Turbo PSI vs. Boost

[forrest11479]

Ok , everyone talks about airflow and max psi for psd (40-45psi before gaskets blow) but psi is pound per sq. in. wouldn't you want to measure max flow in cfm cubic ft. per min.

for ie. if you force 1200 cfm through a 3" pipe you have a psi fo let say 35 pounds. if you pass that same volume of (1200cfm) air though a 4" pipe you should have a psi of some thing like 20 pounds.

my point is that you can't compare psi from one turbo to the next _ htt to gtp38r, to he2. to make a proper selection of turbo for my aplication i need to know cfm right?

[diesel_doctor]

Well no, you need the compressor map and what kind of PSI your looking for.

The air is measured in lbs/min at a certain pressure ratio. That is the outlet pressure/inlet pressure. Then you want the turbo to stay above 65% efficent and under the max RPM limits.

I found a nice little free program that caluates most of this for you, then with a compressor map of the turbo in question you can check it.

[96strokin]

Well you got the right idea for the most part. However most likely the intercooler pipes aren't the restriction that is makeing the pressure so increaseing the size of them isn't going to drop the psi. The biggest restriction is most likely the valves. There are also other factors to factor in. With compressor efficiency you have heat. The more efficient the cooler the air will be so there will be more mass in the same volume. So with a larger turbo that is more efficient at the higher boost levels you may have the same pressure of air but you will have more mass of air as the air will be cooler and denser. So if you have a ton of fuel that isn't being burned due to not enough air the added mass at the same pressure will make more power and lower EGTs.