Here's a way to have the best of both worlds:
Use a large diameter "Y" (like Kenne Bell use to sell), with one leg going to pick up cool outside air under the bumper. The other leg has a large one-way check-valve (like a 3" diameter, spring-loaded flapper dryer vent) open to the engine bay and arranged such that low pressure will open the flapper at low speeds. Once the outside air pressure builds up at speed, it should be sufficient to feed the engine, causing the check valve to close, thereby introducing cool air to the engine.
Use a large diameter "Y" (like Kenne Bell use to sell), with one leg going to pick up cool outside air under the bumper. The other leg has a large one-way check-valve (like a 3" diameter, spring-loaded flapper dryer vent) open to the engine bay and arranged such that low pressure will open the flapper at low speeds. Once the outside air pressure builds up at speed, it should be sufficient to feed the engine, causing the check valve to close, thereby introducing cool air to the engine.