I thought it would look really 'crappy' but actually not so bad! (I must admit a strip of black correx would be even neater - if I can find some) But the pipe insulators do the job...
As I drove home tonight I used Hybrid Assistant to keep an eye on the temperatures - both in town and on the dual-carriageway. Even with the greatly reduced air through the grill, ( the lower grill is almost totally blocked off) the engine temp can drop very quickly if you slow down, or start using the h