I don't think you do because they are not directly related. You can decrease boost and still increase cylinder pressure. You can increase boost and still increase cylinder pressure. Are you getting it yet? A bigger turbo will increase cylinder pressure with lower boost. Remember that the motor makes the power, not the turbo. The turbo just provides the motor with more air to make power with. The turbo, hence boost, does not make power. The motor makes the power and how much power it makes is directly related to volumetric efficiency, capacity, cylinder pressure, etc. Boost is just a measure of how much pressure is in your vaccum lines really. A larger turbo can increase your cylinder pressure (more power if harnessed) with lower boost because it is still moving more air than the smaller turbo. A motor is just an air pump and nothing more. The more air you can get through the motor safely, the more power you can make. Boost is not a measure of this. And it is not directly related.

 

It's never too late.

This may be a little technical, but Jeff Lucius put together a nice page about this: http://www.stealth316.com/2-3s-compflowmaps.htm

Dave

 

I did not know that boost is just a measure of how much pressure is in the vaccum lines.
I was thinking the pressure would equalize itself in all parts of the engine.

 

THANK YOU djh

I just scanned that article quickly and it seems that it does come down to
heat and air density.
A bigger turbo can produce more volume with less heat.
The confusion was between volume and density.
That is a great article, it does a good job of explaining.
Thanks again.

 

No offense taken, should have said density



Good article

 

Another way to look at boost(pressure) vs flow is that you can have a pipe with both ends closed and it can have 30psi of pressure, but you are not getting any flow, nothing goes in and nothing goes out. You can also have a pipe that's open on both ends, and have 30psi of pressure, but now you have flow (from high pressure to low presure).

-ob4

 

EVOTEXAS, if you are referring to post-combustion cylinder pressure, you are correct. If you are referring to pre-ignition cylinder pressure, this is not correct.


This is only true if by "more air" you mean more air mass. You are not moving much more volumetrically into the cylinder. Say for the sake of this "comparison" the two turbines make the same backpressure at the same boost level, but the "bigger" compressor is more efficient. The pre-combustion cylinder pressures will be the same in both cases. You will have moved more air mass into the cylinder with the more efficient turbo however, because at the same pressure the air density is higher with lower discharge temperature. The "bigger" turbo in this case will move more air mass into the cylinder, and assuming the ECU correctly senses this and temperature compensates (increases) the fuel pulse (and assuming the fuel system can keep up), more power will be made. Despite the two pre-ignition cylinder pressures being the same, more reactant molecules were combusted and the post-combustion cylinder pressure will be higher, making more power.


True that boost is neither a direct measure of power, nor is it a direct indicator. However, it is a first-order contributor to things like mass flow rate, which do directly determine how much power the engine can make.

With respect to the motor as an air pump analogy, I think it is misleading, though extremely common. A pump absorbs power to move a volume of fluid by increasing its pressure. On the other hand an internal combustion engine converts energy stored in chemical form in the fuel to kinetic energy, thereby "making" power. For maximum efficiency you would want to leave as little residual pressure in the waste gas as you could while still scavenging and filling the cylinder well. Sometimes, it's best not to oversimplify, otherwise it leads to questions like the one that started this thread (again, no offense meant to anybody). Now, if you look at what a pump is, you'll realize that we have been talking about a "true" pump -- the turbocharger compressor.

 

While I do not disagree with anything you said, sometimes it's best not to overcomplicate things when answering a question such as the one that started this thread. I am a big fan of KISS.