Correct that cowl hoods only work at low speed. At higher speeds you have high pressure at the base of the windshield, making a cowl ineffective. On our race car we have an extractor vent cut right behind the radiator, and a home-fabbed scoop 10" deep that hangs down right in front of the motor, directing the air coming out of the radiator over the top of the hood. If you're trying to fit turbos, there's not much room for that. I do know a guy who turbo'ed the KA24 in his 240SX and was cooking everything underhood. He put some hood louvers over the turbo and it made a significant difference.Cowl hoods absolutely allow hotter air to escape the engine bay at idle and slow speeds. As speed rises cooler air is drawn in through the cowl, but that only happens at speeds high enough to lower air pressure at the base of the windshield.