how-to read a compressor map

[BlueShadow]

<FONT COLOR="blue">EDITED 30 SEP 05: Hey guys, it is almost 3 years since I first wrote this how-to. At the time I wrote it all I was ableto use for reference was my book Maximum Boost. Until today, I couldn't find anything else on the net that said how to read a compressor map. While doing some research on how to swap compressor housings I found thie very detailed explanation by Garrett on how to read a compressor map. http://www.turbobygarrett.com/....html </FONT> (original post) Hey everybody, I was bored so I though I would type up a brief Topic on how to read a compressor map. I know this has been covered before but the threads have already been archived and some people (my self included) are unable to go back and add/change some of the info. so here it is, hopefully I remembered to add all the inportant information. HOW-TO-READ A COMPRESSOR MAP using a map of a T04E 60 trim I will explain all the numbers on the map 1-left side, PRESSURE RATIO (14.7 + amount of boost) / 14.7 = PR so to figure out the PR for 8 PSI (14.7 + 8) / 14.7 = 1.54 PR 2-bottom side, AIRFLOW RATE UNDER BOOST (LB/MIN on this map) Most methods of calculation your engine's airflow rate will give you the answer in cubic feet per minute (CFM). However most compressor maps measure airflow rate in pounds per minute (LB/MIN). As some of you may know the weight of air varies with the temperature. To convert CFM to LB/MIN use the following numbers. @ 48 degrees F : (CFM * 0.078125) = LB/MIN @112 degrees F : (CFM * 0.070318) = LB/MIN @175 degrees F : (CFM * 0.06251) = LB/MIN Say for example our airflow rate is 500 CFM , and the temperature is 112 degrees F. (500 * 0.070318) = 35.16 LB/MIN *For those of you that know anything about ideal gas law, if you know a better way of explaining how to convert CFM to LB/MIN, your input would be appreciated. But please explain it in "laymans" terms, so that everyone can get a grasp on it. 3-dotted line on far left side of "ovals", SURGE LIMIT It is important to try and keep yourself on the right side of this dotted line whenever possible. If you fall to the left of this dotted line you will experience compressor surge. This type of compressor surge will occur when there is too much boost, but not enough airflow through the system, usually this is between idle and the point at which full boost is reached. The chirping sound that can be heard is a result of the oscillating air. This sound is often described as a "Snakelike" sound or a che-che-che sound. *staying in the "surge limit" area for too long could possibly damage your turbo. 4-numbers on far right, 46,020, 69,640, 83,972 etc, COMPRESSOR RPM This is RPM at which the compressor fans will be turning. an average RPM is between 90,000 and 130,000. The line that branches out from each of these numbers that goes towards the surge limit line shows you the RPM range of the compressor fan across the entire compressor map. 5-78%,75%, 74%, COMPRESSOR EFFICIENCY This is related to the temp of air and how much it is being heated up as it is being compressed by the compressor. A low number (60%) means that the compressor is heating the air more a high number (78%) means the air is not heated as much when it is compressed. 6-"Ovals" I you look closely you will see that the compressor efficiency numbers usually sit right on top of one of these Oval lines. These Ovals show you the boundaries of the compressor efficiency at the different percentiles. Think of it as a topography map that shows you different elevations or changes in elevations. The innermost Oval on the sample T04 E 60" is not labeleb but it is probably 79% or 80%, so any where inside that Oval and you would be operating in the 80% range of that compressor. --changed # 1 (pressure ratios) 29 Mar 02 --changed # 2 (airflow rate) 29 Mar 02 --added # 2 (airflow rate) 13 Apr 02 --added # 3 (surge limit line) 13 Apr 02 --changed # 5 (compressor efficiency) 08 Jan 03 [Modified by BlueShadow, 7:09 AM 4/13/2002] [Modified by BlueShadow, 10:08 PM 1/8/2003] Modified by BlueShadow at 2:45 PM 9/30/2005

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[Arturbo]

Awesome post!!

[BlueShadow]

Phew! If anyone has anything to add, or if anyone sees something wrong let me know. Things such as the coversion ratio for CFM to LB/MIN were given to me by a very good source but I havent checked the numbers myself to confirm their accuracy.

[BlueShadow]

Awesome post!!

Thank you! thank very much.

Next Lesson, how to select a compressor for you car.... on second thought

Actually I have though about doing a How-To select a compressor for your car. But not on this site, for the simple fact that there are a LOT of Hondas/Acuras roaming around on this site. There would be to many variations to consider and a how-to-select a turbo on this site might do more harm than good. Besides there are plenty of experienced members that have already "been there and done that" and IMO they would be in a better position to help them on how to select a turbo than I would.

