gt2871r will run okay?

[jspaeth]

Originally Posted by mct3351

Let's run a little thought experiment. Imagine a majic box capable of unlimited mass flow rate at ambient temperature. Imagine we connected this to the throttle body of an sr20 and asked it to pressurize the intake manifold to 7psi, then took measurements of the mass flow rate as the engine rev'd from 4k rpm to 7k rpm. The mass flow rates measured are measurements of the mass flow rate demand/required to pressurize an sr20 manifold as it rev'd through the rpm range with air at ambient temperature. Remove things like cams, intake manifold and ve from the experiment because we arent changing those things. This majic box experiment is the most ideal/upper limit flow rate required to pressurize the intake manifold, becasue regardless of how effieint a real world compressor is it will heat the air because of the idea gas laws. The power output by the sr20, connected to the majic box, at its peak engine speed will be the peak/absolute maximum power possible, because this is the ideal case. The torque curve will be flat through the rpm range because the cylinders are filling with the mass quantity of air regardless of engine speed. When an intake valve opens the most amount of air mass it can take in is equal to 2.0 liters divided by four cylinders that is pressurized to 7psi. Increasing engine speed will increase the mass flow rate required to maintain 7psi in the intake manifold because there are more intake events in a given amount of time. So the difference is turbos will lie somewhere in terms of flow rate underneath this maximum/ideal case.

Assuming we are using air there are only so many parameters that can affect mass flow rate. Pressure, Volume, Temperature and engine speed. If we are talking about 7 psi manifold pressure and the volume of the engine is fixed as well as engine speed rangre the only remaining parameter that can change is temperature. When a small turbo like a t25 is asked to pressurize the intake manifold of an sr20 to 20 psi it will do so reasonably efficiently up to the mass flow rate limit of its compressor wheel. If the intake manfold pressure does not at some point drop after the mass flow rate limit has been reach that means that it has begun heating the air. Usually what happens when the mass flow rate limit has been reached is a combination of unnecessarily heated air and the intake manifold pressure will drop.

So when comparing the two turbos a t28 and a 2871 as installed on an sr20, they are both capable of producing adequate mass flow rate to pressuize the intake manifold of an sr20 to 7 psi without adding excessive amounts of heat or droping the intake manifold pressure near redline. Yes there will a slight neglible difference in the neighborhood of < 3-5% percent probably (I hope thats not what you are arguing for). But, at 7psi the engine's "pumping" ability will be the mass flow rate limitation. Between the two turbos this is not true at 14 psi. That is when your absolute rule that Xpsi is not equal to X psi becomes true when comparing these two turbos or any other turbo at least as big as a t28 upto and including a t88 on a 2.0 liter.

How much horsepower does a SR20 with a T25 make at 15 psi?

The same stock SR20 with a 3071R at 15psi?

[mct3351]

Originally Posted by jspaeth;

How much horsepower does a SR20 with a T25 make at 15 psi?

The same stock SR20 with a 3071R at 15psi?

Assuming you're asking sarcasticly (if ur not being sarcastic I could plot you hypothetical dynos in excel comparing the expected output of those two scenaros)...Not any where near the same amonut of peak power which is exactly what I said in the two previous explinations, for reasons which I have also already explained. Did u read it? FYI they'd produce similiar peak torque numbers though. Because for a shot period the t25 could provide adequate mass flow rate to achieve 15 psig manifold pressure. Then, as the revs increase, it would no longer be able to sustain the mass flow rate required the keep the manifold pressurized to 15psig. The torque curve will dive and the hp will flat line.

Are you still disagreeing with my original statement that the stock fuel system on an sr20, as installed on an sr20, can handle any turbo known to man at 7 psig manifold pressure?

[jspaeth]

Originally Posted by mct3351

Assuming you're asking sarcasticly (if ur not being sarcastic I could plot you hypothetical dynos in excel comparing the expected output of those two scenaros)...Not any where near the same amonut of peak power which is exactly what I said in the two previous explinations, for reasons which I have also already explained. Did u read it? FYI they'd produce similiar peak torque numbers though. Because for a shot period the t25 could provide adequate mass flow rate to achieve 15 psig manifold pressure. Then, as the revs increase, it would no longer be able to sustain the mass flow rate required the keep the manifold pressurized to 15psig. The torque curve will dive and the hp will flat line.

Are you still disagreeing with my original statement that the stock fuel system on an sr20, as installed on an sr20, can handle any turbo known to man at 7 psig manifold pressure?

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.

[mct3351]

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).

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.

[codyace]

Originally Posted by mct3351

Couldn't have said it better myself. Good call on bringing up density.

Agreed...I think Justin and I sort of mistook your generic comment at first. We all are saying the same thing, but form different perspectives. That's why we were bringin up the total mass flow (lbs of air) that each compressor makes at specific PSI...all to often people on here just say 'well 8 psi is 8psi' and we took it that way.