With each individual power pulse the crank is accelerated, then slows. As the cam comes up on a lobe it wants to slow down, and then as it comes over center heading off the lobe it wants to accelerate. These forces are at their worst at low RPM. As speed (RPM) increases they tend to smooth out, but they are there. Obviously with a V-8 firing four times per revolution these fluctuations aren't as great, but now the frequency is X4. I'm pretty sure most cam drives are slightly less than perfect (although belt is a pretty good bet), so a tensioner is likely a good bet...
