Fabio
What engine are you using ? as I am using G-62's in both of my birds with really short volicity stacks (spelling) and have not had any problems with the cowl, also do you have a dummy engine in the cowl? if not you may have to put in an air dam. one other thing you may not have enough air exit area.
Pic's of your set up might help in getting a good answer.
Just my thoughts on your ?
Cheers Bob T