Couldn't have said it better myself. Good call on bringing up density.
The ABSOLUTE temperature of air greatly affects its density (when volume isnt fixed) and thus the mass flow required to pressuize a given volume at a particular engine speed. Hotter air from the compressor will mean less mass flow rate is required to pressurize the intake manifold (loss of torque). The boost gauge will be reading constant which is what you guys were talking about in saying 14psi is NOT 14 psi, when comparing the t28 to the 2871.
All you can infer from a boost gauge is volumetric flow rate. Volumetric flow doesnt say much for the imperical quantity (moles) of air going through the engine (thats why MAP efi requires a temperature sensor). Mass flow rate does equate to air molecules. More mass means more air molecules which means more fuel which will equate to more power. Word of warning though: If that mass is too hot when it gets injested by a cylinder and compressed again (gets even hotter) the propensity to knock will increase (engine damage).
Quote:
Originally Posted by jspaeth
After doing some further reading and rethinking things, I want to offer an apology and say that I agree with you about your explanation.
If I understand you correctly, you are suggesting that a T25 vs a 2871R (for example) are SUFFICIENTLY efficient enough at 7 psi that the difference in output air temperature shouldn't be much......thus, the mass air flow from the two WOULD be the same.
However, at 14 psi and higher RPMs, I believe you suggest that the T25 is basically INEFFICIENT enough relative to the 2871R that it will not flow as much mass (air density goes up).
I believe your explanation was a good one, but I was not thinking clearly (LONG FUCKING DAY, trust me).....
So, hypothetically, the T25 at 15 psi and GT2871R at 15 psi....comparison:
Assume that they both spool by 3500 RPM and fix them at 15 psi (JUST SUPPOSE....).....so then (as an example), they may produce the same mass air flow (and thus torque) up to say 5000 RPM (JUST AN EXAMPLE again), but beyond this point, the T25 starts becoming so inefficient that even though they are still both at 15 psi outlet pressure, the T25 just can't keep pumping air at that pressure and at the same DENSITY as the GT2871R....thus, the T25 torque curve begins dropping off earlier, even though the boost pressure is the same in the 2 cases....
Sound correct now?
So okay, I give in, you are correct :-)
Sorry, again.
|