I did not tested it by myself. But from official documentation and some forum posts by official staff, over_voltage bigger than 6 can be only activated when force_turbo is enabled. It is even possible to go over official limits by using parameter current_limit_override. But you are risking voiding your warranty if anything goes terribly wrong...
Source: https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=6201&start=325#p170793
@sergioad Yes, I can confirm arm_freq=2100 is not viable too. measure_clock arm is reporting same strange frequency of 600 MHz as 2147 MHz OC for me.
Pi Case 40 is amazing. Totally passive and well build solution. It has slick power button from factory. Can throw it into my backpack without worries that something broke and play during brakes at work, during party at my friends palace... Just love it, worth of every penny.
I am observing really strange behavior when I am applying overclock in Recalbox using Pi 4 4GB and looks like @sergioad report confirms my findings.
If I set values below in config.txt
arm_freq=2000
gpu_freq=750
over_voltage=6
then I am getting correct cpu clock when checking back using terminal and framerate especially in PSP games is noticeably better
# vcgencmd measure_clock arm
frequency(48)=2000531200
But if I set maximum allowed Pi 4 OC like this
arm_freq=2147
gpu_freq=750
over_voltage=6
then I am getting only 600 MHz when checking back using terminal, that is significantly lower cpu clock than stock value and it confirms @sergioad report. Emulation is slow like hell in this case. I am using Pi Case 40 from Cooler Master and temps are below 60°C all time and boosting over_voltage is not helping at all.
# vcgencmd measure_clock arm
frequency(48)=600169920
I tried to update to last stable eeprom released 2020-09-03, then applied maximum OC using same method in newly installed Raspberry Pi OS and it was working like charm. Reported clocks were correct and stable. I was testing stability using synthetic load (sysbench) on all 4 cores and temperature did not exceed 65°C after 20 mins of continuous stress testing.
When I returned back to MicroSD card with Recalbox, then I got same strange behavior when applying max. OC. It looks like overclocking in Recalbox still have some issues and needs some tuning by devs.
But overall I am really satisfied with this OS. UX is on totally different (better) level compared to for example RetroPie.
@myjosito Just FYI: It should be possible to temporarily disable all overclocking by holding SHIFT key when Pi is booting according to official OC guide, no need to reinstall whole Recalbox.
→ https://www.raspberrypi.org/documentation/configuration/config-txt/overclocking.md