go ahead a block them off, i always used them, but without the heat riser you want know the dif. we may be talking 2 dif. things, but the exhaust is gonna take the path of least resistance and that would be out of the tail pipe. the only reason the crossover is there is to heat the gas up on cold starts, and after the motor warmed up the exhaust goes out of the tail pipe. with the heat riser closed the exhaust had to go somewhere, so it crossed over to the left side.