This K&N Stubstack is designed to be used on Holley 390-850 cfm square bore carbs (4150/4160), that have had the choke horn milled off, but do not use an air cleaner. The full radius design helps to increase the airflow for drag racing. Will also hold down the base plate of a cold air scoop. Will not work with K&N Custom Chrome Racing Air Cleaners. The K&N Stubstack is designed to increase the airflow of carburetors by decreasing the restriction around the choke horn. Stubstacks fit inside the air cleaner housing and slide snugly down over the choke horn.