Turbo guru question - effect turbine speed

Question for the turbo guru's

I'm interested to understand the effect higher turbine speed has when comparing 2 different compressors

For example a G30 660 vs G30 770 turbo both have the same turbine, but at 2.5 P/R (1.5bar boost sealevel) and 55 lbs/min airflow, one is doing ~145k rpm while the other is doing ~130k rpm.

What effect would this have on turbine flow and EMAP?

With the intake airflow being the same would the effect be the faster spinning turbine wheel will be flowing more and so have reduced emap ?

(Both compressor maps attached)

E Map will be higher, as you are using more drive pressure from the exhaust gas to get the turbocharger to a higher speed. The turbine is driven by the exhaust gas - it isn't working as extraction.

The other item that you aren't considering here is the affect on the intake side. With the G series turbos - particularly when you start approaching the turbo shaft speed limit is that the compressor side really starts to heat up the charge air.

In a recent project we were running G35-900 turbos, we ran into an issue where to run our ideal scramble boost, we were exceeding our shaft speed limit. Worse than this, we were seeing intake air temps of 65C by the end of the front straight at SMSP, despite the intercooler being rated well over the power of the engine.

We switched over to G35-1050 turbos, we shed 20,000rpm of shaft speed and almost 25C of inlet charge temp for the same boost pressure down the front straight.

Thanks, yeah that was also a factor I was also unsure how it fits into the equation.

How much did the emap change when you did the g35 swap.

Yes agreed the compressor efficiency is also a factor, I am just trying to get an idea of the effect on the exhaust side.

According to compressor and turbine maps the difference at outlet air temperature isn't that big. At 2.5PR and 55lbs air mass flow 30-770 would be working at 75 percent adiabatic efficiency whilst 30-660 would be working at 70 percent. If ambient temperature is 25 degrees C it would result in 144 degrees C compressor outlet air temperature for 30-770 and 152 degrees C for 30-660. Providing that turbine housing AR is the same on both turbochargers there shouldn't be any significant difference in EMAP. But this would be true only for PR2.5. The biggest advantage of 30-770 is that it has higher potential for higher PRs. This is where it would shine. With 30-660 you could face a situation as described above when you need more boost but limited by adiabatic efficiency and you get nothing but very hot air not making any power and promoting knock.