Recent flood of questions on turbo sizing....

[nissanfanatic]

I've noticed a lot of people asking about what turbo to use on their car(s) lately. And its hard to find out on paper. I know, I did it. And there are a lot of unforseen factors that can be left out. This calculator can make life a little easier.

Turbo map

[C-Kwik]

I've used that calculator in the past. Frankly, I'm not a fan of it. It tells you nothing of how an individual motor will actually plot out on it. All it really does is plot the graph for you. The input is still just as uncertain as if you calculated it by hand.

The easiest way I find to do this is to simply draw a straight line(horizontal) through the pressure ratio you are planning on. Then plot your boost threshold(the estimated airflow that will occur when you hit the boost level you are plotting for. Then plot the airflow at redline. The area of the horizontal line between these two points is the most important part of your plot on this map. The area leading up the the boost threshold is complicated and hard to guess at without actual data from a working system using that turbo on that motor. For the most part, you can aim it at or near the (0,0) points on the graph and it should give you a conservative look at how it will relate to the surge limit. While plotting several points between either end of the relevant curve may be useful, most don't have the airflow data to get an accurate account of it. As the VE of a motor varies with RPM, it would be difficult to simply calculate this. You can try to aproximate the changes using the torque curve as a guide, it will not be perfect, and for the most part, a waste of time. A modern 4 valve/cylinder motor will have a flat enough VE that it's negligible.