If your car is primarily driven on the street and used during colder weather, I personally would leave the cross-over open. I blocked the cross-over in a 383 powered car I had. The change made it very "cold blooded." If the car is equipped with a factory type thermostatically controlled choke, the choke will take way too long to pull off. If you have an electric choke, it will pull off too soon. Plus, blocking the cross-over didn't make any noticeable improvement after the engine was "warmed up". I think this is one of those questions to which the answer is, it depends.