Quote:
Originally Posted by frostmotorsport
Frosty will be fine!! 
however, I do not agree with your 1st statement regarding 1.1 vs 1.4 bar. As I said in my 1st post, ALL other factors being the same, regardless of what model turbos you have bolted on the side of the engine, you will require exactly the same boost pressure to make a given HP at a given RPM - regardless of whether that said boost pressure is at the bottom, top or middle of said turbo's compressor map - ie it doesn't matter if the turbos are flat out making, say, 1.4 bar or if it's in the middle of its range making 1.4bar.
Remember tho, this only matters for a "snap shot" - i.e. 1.4 bar of boost at 6000rpm makes 320atwkw on this motor - regardless of what turbos are making that 1.4 bar.
What WILL cause variance, is the how the turbos spool, their efficiency etc etc.
make sense? |
Sorry, this is not true at all. Did you learn how turbos work before making that post? Turbochargers compress air. What happens when you compress a gas? That's right, it heats up.
Two turbos running the same amount of boost, one larger than the other, will NOT result in the same mass of airflow going into the engine because the more efficient compressor will heat the air less.
You can model the amount of airflow into an engine using the ideal gas law; PV=NrT where P is pressure, V is volume, N is the number of molecules, r is a constant and T is temperature. From this you can see that increasing the pressure will increase the molecules or Mass of air, increasing the volume will increase the mass of air, and decreasing the temperature will also increase the mass of air! Pretty basic high school science concept.
Now, apply it to the sizing of turbochargers:
Take a GT2860-5 at 30lb/min airflow and a pressure ratio of 2.5 (1.5 bar boost). That compressor wheel will be operating at 76% efficiency (look it up on the compressor map at turbobygarrett.com
Take those same specs and look at a GT2860-7. You are going to be outside of 60% efficiency - it's pretty close to the edge of that 'land' so we'll call it 58% efficiency.
76% vs 58%.
Now if you look up compressor efficiency vs ΔTemperature, you can see how the inlet air temperature will change. The chart I am referencing (page 29 of Maximum Boost) is in Fahrenheit, so bear with me.
According to the chart at 75% efficiency you would be heating the air about 305 degrees. And at 58% efficiency you'd heat it about 365 degrees. So that's 60°F hotter air at the same pressure! Your intercooler is going to work to bring that charge temp down, but it makes sense that if you start with colder charge air in the first place, the intercooler will be able to bring the charge air temp that much more.
What does all this mean? Everyone knows that colder air is denser air and denser air makes more power. There it is, your incontrovertible proof that larger (more efficient) turbochargers will flow a greater mass of air, and therefore make more horsepower, than a smaller turbo at the same boost pressure.