Turbo's have a range of efficency. This is what compressor map's show you. As you leave a turbo's efficency range, exhaust temp's rise. As exhaust temp's rise, the intercooler has to work harder to drop the charge temp down.
As charge temp's increase, power begins to drop as the engine will start to pull timing to prevent detonation.
It is actually possible to increase boost on a car, and increase power in one section of the graph (where the turbo is most efficent), and subtract it everywhere else.
Here's an example. This compressor map is "similar" (
or the same as, depending on who you argue with) to the turbo in the MPS 6.
(
Hitachi-Warner K04 single-scroll turbocharger)
Pressure ratio is calclulated by Absolute pressure/Atmospheric Pressure or 15.7 PSI+ 14.7 PSI/14.7 PSI
CFM need is...
CFM = (L x RPM x VE x Pr)/5660
CFM= (2.3 x 6500 x 90 x 2.068)/ 5660
CFM= 491.6
To convert CFM to CMS
http://www.turbos.bwauto.com/en/products/passenger_gas.asp
If you calculate for 19 psi (about a 4 psi jump...), you get a pressure ratio of about 2.3 (vs 2.06 stock). Plug this in to yee old formula in below mentioned thread...(and below mentioned books)...you get a CFM requirement of around 546, or 55 additional CFM required by the turbo.
Convert CFM to CMS and you get 0.257.
Plug in 2.3 on the Y axis, and 0.257 on the X axis, and you'll notice you aren't even on the chart anymore.
So lets drop it back down to 5,000 rpm at the same psi levels. 378 CFM stock, (.178 CMS), and 420 at 4psi higher (0.198).
Lets bring up the graph...
Edge of the efficency chart stock, off the chart "pumped up". On this particular compressor map, you'd have to drop all the way to around 3500-4000, to even get any benefit from more boost. Otherwise, you're so far out of the efficency range, all the turbo is doing is blowing hot air. Increasing psi out of a turbo's efficency range results in minor increases in power, but massive increases in exhaust and charge temps, increased chance of detonation, and increased wear to the turbo.
Sources of more information.
Forced Induction Performance Tuning
https://www.amazon.com (commissions earned)https://www.amazon.com (commissions earned)
Maximum Boost
https://www.amazon.com (commissions earned)
https://www.amazon.com (commissions earned)
I highly recommend anyone even considering modding the MPS6 pickup or borrow these books (check your local library). The concepts behind a turbo system are relatively simple, but jumping into tuning without understanding the result of your actions is always a dangerous approach.