turbo: blow thru vs. draw thru
Can you explain 1.8:1 pressure ratio for me?
There are two things you need to know when you are looking at a compressor map, pressure ratio & flow.
The pressure ratio is the ratio between atmospheric pressure and your desired manifold pressure. To calculate it you would take atmospheric pressure (usually 14.7 psi unless you live in Denver) and add the desired boost pressure. Then divide by atmospheric pressure. So 8 pounds of boost + 14.7 = 22.7 Divided by 14.7 = 1.544 That would be a 1.544 pressure ratio.
Turbocharger compressors are rated in mass flow. Usually pounds of air per minute. There is a fairly close relationship between mass flow & horsepower. Plenty of calculators out there to help with that.
For an arbitrary target of 400 horsepower, you will need 44 pounds of air per minute. Boost & engine displacement can change, but the mass flow will still be 44 lbs per minute. A 318 with 9psi of boost or a 225 /6 with 19psi - both 44 lbs per minute mass flow.
When you look at a compressor map, there will be pressure ratios on one axis, and flow in pounds per minute on the other. Where those lines cross is where you wind up on the map.
B.
