Really trying to learn this but sometimes I can be a rock.


Mass Flow Rate is the mass of air flowing through a compressor (and engine!) over a given period of time and is commonly expressed as lb/min (pounds per minute). Mass flow can be physically measured, but in many cases it is sufficient to estimate the mass flow for choosing the proper turbo.

Many people use Volumetric Flow Rate (expressed in cubic feet per minute, CFM or ft3/min)) instead of mass flow rate. Volumetric flow rate can be converted to mass flow by multiplying by the air density. Air density at sea level is 0.076lb/ft3

What is my mass flow rate? As a very general rule, turbocharged gasoline engines will generate 9.5-10.5 horsepower (as measured at the flywheel) for each lb/min of airflow. So, an engine with a target peak horsepower of 400 Hp will require 36-44 lb/min of airflow to achieve that target. This is just a rough first approximation to help narrow the turbo selection options.


Someone breakdown the formula and show me how they got 36-44lb/min.

 

Finally, a new person that has an insightful question.

The peak goal of 400 hp needs a turbo in that 40-44lb/min range, but this is an approximation of flywheel or brake horsepower. It will not translate into 400whp due to transmission and drivetrain losses (figure 20%).

My math is just this: 400hp / 10hp (per lb/min) = ~40lb/min

This serves as a rough estimate for your needs. Of course there are more in-depth ways of figuring out exactly what you need...for example's sake, this should suffice

 

Great thanks, thought my math was off.

next question

why wouldn't recirculate the BOV back to the compressor to minimize surge?
Or is the result the same venting it out?

 

As turbo speed increases, the pressure ratio increases and/or mass flow increases.


Now is this where the wastegate comes into play to maintain turbo speed, pressure ratio and/or mass flow increases.

 

Recirculation won't matter, as long as you have a properly sized and adjusted bov, surge will be almost non existent. Now at low low pressure or at the positive pressure threshold there will be light surge do to there not being a big enough pressure differential but it's minimal and of little concern

 

Ok, understand.

 

This one's more for clarification. I really know it is. thought it might help someone else.

 

With a MAP sensor, like Hondas use, there is no real need to recirculate the bov. For cars like my STi, which uses a Mass Airflow system, recirculating the bov avoids running rich during shifts (since the air has already been registered and appropriate amount of fuel added). The air charge is vented into the atmosphere, rather than the engine, but the fuel is added regardless.

 

And you dont have a MAF before the turbo.

 

On maf cars I run blow through, did it on my s13 with an sr in it... ran better then draw through... and blow through after the intercooler also means the maf more accurately ,measures the volume of air entering the motor as well as air temp... we saw a power increase when we swapped to blow through and the car had much more stable air/fuel ratios before I even touched my safc.