It's like gettin' boosted with out the huffer. Ram effect in simple terms is the air and fuel that continues to enter the cylinder even after the piston has reached BDC and is on its way up. All along while the intake valve is closing. This stuffing of the cylinder is caused by velocity of the air fuel column. The more velocity we have the more mass this column has which allows it to over come both items that are trying to pervent it....intake closing and piston coming straight at it.
All engines have this some what but 2 things will always inhance this: Piston speed and restricted inlets. Both of which increase the velocity of the column coming into the cylinder.
RAM happens in different rpm ranges depending on piston speeds at given rpm. The % of increase that RAM will affect the engine is also enhanced by the amount of compression. Effectively engines in the 9 to 10.5 compression range with a piston velocity of 3500 to 4000 FPM will gain power over an enigne that is 10.5 to 12 to 1 with the same piston velocity. Reason is this type of mechanical compression neturalizes the column much quicker.
So with low compression boat engines that are strokers, a 4.25" stroke BBC takes advantage of this around 5000 rpm, RAM can play a big part in making extra FREE power in the mill. Simple valve event adjustments on cam timing can play a big part in gaining 5% or more in power.