DyNaMiTe KiD

Hey
1990 turbo rx7

Can someone give me a explanation to what a turbos efficency means? An example is a t04b 60-1 efficency is 10-18 a gt35/40 is like 17psi+ and i think the apexi rx6 as well is 17+psi. so what would happen if i took something like a gt35/40 and drove it daily at 13psi what does that mean?does it affect air velocity at lower psi?increased temps at lower psi? basically i want all i can get tourqe, low inktake temp, good hp and awesome spool around 13-17 psi and maintain 400rwhp 15-17 psi. I know i wont be boostin past 18.I also dont want all my tourque and hp sittin past 6500rpm with a mild streetport.

Thanks

XSTransAm

you will be fine running lower boost... efficiency range is basically where the turbos sweet spot is... if you start going above it you run out of its efficenciy range and heating the air too much for the boost created which makes the final charge "inefficent"

XSTransAm

heres a better explaniation i found:

Additional important factors to consider are efficiency (or how little the compressor heats the air over adiabatic compression alone), surge (when the compressor cannot supply enough air at low engine speeds), and the maximum pressure ratio (PR) the compressor is capable of producing. Compressor flow maps are required to accurately determine efficiency, surge limits, and maximum PR. Most modern turbos operate with maximum efficiencies in the 70% to 80% range. The peak efficiency "island" usually occurs somewhere near the middle of the both the flow range and the pressure ratio range. Surge limits are generally not of much concern for our cars except for the very-largest turbos listed below. Maximum pressure ratio is very important because the relatively small displacement of our 3-L engines requires very-high density ratios (for high effective volume airflow) to achieve performance at the 500 bhp level and beyond.

maxcooper

Compressor Efficiency defined (without the math): Compressing air always heats it up, but a turbo always heats it up a little more than the gas laws (PV=nRT) tell us it should heat up just from the compression. A turbo operating in its most efficient range will heat it up only a little more than you would expect just from the compression. A turbo operating outside of its efficiency range will heat it up a lot more than you would expect just from the compression. Some turbos are more efficient than others in general, but most are in the 75-80% range of efficiency in their sweet spot (flow and pressure ratio).

Pressure Ratio defined: ratio of absolute compressor output pressure to absolute compressor input pressure. The atmosphere is 14.7 psi absolute. You would have a pressure ratio of 2.0 if you are running 14.7 psi of boost on your boost gauge (which is 14.7 + 14.7 psi absolute). In reality, you've probably got an air filter dropping the pressure at the turbo inlet down to 14.3 psi, and you lose 2 psi in the IC and piping before your boost gauge reads 14.7, so the compressor outlet pressure is really 16.7 psi (+14.7 = 31.4 psi absolute). So, you're really running a pressure ratio of about 31.4 / 14.3 = 2.2 when the boost gauge reads 14.7 psi.

Some turbos are most efficient at higher pressure ratios, some are most efficient at lower pressure ratios, and some do pretty well across a wide range of pressure ratios. This is what people are talking about when they say a turbo is "best above 17 psi", etc. If you run a turbo out of its efficiency range, it heats the air more than it should.

-Max