Attachment:
GTX2867R_816366-1_comp.jpg
Sizing a turbo:
To make a quick pass at sizing a turbo, you need to assume a few more things. You need to estimate the rate at which your engine can swallow air, and this is done by picking a volumetric efficiency and calculating the airflow from that and from how fast the engine is spinning. Simply put, a 122 cubic inch engine draws in 61 cubic inches per revolution if it’s operating at 100% efficiency. When all the turbo books were written, a healthy VE was about 80-85%, but I believe that we’re running much closer to 100% nowadays. For this example, that’s exactly what I’ll assume. 61 cubic inches at 6000 RPM = 212 CFM. That part is simple.
You also need a compressor map – I will be using the GTX2867R map from ATPs website. Our earlier goal is 22 psi boost pressure, or a pressure ratio of 2.5:1, and if we look along the horizontal line that corresponds to a PR of 2.5, we see that the peak efficiency of this turbo is 78% for that pressure. We will use that in our calculation as well, and will tweak things to suit.
The first step is to estimate the discharge temperature for our pressure ratio, and the spreadsheet that I built tells me that this value is 268*F. To calculate the pre-turbo airflow we must apply a fairly simple calculation, which is (absolute ambient temp / absolute compressor discharge temp) x pressure ratio x post turbo airflow. Plugging in numbers, this is (527.67 R / 727.67 R) * 2.5 * 212 CFM = 383.4 CFM. Then we must convert cubic feet of air into a mass, each cubic foot weighing approximately 0.076 lbs at sea level. 383.4 x 0.076 = 29.14 lb, so our engine is consuming 29.14 lbs of uncompressed air every minute. Plotting that airflow on our map against the pressure ratio of 2.5, we find ourselves right around the 76% efficiency range. We would then use this to tweak our numbers, but the difference is going to be pretty much fractional: after tweaking the calcs with the new efficiency, the airflow is 28.93 lb/min.