Boost questions?

Question

So boost is measured by how much the air is compressed because when the volume decreases the pressure increases and if you increase the volume of air and compress it you have a higher boost pressure? Is this correct? How can you be producing more boost when at lower air masses than at higher air masses? Example 9psi@30lb/min 20psi@20lb/min How is this possible? When there is more pounds of air flowing wouldnt there be more boost because more air is flowing? Or does it depend on volume rather than pounds? Can you please explain this to me in detail?

Answer 1

Here goes.

Normal atmospheric pressure is about 14.7 psi (pounds per square inch). When a car is at wide-open throttle (WOT), it is getting air delivered at nearly 14.7 psi (but not quite, since no air delivery system is perfect).

When someone talks about turbocharger boost pressure (in the intake manifold rather than the exhaust), they are referring to the amount of pressure BEYOND atmospheric pressure that's being delivered into the intake manifold. So 9 psi of intake boost means the air is being delivered at a pressure of 14.7 + 9 = 23.7 psi into the intake manifold.

Now, for a given engine, more pressure in the intake manifold always means a higher mass flow rate of air (in pounds per minute). However, a 5-liter engine sucking air in at 14.7 psi has a higher mass flow rate than a 2-liter engine sucking in air at 20 psi, simply because it intakes 2.5 times as much air volume as the 2-liter engine at any RPM. The large displacement difference overwhelms the small pressure difference. However, if the 2-liter engine were somehow (miraculously) operating at 100 psi, it would have a higher mass flow rate than a 5-liter engine operating at only 14.7 psi.

So, as a rule, we can state that mass flow rate is

1. Proportional to displacement
2. Proportional to RPM
3. Proportional to intake manifold pressure

If you double one of these while cutting another in half, the mass flow rate will stay the same.

Hope this clears things up.

Answer 2

Lithium pretty well has it covered. In addition, you have to account for the temperature of the air for actual mass, when air is heated it expands, meaning you can be boosting 20 lbs of hot air or 15 lbs of cool air and the cool air will probably have more mass/make more power... depending on the difference between the two temps of course. Turbos, while being extremely efficient have a problem with heat, since theyre driven by 2000 deg exhaust. This is the purpose of intercoolers.

Basically all of this together means that boost is an arbitrary number. A turbo has an "optimal efficiency" range, and upping boost may not necessarily equate to more power, if the turbo is just dumping out hot air, or if it suddenly outflows the intercoolers ability to cool it, etc, etc, etc. Usually you can get a compressor flow map to see where (generally in cfm) your turbo will be most efficient.

Hope that helps.