There are seven studs on top and six on the bottom. The tops are no-brainers, you can see them. You are probably missing one of the lower ones and likely in the middle. They are a bit of a booger to access. It takes a socket with a universal joint and an extension to reach them. I have to do them by braille to get them out.
Also, that intake/exhaust is very heavy, right at fifty pounds. If you don't have a helper to support the weight it can bind on the last nut or two. I leave the upper center nut in place till all the others are off so it holds it in place, then remove that last nut.
Using your cell upside down to take a video of the lower bolts should show you where the last one is hiding.