Quick (noob) question. I have thoroughly confused myself about compressor maps.

Running to the right of the choke line will create excess heat and cause trouble

 

Quick (noob) question. I have thoroughly confused myself about compressor maps.

Running to the right of the choke line will create excess heat and cause trouble.

When plotting points on a compressor map, where are the pressure ratio measurements taken? I'm assuming inlet and outlet.

The reason I ask is because my intake increases in size substantially along the way - from a 60mm (ish) outlet to a 3.5" pipe feeding an 82mm TB. I anticipate that the pressure will drop along the way as the diameter increases.

My goal is to have a high flow (26lbs/min at 6800rpm) at low pressure (5psi) to create 300hp on a 2.7L 6 cylinder. My concern is that the higher flows at that pressure will put the smaller (and faster spooling) turbos past the choke line. But I plan on measuring boost just before the TB, since that's what the engine will actually see.

So am I completely overthinking this? Will the smaller turbos be fine since the pressures at their outlet will be higher then at the TB and that will keep them from choking? Or am I going to have to go with a larger turbo and sacrifice spool time to keep pressure low and flow that much air properly.

As always, advice and criticisms are welcome.

Best,
Ike

 

Correct. The right side of the choke line is low efficiency, meaning more of the turbine power is converted to heat and less to compressing the air. It also runs to higher shaft speed which means higher exhaust back-pressure.

 

There will be neglegible pressure drop in the pipe you have described.
Make sure you are left of the choke line at 1.3 P.R. and 26 lb/min and you will be fine. (30 lb/min would be safer to ensure 300 hp)

 

Thanks grunt! I appreciate the help.