[Ltech]

Anyone need me to scan the Maximum Boost book? I can put it on here lol. Good post.

[Stock@$$GSR]

For compressor efficiency, at what percentage should you start to stay away from? 60%? 65%?

[gh32]

Hmm...not sure how legal it is, but if others wouldn't object, I say throw that book up here. (I'm going to buy it anyways, but still, great reading material)

-George

[Arturbo]

I know Corkey Bell personally and I dont think he would like that.

art

[bruthaboost]

very edifying post

[redline]

1-left side, PRESSURE RATIO
(14.7 + amount of boost) * 14.7 = PR
so to figure out the PR for 8 PSI
(14.7 + 8) * 14.7 = 1.54 PR

should be (14.7 + boost ) / 14.7 = PR (not *)

5-78%,75%, 74%, COMPRESSOR EFFICIENCY
This is related to the temp of air and how much it is being heated up as it is being compressed by the compressor. A low number (60%) means that the compressor is heating the air more a high number (78%) means the air is not heated as much when it is compressed. If there was such a thing as a compressor with 100% efficiency then the air would not be heated up at all.

It has to heat up if you compress it, its the law! In threoy anyway!!

[dbman96]

Very informative post.

A couple things I can think of offhand:

Lbs/min of air is not just dependent on CFM (as calculated based on engine displacement and speed) and temperature - you also need to multiply it by pressure ratio. Unless you're already using the pressure ratio to calculate CFM which in my opinion is misleading at best. The CFM your engine consumes at a given RPM and throttle position is fixed - the mass airflow is VERY variable.

If your engine is consuming 200 CFM of air at 112 F, at ATMOSPHERIC pressure, the mass is

200CFM *0.070318 lbs/ft^3 = 14.06 lbs/min of air

If you're boosting, say, 8 psi... it's 200*0.070318*pressure ratio

= 200 CFM * 0.070318 * (14.7+8)/14.7 = 21.7 lbs/min

- this makes a BIG difference when you're reading a compressor map since you're interested in the mass of air flowing at full boost.

Also regarding efficiency - 100% efficiency would mean that the air is heated exactly as much as it would be in an isentropic compression (only compressive work is done on the gas - this still increases the energy of the gas so there IS temperature rise). So as mentioned, by physical law, any compression results in a temperature increase - but a 78% efficient turbo is a hell of a lot better for temp rise than a 55% efficient Eaton blower.

[BlueShadow]

1-left side, PRESSURE RATIO
(14.7 + amount of boost) * 14.7 = PR
so to figure out the PR for 8 PSI
(14.7 + 8) * 14.7 = 1.54 PR


should be (14.7 + boost ) / 14.7 = PR (not *)

OHH CRAP!!! I cant believe I missed that!, I'll go back and change it right away.

5-78%,75%, 74%, COMPRESSOR EFFICIENCY
If there was such a thing as a compressor with 100% efficiency then the air would not be heated up at all.

It has to heat up if you compress it, its the law! In threoy anyway!!

that's why I said "IF THERE WAS SUCH A THING..." But there isn't. any time you compress air then yes you are raising its temperature (Thermodynamics).

[BlueShadow]

Lbs/min of air is not just dependent on CFM (as calculated based on engine displacement and speed) and temperature - you also need to multiply it by pressure ratio. Unless you're already using the pressure ratio to calculate CFM which in my opinion is misleading at best. The CFM your engine consumes at a given RPM and throttle position is fixed - the mass airflow is VERY variable.

If your engine is consuming 200 CFM of air at 112 F, at ATMOSPHERIC pressure, the mass is

200CFM *0.070318 lbs/ft^3 = 14.06 lbs/min of air

If you're boosting, say, 8 psi... it's 200*0.070318*pressure ratio

= 200 CFM * 0.070318 * (14.7+8)/14.7 = 21.7 lbs/min

- this makes a BIG difference when you're reading a compressor map since you're interested in the mass of air flowing at full boost.

EDIT: BTW the 500 CFM was supposed to be airflow under boost...

I dont know where you're going with that..... but you just made me realize something else. If a "NEWBIE" was to read #2 airflow rate he might get confusedm cause I didn't specifically say "BOOSTED ENGINE"S CFM" I just said "ENGINE CFM". I used 500 CFM cause that is the typical CFM for a H22 with 9-12 PSI of boost. I guess I assumed that everyone would know that the 500 CFM is from a boosted engine. But I think it might be possible that someone would mistake 500 CFM for a NA engine, I'm gonna change #2 to say CFM for a boosted engine.

