The Mini’s a lot louder than I expected, with a couple of really nasty resonant frequencies; at idle, of course, and around 80kph in top gear. A closer inspection reveals that the clearance between the air box and firewall at the back of the motor has disappeared, and reappeared at the front between the starter motor and oil cooler. Looks like the engine has settled a bit further back than I anticipated, and I suspect engine vibration is being transferred from the air box directly into the firewall. Gah.
Fixing it is (relatively) easy- make a longer top engine steady. But first, I need to know how much the engine moves while driving, so I don’t end up damaging anything else.
I’m sure this is how professionals do it:
After a short, but enthusiastic drive:
Ok, the air box moves about 5mm, mostly backwards, pivoting about the engine mounts which are near the bottom of the engine. Easy. Make new engine steady, install new engine steady, freak out a bit about how tight everything is.
The old one was painted. Paint takes too long to dry. So the new one is zinc plated. Bling!
…and about 6mm between starter and oil cooler. Which is halfway down the engine, so should only move half as much as the airbox. I hope. Maybe I’ll look at moving the oil cooler forward a gnats.
The result is a bit less impressive than I hoped. The vibration is a lot better when accelerating hard, and a couple of the more strident rattles have disappeared from the instrument cluster, but there’s still a nasty boom at 3000rpm and at idle. Hmm.