it is the limit of efficiency of the turbo/supercharger

 

Efficiency Islands- They are denoted on the map itself in percentages and is read like a topographic map of a mountain. For example, on the map above, within the center island the compressor is working at 74% efficiency. In the island around that, it is working at 70% efficiency, and around that, 64% efficiency.

 

The greater the percentage, the more effiecient the turbo is working. Ideally, you want to be in the highest percentage island, but often compressors are pushed beyond that to make more power.

A 70% efficient compressor means that 70% of the power put INTO the compressor is used to build air pressure. The remaining 30% is used to heat up the air. No compressor is 100% efficient

 

 

Now, i think the best way to explain how to use these graphs is by example.

 

How about i want to make about 250bhp on my legacy with this turbo.

 

250hp is the goal, so id need to flow 25 lb/min basically.

 

remember, HPgoal/10= mass flow (lbs/min)

 

now you also need to know how much boost pressure you are going to have to run at this much power to meet your goal (this varies car by car of course)

 

lets say we want to run 15psi to achieve this. that equates to a 2.02 pressure ratio

 

It is now possible to find the spot on the map that the compressor will operate at the instant your engine is at the given (power & boost) operating parameters.

 

well if you plot the point with x=25lbs/min and y=2.02 then we land in a decent effieciency spot. Basically this turbo would be good for what we are trying to do

 

 

The idea is to stay to the right side of the surge line and stay within the highest (average) efficiency islands in the operating range of the engine...

 

so thats how you decide what turbo is right for your application with a compressor map:icon_mrgr

 

some other maps for reference so you can see why bigger turbo=bigger power with better efficiency

 

http://www.rbracing-rsr.com/turbo/TurboMaps/GT35compress.jpg

 

http://www.rbracing-rsr.com/turbo/TurboMaps/GT60compress.jpg

 

http://www.rbracing-rsr.com/turbo/TurboMaps/GT12compress.jpg

 

Warning, the next section is very nerdy and probably not that useful to most

 

As i said before, a 70% efficient compressor release 30% as heat. How can you figure out the value of this heat? Well I'll tell you

 

Temperature out = Temperature in + {[Temperature in*(-1+ pressure ratio^0.263)]/efficiency}

 

Temperature in is the ambient air temperature in Rankine® (R= farenheit+460)

Pressure Ratio is the same as i said above:

(Atmospheric pressure+ Manifold Pressure)/(Atmospheric Pressure)

 

Efficiency is what island of efficiency you are running the compressor at

 

plug all of this into the formula and voila! you get your temperature out in degree rankine. subtract 460 to get the temperature in farenheit

 

hope this helps someone out there, if anyone has anything to add, feel free and i can add/edit as needed. im no expert after all.

nice job. bosco

There's also some good info on some DSM sites.

 

http://www.stealth316.com/2-3s-compflowmaps.htm

 

It's useful to overlay the engine demand on to the compressor map to see where the boost and rpm should be.

 

Unfortunately, IHI doesn't seem to like to publish compressor efficiency maps. Only the surge line and choke lines are available. I'm going to assume that the choke line for the IHI turbos are also at 60% efficiency.

I think you should measure pre-compressor absolute pressure before just assuming its 14.7 psi.

 

wouldn't want to pick the wrong turbo because you guessed on your inlet pressure, eh?

^ right-o, i will make that more clear

Excellent information!

 

I would like to add that you also have a pressure drop thru the IC which effectively raises the PR seen at the turbo. Generally the better IC's will yield around 1-2psi, whereas a stocker pushed to its limits will be more like 4-6psi.

HERE is some more good info and the map for a 20g

added explanation on the efficiency islands

added some geeky info on how to estimate the heat put out by the turbo at a certain efficieny range and pressure

HERE is some more good info and the map for a 20g

 

mmm...at 17psi you can run 450bhp and be in the best efficiency island. drivetrain loss at 25%, thats 340whp!!

mmm...at 17psi you can run 450bhp and be in the best efficiency island. drivetrain loss at 25%, thats 340whp!!

