exhaust setup?

My apologies up front if this is not the correct forum for this question.If that is the case please can you advise where to post this question.

I have always understood that on any engine, and especially a petrol powered engine, that back pressure is required to avoid the exhaust valves burning on there seats.
In fact I have seen engines where this happened when they were run on a open pipe with out any type of box or restriction.
What I don’t understand is that as far as I can see, the engines that were in use during the 2nd world war in aircraft never seemed to have any silencing and therefore no apparent back pressure.Is this correct? and if so why did these engine not suffer from burnt exhaust valves?
thanks
Alistair