How to read compressor maps, and size up a turbo.

This post is to help you get past the Honda-Tech mentality of “bigger is better” when it comes to turbos, and get you thinking “properly sized is better”. Who’d a thunk it?
This will make some assumptions that you under stand some basics about what a turbocharger is and a what it does.

So, lets take a look at a compressor map. This is the map for the T3 super 60.

Ok, thats it go build a turbo system. :stuck_out_tongue:

You can see this is a basic 2 axis graph (x and y)… but what does it all mean???
It’s really pretty simple.
Pressure ratio is just that a ratio of pressure.
Pr= (Bp+Ap)/Ap
Bp = boost pressure
Ap = air pressure (14.7psi you should know this, if not go back to school) :slight_smile:

so if we want 10 psi PR= (10 + 14.7)/14.7
or 1.68 (we will round to 1.7 for ease)

pretty simple right?

Now the column on the bottom is Air flow lb/min, but sometimes they will be in CFM.
Oh no’s what to do?
Here is how to calculate CFM

CFM = (L x RPM x VE x Pr)/5660

and to convert CFM to lb/min simply multiply CFM x .07

L = engine size in liters
RPM = what rpm your plotting the point for
VE = volumetric efficiency
2 valve engines 85%
4 valve engines 90%
ported 95%
race heads like whoa 103%
these are just estimated numbers, but should get you pretty close to what you need.
Pr is taken from the calculation we did earlier.

So if we have a d16 what kind of CFM do we need?

CFM = (1.6 x 7000 x 90 x 1.7)/5660
CFM = 302
and that in lb/min is 21

so lets plot that point.

Not bad, at redline that turbo at 10psi is just at the end of the center island. (that’s a good
thing) but there has to be more right? Yep, we are going to have to plot 2 more points. With these next 2 points we are going to make a few assumptions. (but that’s ok, because they are almost always right.)

The 2nd point we need to plot is 50% max rpm.
Assumption 1. The turbo will make full boost by this rpm. Usually it will, or it will be really close. This is easy oddly enough the engine flows 1/2 the CFM at 1/2 the rpm (yea yea, that’s an assumption too, but again its fine)
So to plot this point we keep the Pr the same, but divide the CFM or lb/min number in half. That gives us a new lb/min of 10.5.

this is good, the point falls on the right side of the surge line. Had it been on the other side, all hell would break loose, cats and dogs living together, real wrath of god kinda stuff.

A quick note about compressor surge… If that point (or any point) falls on the other side of the line, it is similar to letting off the throttle to shift and not having a BOV. (only a little different) The other side of that line is where the turbo isn’t pushing air out of the compressor housing. Instead the air is just spinning with it, and with the exhaust side still spinning it, it can/will create pressure build up at the turbo outlet. This is where damage to the turbo can occur, as the air can/will try reverse flow and go back though the impeller.

The last point we need to plot is the 20% air flow to make sure we don’t cross that surge line between then and 50%.
We will plot this point at 1 Pr (atmospheric pressure, no boost) and take 21 and divide that by
5 (20%) roughly 4, and then run a line from there to the 50% point.

By looking at that map with these points, you can see that this turbo on a D16 is a pretty damn good match.

So figure those 3 points out, and go to town plotting compressor maps and find the right turbo for your application.

cool, I certainly did not understand how to read them before…

good stuff

Interesting, you should write one on the basics of how to go about tuning that setup. Good write up we need a fuggin :TUP: emoticon

nice write up

step 1
call maxpowertuning

step 2
sit back and watch us make power

man i need fucking engine management so i can GET tuned :frowning:

yes…but then we dont learn anything hehe

And your wallet is significantly lighter than before… Hhahaha… J/k, ZD, MaxPower Tuning does great work. John has an outstanding grasp of the entire concept of Tuning. Not a DIY, have them get you started. [/KissingZer0DazEAss]

Awesome stuff. I had meant to try and figure out how those maps are ready for a while, but you saved me the research. :tup:

Good link for matching turbos:

awsome link

wow, best post ever.

I just wish they had more maps.

AHHH i figured it out but it keeps going off the chart when i do 50% max. Can some one help me out? PLEASE. Ok a guy with a t3/t04e turbo 57 trim made 438rwhp at 24psi. So would I put (24+14.7)/14.7? Thats wha ti put and it came out to 2.6. So 2.6 is part of the first point. Second (2.0x7400x95x2.6)/5660=646. 646x.07=45. half of 45 would be off the chart. I DONT GET IT.

+1or2gadjillion…2 GREAT posts right here

heres a great little tool my buddy Frank has been working on. many maps in here. We are working on making sure the newest version is ready before we release it. the new one has the same stuff, its just a little easier to use. Any questions lemme know.


Very intersting calculator. We have a similar program that we devised to calculate all that and plot your map. You are missing a few things there but all in all looks like a really nice free tool.

what things would you recommend adding? we are always looking to making it more accurate. we actually had a few guys over at Borg Warner check it out and they thought was pretty decent.