That dosnet add up to what I have seen on a dyno graphs but I will take it

 

I typically see 320whp with tq a little higher. Eather way its fun

Just because the turbo can flow the required airflow at set psi, doesn't mean the engine can. Hence why you generally need more boost than what a comp map shows.

Now if we wanted to increase the output of the stock motor from 250chp to maybe 375chp for easy math (and close to 300 whp) we can determine the necessary boost required.

 

target chp/current chp = 375/250 = 1.5

 

If we multiply this ratio by the stock boost (13.5 psi) then we determine we will need approximately 21 psi of boost in order to reach our goal.

 

We can now use the new pressure ratio of (14.7+21)/(14.7) = 2.43 to determine how much flow we must spec. the turbo at.

 

CFM = ((Displacement(cid) * rpm * .5 * Ve (assume 90% or .9))/1,728) * Pressure ratio = ((150*6500*.5*.90)/1,728) * 2.43 = 617 CFM

 

So... in order to produce peak boost at redline we need a turbocharger capable of flowing 617 CFM (or ~50 lb/min) at a pressure ratio of 2.43.

 

Obviously a very simplistic example as it makes no note of losses and is based on only a single parameter (target hp at redline) but hopefully it helps someone.

That dosnet add up to what I have seen on a dyno graphs but I will take it

 

I typically see 320whp with tq a little higher. Eather way its fun

 

haha, i was close

 

like i said edmundu, its an estimate:lol:

 

the whole compressor map thing is based on 100% volumetric efficiency, which is not really possible

The compressor map is just a description of how well the compressor works. It doesn't represent anything that happens at the engine.

 

It's only when you overlay the engine demand curves onto this curve that the losses at the IC need to be accounted for.

 

And the engine demand curve really needs to be overlaid to see if boost crosses the surge line or how close to the choke limit the engine can approach. It can also tell you if the turbo is too large.

 

You can put a HUGE turbo on a little engine and it won't do you any good.

 

Take a look at this.

http://members.tripod.com/~cherrypicker/sitebuildercontent/sitebuilderpictures/td06h-20g-flow.gif

 

The above is the equivalent of a 1.5 liter (it's actually 1/2 a 3.0 liter) engine load overlaid on a TD06-20G turbo. Even though the turbo is capable of 640 cfm at PR=2, this particular engine will need to spin very fast to get there.

^ good info

 

like i said, the maps are only useful when you have a hp goal and pressure goal in mind for sizing a turbo

I would say they are also good for tunning so you know what boost to target or when you are asking her for too much.

Now if we wanted to increase the output of the stock motor from 250chp to maybe 375chp for easy math (and close to 300 whp) we can determine the necessary boost required.

 

target chp/current chp = 375/250 = 1.5

 

If we multiply this ratio by the stock boost (13.5 psi) then we determine we will need approximately 21 psi of boost in order to reach our goal.

 

We can now use the new pressure ratio of (14.7+21)/(14.7) = 2.43 to determine how much flow we must spec. the turbo at.

 

CFM = ((Displacement(cid) * rpm * .5 * Ve (assume 90% or .9))/1,728) * Pressure ratio = ((150*6500*.5*.90)/1,728) * 2.43 = 617 CFM

 

So... in order to produce peak boost at redline we need a turbocharger capable of flowing 617 CFM (or ~50 lb/min) at a pressure ratio of 2.43.

 

Obviously a very simplistic example as it makes no note of losses and is based on only a single parameter (target hp at redline) but hopefully it helps someone.

 

this is way too pessimistic...

the FP green is a 49lb/hr compressor, and greens make a lot more then 300whp.

It's pessimistic because its for target boost at redline. In practice, we would expect full boost by what? 4000 rpm? So if we substitute 4000 rpm in place of the 6500, you can see we would only need 30.7 lb/min.

 

Edit: you say lb/hr... I believe it's lb/min.

Now if we wanted to increase the output of the stock motor from 250chp to maybe 375chp for easy math (and close to 300 whp) we can determine the necessary boost required.

 

target chp/current chp = 375/250 = 1.5

 

If we multiply this ratio by the stock boost (13.5 psi) then we determine we will need approximately 21 psi of boost in order to reach our goal.

