Yellow Rose
Suspended
- Joined
- 22 November 2001
- Messages
- 2,256
Ever wonder what the inlet air temperature is on a forced induction engine? From thermodynamics, below are various compressor discharge temperatures based on 100 °F ambient temperature.
6 PSI - 130 °F
8 PSI - 138 °F
10 PSI - 146 °F
12 PSI - 153 °F
14 PSI - 160 °F
16 PSI - 167 °F
18 PSI - 173 °F
20 PSI - 179 °F
Somewhere I read that the NSX's ECU objects to throttle body intake temperature greater than 150 °F. (I wonder if this is because of factory fuel map limitations or engine component (valve stem guides come to mind) temperature limitations.) Therefore, from the above theoretical temperatures an aftercooler is required on an NSX with more than 11 PSI of boost. As a data point, I am at 11.7 PSI at 7440 RPM and have measured approximately 30 °F of aftercooling on my setup on an 80 °F day. From another thread, a 12 PSI BBSCer reports maximum intake temperature on an 80 °F to be 99 °F. From the table above, that is a phenomenal 54 °F of aftercooling!
The above temperatures are theoretical. While not exact because the formula is a polynomial, you can approximate resulting temperatures by subtraction. For example if it was only 60 °F outside, the resulting temperature with 20 PSI of boost calculates to 133 °F. Remember, these are textbook numbers for heat of compression only. While some would argue that heat soak would also have an affect, I do not subscribe to that theory and below is why.
The piping on my BBSC is 3". From Paxton's website, the Novi 2000 has a rating of 1700 CFM. In the engineering world, this is meant to be SCFM. To calculate velocity, the flowrate has to be in ACFM. Converting yields 947 ACFM at internal pressure of 11.7 PSI. The cross-sectional area of a 3" is 7.065 IN^2. Crunching the numbers yields internal air velocity at redline of 326 FPS. To put this into perspective, that is 222 MPH! Considering the relative short plumbing of the compressor discharge piping, the air molecules are not in contact with the hot pipe long enough to conduct heat into the charged air. Last weekend I had the aftercooler off of my car. At idle with the BBSC just blowing, I was amazed of how much air was coming out. I had my wife rev the engine a few thousand RPM and it was an effort to hold my hand in place. And the noise! The airflow from the Paxton is much more than my gas-powered leaf blower, which is rated at 120 MPH. Simply put, the compressed air is flowing too fast to be additionally warmed by metal heat soak.
6 PSI - 130 °F
8 PSI - 138 °F
10 PSI - 146 °F
12 PSI - 153 °F
14 PSI - 160 °F
16 PSI - 167 °F
18 PSI - 173 °F
20 PSI - 179 °F
Somewhere I read that the NSX's ECU objects to throttle body intake temperature greater than 150 °F. (I wonder if this is because of factory fuel map limitations or engine component (valve stem guides come to mind) temperature limitations.) Therefore, from the above theoretical temperatures an aftercooler is required on an NSX with more than 11 PSI of boost. As a data point, I am at 11.7 PSI at 7440 RPM and have measured approximately 30 °F of aftercooling on my setup on an 80 °F day. From another thread, a 12 PSI BBSCer reports maximum intake temperature on an 80 °F to be 99 °F. From the table above, that is a phenomenal 54 °F of aftercooling!
The above temperatures are theoretical. While not exact because the formula is a polynomial, you can approximate resulting temperatures by subtraction. For example if it was only 60 °F outside, the resulting temperature with 20 PSI of boost calculates to 133 °F. Remember, these are textbook numbers for heat of compression only. While some would argue that heat soak would also have an affect, I do not subscribe to that theory and below is why.
The piping on my BBSC is 3". From Paxton's website, the Novi 2000 has a rating of 1700 CFM. In the engineering world, this is meant to be SCFM. To calculate velocity, the flowrate has to be in ACFM. Converting yields 947 ACFM at internal pressure of 11.7 PSI. The cross-sectional area of a 3" is 7.065 IN^2. Crunching the numbers yields internal air velocity at redline of 326 FPS. To put this into perspective, that is 222 MPH! Considering the relative short plumbing of the compressor discharge piping, the air molecules are not in contact with the hot pipe long enough to conduct heat into the charged air. Last weekend I had the aftercooler off of my car. At idle with the BBSC just blowing, I was amazed of how much air was coming out. I had my wife rev the engine a few thousand RPM and it was an effort to hold my hand in place. And the noise! The airflow from the Paxton is much more than my gas-powered leaf blower, which is rated at 120 MPH. Simply put, the compressed air is flowing too fast to be additionally warmed by metal heat soak.
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