I've been playing around with this for a while now, and here's my opinion. For driving around town or a B road blast, low-ish revs and many gear changes, removing both pieces of foam sounds the best. This is almost unbearable at normal motorway speeds (70mph), where removing only the bulkhead foam works out to be the best option. Anything over about 115-120mph, however, and it's best to have both pieces fitted, or wear ear-plugs. It depends what sort of sound you're after, and that will be affected by how and where you drive your car the most. Take from this what you will, but I've just had to refit BOTH pieces of foam... :driving: