Dirty_SOHC
02-01-2005, 05:54 PM
Can any one help me understand and read compressor maps?
I have figured out the PR- Pressure Ratio and how to read the map. But I cannot seem to be able on how to calculate the corrected air flow "lbs/min". Here is a quote on honda tech, that I have cut and pasted.
Any help will be apperciated.
Thanks Rolland
quote:
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Lbs/min of air is not just dependent on CFM (as calculated based on engine displacement and speed) and temperature - you also need to multiply it by pressure ratio. Unless you're already using the pressure ratio to calculate CFM which in my opinion is misleading at best. The CFM your engine consumes at a given RPM and throttle position is fixed - the mass airflow is VERY variable.
If your engine is consuming 200 CFM of air at 112 F, at ATMOSPHERIC pressure, the mass is
200CFM *0.070318 lbs/ft^3 = 14.06 lbs/min of air
If you're boosting, say, 8 psi... it's 200*0.070318*pressure ratio
= 200 CFM * 0.070318 * (14.7+8)/14.7 = 21.7 lbs/min
- this makes a BIG difference when you're reading a compressor map since you're interested in the mass of air flowing at full boost.
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EDIT: BTW the 500 CFM was supposed to be airflow under boost...
I dont know where you're going with that..... but you just made me realize something else. If a "NEWBIE" was to read #2 airflow rate he might get confusedm cause I didn't specifically say "BOOSTED ENGINE"S CFM" I just said "ENGINE CFM". I used 500 CFM cause that is the typical CFM for a H22 with 9-12 PSI of boost. I guess I assumed that everyone would know that the 500 CFM is from a boosted engine. But I think it might be possible that someone would mistake 500 CFM for a NA engine, I'm gonna change #2 to say CFM for a boosted engine.
I have figured out the PR- Pressure Ratio and how to read the map. But I cannot seem to be able on how to calculate the corrected air flow "lbs/min". Here is a quote on honda tech, that I have cut and pasted.
Any help will be apperciated.
Thanks Rolland
quote:
--------------------------------------------------------------------------------
Lbs/min of air is not just dependent on CFM (as calculated based on engine displacement and speed) and temperature - you also need to multiply it by pressure ratio. Unless you're already using the pressure ratio to calculate CFM which in my opinion is misleading at best. The CFM your engine consumes at a given RPM and throttle position is fixed - the mass airflow is VERY variable.
If your engine is consuming 200 CFM of air at 112 F, at ATMOSPHERIC pressure, the mass is
200CFM *0.070318 lbs/ft^3 = 14.06 lbs/min of air
If you're boosting, say, 8 psi... it's 200*0.070318*pressure ratio
= 200 CFM * 0.070318 * (14.7+8)/14.7 = 21.7 lbs/min
- this makes a BIG difference when you're reading a compressor map since you're interested in the mass of air flowing at full boost.
--------------------------------------------------------------------------------
EDIT: BTW the 500 CFM was supposed to be airflow under boost...
I dont know where you're going with that..... but you just made me realize something else. If a "NEWBIE" was to read #2 airflow rate he might get confusedm cause I didn't specifically say "BOOSTED ENGINE"S CFM" I just said "ENGINE CFM". I used 500 CFM cause that is the typical CFM for a H22 with 9-12 PSI of boost. I guess I assumed that everyone would know that the 500 CFM is from a boosted engine. But I think it might be possible that someone would mistake 500 CFM for a NA engine, I'm gonna change #2 to say CFM for a boosted engine.