First which OS .18 the CV-R or the TZ? All so what have you done as far as upgrades? I am guessing you have the RTR MT2? The TZ is a MUCH more powerful engine then the CV. There are things I would recommend if you put either in if you have not done so far and a whole lot if you run the TZ. But it depends what upgrades you have done. I have never own the HPI .18SS engine but I have owned quite a few HPI and OS engines. OS is about as user-friendly engine as you can get. I have heard that the HPI .18ss runs on the hot side but do not know for fact. Here in the states we can get the OS cheaper then the HPI. All and all I have had good luck with both OS and HPI engines.

I am getting ready to drop an OS .18 TZ in my HPI MT 1 ¾. I now have an OS .15 CV-RX in it which they made before they started making the OS. 18 CV-R.