Also regarding efficiency - 100% efficiency would mean that the air is heated exactly as much as it would be in an isentropic compression (only compressive work is done on the gas - this still increases the energy of the gas so there IS temperature rise). So as mentioned, by physical law, any compression results in a temperature increase - but a 78% efficient turbo is a hell of a lot better for temp rise than a 55% efficient Eaton blower.

In English? I tried to explain things so that even your average person without a thermodynamics degree would understand.

I should probably take out that statement "IF THERE WAS SUCH A THING AS A 100% EFFICIENT COMPRESSOR", It wasn't meant to be taken to heart. To many people might misinterpret that, or they might not get what I was trying to say.


[Modified by BlueShadow, 3:23 PM 3/28/2002]

[VTC_CiViC]

Excellent post dude!! I'm still a little hazy on it, but ALOT better off than where I was before I read your post

[BlueShadow]

Excellent post dude!! I'm still a little hazy on it, but ALOT better off than where I was before I read your post

thanks dude! I'm here to learn and to help in whatever little ways that I can :D

[Muglit]

Shouldn't the pressure drop across an intercooler be included in the PR calculations? I think pretty much think every body reads their boost in the IM which is the amount of boost after the intercooler but the turbo is working at the boost before the drop.

Meaning for 1.5psi intercooler drop: PR = (14.7 + 1.5 + boost)/14.7

[BlueShadow]

Shouldn't the pressure drop across an intercooler be included in the PR calculations? I think pretty much think every body reads their boost in the IM which is the amount of boost after the intercooler but the turbo is working at the boost before the drop.

Meaning for 1.5psi intercooler drop: PR = (14.7 + 1.5 + boost)/14.7

sounds like that would make sense, you would also have to include air filter loss, IC piping losses (to many abrupt bends can lower psi), and I forget what else. I think those three are the ones you take into consideration. The 2 I mentioned will have small losses or none at all. The one you brought up will be the most important one.

Keep in mind though that the basic calculation for compressor selection assumes that it is non-intercooled. I dont think the amount of PSI lost makes a difference though (IMO). unless you are running real low boost (5-7 PSI).

[Prelussion]

I would like to read an article on how to select the right compressor.

[BlueShadow]

I would like to read an article on how to select the right compressor.

Maximum Boost $30-35 at your nearest Barnes and Nobles, or http://www.amazon.com

you'll learn how to select a compressor and a WHOLE LOT MORE!

[Prelussion]

So on a t04 turbo w/ 60 trim if you wanna run the highest efficiency with the highest boost you would want to run roughly 22 lbs of boost and have an airflow of roughly 35 lbs per minute?!?!

[BlueShadow]

So on a t04 turbo w/ 60 trim if you wanna run the highest efficiency with the highest boost you would want to run roughly 22 lbs of boost and have an airflow of roughly 35 lbs per minute?!?!

only if your intended goal is to run 22 lbs of boost. you select the compressor based on your specific boost and flow. if the compessor doesnt't fall in the highest efficiency from 6000-7500 RPM its ok, as long as the number is above 65% your ok. IF it falls below 65% you can try searching for a map with a better efficiency rating.

Remember, that would be how much the air is heated up for non-intercooled boost, by adding an intercooler you are further cooling the intake charge.

[Prelussion]

only if your intended goal is to run 22 lbs of boost. you select the compressor based on your specific boost and flow. if the compessor doesnt't fall in the highest efficiency from 6000-7500 RPM its ok, as long as the number is above 65% your ok. IF it falls below 65% you can try searching for a map with a better efficiency rating.

Remember, that would be how much the air is heated up for non-intercooled boost, by adding an intercooler you are further cooling the intake charge.

you would also add a bit to the pressure number for the i/c right? So if i wanted to run about 9psi and have an efficiency rating of at least 70% how would i know what air flow i would have before i knew what turbo i would be running?



[Modified by Prelussion, 2:45 AM 4/8/2002]

[BlueShadow]

you would also add a bit to the pressure number for the i/c right? So if i wanted to run about 9psi and have an efficiency rating of at least 70% how would i know what air flow i would have before i knew what turbo i would be running?

What car/engine? H22?

[Mikey3000]

I know Corkey Bell personally and I dont think he would like that.
art

how do you know Corkey Bell?

[Prelussion]

B18b