Remember that boost is a measure of restriction, not air flow.

Also a larger turbo at the same psi as a small turbo will flow more CFM of air.

 

Part of the reason the hx35 doesn't do very well at higher boost pressures(doesn't cool well) is because the stock exhaust housing is too small to allow that much airflow. The housings are small so the turbo has good spool up and better low rpm performance. However that hurts you when you start running higher boost pressures and running high rpm both increasing airflow. The exhaust gases can't get OUT of the engine as there is a lot of restriction in the EXH housing.

Back pressure and drive pressure are basically the same thing. The difference is drive pressure is back pressure between the combustion cylinder and the turbine wheel. It is called drive pressure because it is the force that turns the turbo. Most people claim that a 1:1 ratio from boost to drive pressure is ideal. but anywhere from 1:1.2 to 1:1.5 is probably acceptable. Especially on a street driven truck.

If you watch a boost pressure gauge and a drive pressure gauge and start to increase boost pressure(more fuel putting the engine under load yada yada) you'll see the ratio of them stay in the 1:1 to 1:1.2 range. However as boost pressure and airflow increases the size of the exhaust housing as well as the design and size of the turbine wheel starts to restrict to the airflow coming out of the engine. This is when the ratio starts to change rather rapidly.

This is usually where a lot of people set their wastegates as increasing the boost pressure much beyond this point doesn't gain much for HP because of the restriction caused in the exhaust manifold. Make sense?

But remember that point changes based on rpms and boost pressures. For instance, an engine at 30 psi at 2000 rpm is not flowing the same amount of air as 30 psi at 3000 rpm, assuming Volumetric efficiency doesn't change. But I really have no idea of what the volumetric efficiency of a cummins head is at different rpms. All I know is that it sucks.

 

very good info fellas. think i know know. very confusing when Drive Pressure is talked about but once explaind very simple. Should put some of these explinations in a sticky for the others that might be as slow as me ha ha.