 

We can now use the new pressure ratio of (14.7+21)/(14.7) = 2.43 to determine how much flow we must spec. the turbo at.

 

CFM = ((Displacement(cid) * rpm * .5 * Ve (assume 90% or .9))/1,728) * Pressure ratio = ((150*6500*.5*.90)/1,728) * 2.43 = 617 CFM

 

So... in order to produce peak boost at redline we need a turbocharger capable of flowing 617 CFM (or ~50 lb/min) at a pressure ratio of 2.43.

 

Obviously a very simplistic example as it makes no note of losses and is based on only a single parameter (target hp at redline) but hopefully it helps someone.

 

Lots of good info here guys. I really like the formula by Underdog above. Utilize this method for multiple pressure ratios and rpm points and plot on the compressor map to get something that looks similar to the graph posted by Mikeyd. You want to size the turbo so that compressor efficiency is highest in the range you will mostly be driving in.

 

Also for Pin (inlet pressure) I typically use -.5psig (14.2psia) instead of atmospheric.

CFM to Lbs/Min:

 

CFM*.069=Lbs/Min

 

617CFM = 42.5 lbs/min.

 

This is more along the lines of an 18G turbo and 375CHP sounds reasonable.

coool

CFM to Lbs/Min:

 

CFM*.069=Lbs/Min

 

617CFM = 42.5 lbs/min.

 

This is more along the lines of an 18G turbo and 375CHP sounds reasonable.

 

 

Thanks for the support.

 

I was using the conversion of .08 lb/ft^3 for air around 30 degF. In the summer, the specific weight of air would be more likely around the low .07's. .069 would fall somewhere around 110 degF.

 

Also, it wouldn't be unreasonable to assume 1-2 psi lost due to the intake if you want to be on the conservative side.

 

I've been meaning to do a thorough write-up but haven't been able to test things like P and V at turbo inlet/outlet, IC inlet/outlet and throttle-body.

This thread kicks ass. Thanks guys.

Here is an example I threw together. The attached thumbnail is of an excel spreadsheet I made that allows me to specify what my boost curve will look like. For my purposes (a daily-driver) this may be excessive but I think it offers a good combination of economy and performance. I desire a higher boost output in favor of a low boost-threshold or what many people consider "lag."

 

That is something that should be cleared up right away.

 

Lag is the time it take for the turbo to spool up given that there is the necessary airflow to power the turbocharger. A larger turbo has more lag because the compressor wheel is larger (more massive) and generally the A/R ratio is chosen for high-end power. Lag also changes based on RPM, with a large turbo, lag is much greater at say, 2000 rpm, versus 5000 rpm. Things like ball-bearings and tuning your A/R for the desired response will reduce lag but it can never be eliminated!

 

Boost threshold is the minimum rpm that the turbo is capable of creating boost. Basically, a turbo requires exhaust airflow to spin. At low rpms, there isn't a significant amount of exhaust flow. A turbo with a low boost threshold is designed to operate with small amounts of exhaust gas to power it, whereas a large turbo will not begin to spool until higher RPM's when there is more exhaust gas energy to be had. The small turbo may produce boost quickly but will feel choked at higher rpm since it has already reached maximum operating speed.

 

Now, with that out of the way.

 

I desire higher boost-output over boost threshold because a) more boost=more ponies and, b) by having boost build from 2500+ rpm, I have a window of non-boosted engine speed that I can stay in if I'm feeling conservative (thus improving gas mileage ).

 

So, I decided that I will break up my rpm-band into three sections (denoted by shading in the table. The boost will begin to build at ~2300rpm and will increase linearly at roughly 2.1psi/250rpm until we reach target boost for the upper third of the engine speed range.

 

Using the formula I provided:

 

[(Displacement (in^3) * Engine Speed (rpm) * .5 * .9)/1,728] * [(14.7 + Boost (psi))/14.7] = CFM

 

We can now determine what the flow demand will be for this particular engine/boost map. Plotting pressure ratio in relation to CFM will give us data points we can plot over a compressor map to see which compressor will handle the demand most efficiently. As you can see, a turbo that could fulfill these requirements would be capable of pushing near 430cHp at redline which would work out to rough 340wHp.