(Sorry if this question is posted in the wrong forum, I wasn't sure where best to post it.)

I am currently trying to work out a climate map for my world in as 'realistic' a way as possible (in other words using the processes that seem to the determine the earth's climate as a guide). However, a few questions occurred to me regarding the placement of rainforests.

According to most climate maps, the rainforest in South America seems to be restricted to the West Coast-side, and becomes Savannah-like nearer the East Coast. Is there any reason or geographical feature which prevents the 'tropical rainforest' from covering the whole of the equatorial area there? (I've read that in East Africa the rainforest is limited because of the high altitudes in the East).