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RotaryProphet · 12-11-2009, 11:24 AM

Again, we're not talking about total potential, we're discussing turbos in usage. Any two turbos that, on the same motor under the same conditions can make the same boost are flowing the same amount of air. Putting a bigger compressor onto the same engine doesn't flow any more air unless you raise the boost level. This is because the engine is only capable of consuming a static amount of air/fuel mix per revolution.

Let's take a hypothetical 12a engine, which has a static displacement of 550cc x 2 per revolution. With a hypothetical 96% VE (which means that in any given revolution, the chamber is 96% filled with a "good" air charge; both inadequate or over-aggressive port timing will hurt this, as the chamber either doesn't have time to properly fill, or as exhaust finds it's way into the chamber in place of fresh air.), at 7000 RPM, the engine is consuming 7000 * 550 * 2 * 0.96 cc of air per minute, or 7,392 liters, or 261.046 CFM.

As volume and pressure are two different measurements, this volume will be reasonably constant at a given pressure and engine operating range; that is to say, no matter what the compressor, at a given 15 psi and a given 7000 RPM, the engine will consume the calculated volume.

So what we have is, 260 CFM at 15 PSI (~200 kpa), which is of course much denser than 260 CFM at atmospheric pressure (~100kpa). Of course, the turbo has to ingest and compress somewhere less than twice that much air to pressurize it to that level, and this number changes. A more efficient compressor will have to consume -more- air to achieve a given pressure than a less efficient one, by virtue of the fact that the less efficient one is heating the air more, resulting on a less dense charge, which takes up more space, and thus achieves higher pressure with less airflow.

Perhaps this is what you were trying to imply, and if so, I apologize, but if so, you didn't make your point well.

12-11-2009, 11:24 AM	#50
RotaryProphet Rotary Fanatic Join Date: Sep 2008 Posts: 147 Rep Power: 17	Again, we're not talking about total potential, we're discussing turbos in usage. Any two turbos that, on the same motor under the same conditions can make the same boost are flowing the same amount of air. Putting a bigger compressor onto the same engine doesn't flow any more air unless you raise the boost level. This is because the engine is only capable of consuming a static amount of air/fuel mix per revolution. Let's take a hypothetical 12a engine, which has a static displacement of 550cc x 2 per revolution. With a hypothetical 96% VE (which means that in any given revolution, the chamber is 96% filled with a "good" air charge; both inadequate or over-aggressive port timing will hurt this, as the chamber either doesn't have time to properly fill, or as exhaust finds it's way into the chamber in place of fresh air.), at 7000 RPM, the engine is consuming 7000 * 550 * 2 * 0.96 cc of air per minute, or 7,392 liters, or 261.046 CFM. As volume and pressure are two different measurements, this volume will be reasonably constant at a given pressure and engine operating range; that is to say, no matter what the compressor, at a given 15 psi and a given 7000 RPM, the engine will consume the calculated volume. So what we have is, 260 CFM at 15 PSI (~200 kpa), which is of course much denser than 260 CFM at atmospheric pressure (~100kpa). Of course, the turbo has to ingest and compress somewhere less than twice that much air to pressurize it to that level, and this number changes. A more efficient compressor will have to consume -more- air to achieve a given pressure than a less efficient one, by virtue of the fact that the less efficient one is heating the air more, resulting on a less dense charge, which takes up more space, and thus achieves higher pressure with less airflow. Perhaps this is what you were trying to imply, and if so, I apologize, but if so, you didn't make your point well.