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 I know this has been covered before but the threads have already been archived and some people (my self included) are unable to go back and add/change some of the info.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

<u>1-left side, PRESSURE RATIO</u>
(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


<U>2-bottom side, AIRFLOW RATE UNDER BOOST (LB/MIN on this map)</u>
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.


<U>3-dotted line on far left side of "ovals", SURGE LIMIT</U>
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.


<U>4-numbers on far right, 46,020, 69,640, 83,972 etc, COMPRESSOR RPM</U>
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.


<U>5-78%,75%, 74%, COMPRESSOR EFFICIENCY</U>
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.


<U>6-"Ovals"</U>
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
--changed # 2 (airflow rate) 29 Mar
--added # 2 (airflow rate) 13 Apr
--added # 3 (surge limit line) 13 Apr