Originally Posted by Jeff@TopGearSolutions
Boosting at 19 PSI at 3500 RPM and boosting at 19 PSI at 7000 RPM are two very different values in the realm of the turbo world and efficiency.
We have to remember 19 psi doesn't tell us anything about how fast the turbo's are actually spinning.
Remember that 19 psi is actually a so called back pressure in the engine.
Here is a basic way of looking at boost pressure:
At low RPM there is only so many revolutions where the engine can use the air given. By no accident as the RPM's increase the ability to feed more air is possible, and more of it and at increasing rate.
Lets say you have a fan blowing into the engine at a set rate of 300 CFM.
Using arbitrary numbers at 3500 RPM you may see 3 PSI.
However, if you increase the engine speed to 7000 RPM, keeping that same CFM you might only see 1 PSI. That is because there is less back pressure as engine RPM increases you can utilize that air.
The same principal is here.
The turbo's arent actually spinning any faster to make 19 PSI at 3500 RPM. In fact they may be spinning less at 19 PSI 3500 RPM then at 14 PSI at 7000 RPM. (I dont actually know I'm not looking at an efficiency graphs, just trying to make a point).
In a real world scenario:
My last car was supercharged.
As RPM's increased so did the speed of the blower.
Under normal circumstances so would the boost pressure.
However, if you altered the intake valve timing for more or less overlap that would change the actual boost pressure the motor was seeing. This overlap or lack there of is a restriction and by no accident you could inflate or deflate this number depending on the restriction.
I hope one of these examples can illustrate that boost pressure is a measure of restriction and not a measure of actual turbine speeds.
Same can be said with full throttle at XXXX rpm in 2nd gear versus full throttle and the same XXXX rpm in 5th gear. The loads on the engine are completely different because of the gears and the speed of the vehicle, so therefore the exhaust pressure to spool the turbo will be very different as well.