We dynoed my engine with a 1” open spacer but when I was putting the hood on I had to remove it for clearance then I asked the builder what I lost, he said the spacer was worth around 20hp, BBCAn old friend in the local Chevelle club had an 18k mile unrestored original LS6. They pulled and freshened up the engine since it had sat for 35+ years, and put it on the dyno before they put it back in the car.
All stock it made 452hp.
Headers and a 1" carb spacer made 60hp gain to 512hp!