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.
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)
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.
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Helped me understand compressor maps for sure. Should be all the info you need to pick the correct turbo for your car
