The aircooled flat-four in the Citroën GSA had a grapefruit-sized resonance chamber on one exhaust manifold to match up the exhaust pulse timings between the le

The aircooled flat-four in the Citroën GSA had a grapefruit-sized resonance chamber on one exhaust manifold to match up the exhaust pulse timings between the left and right bank. It also revved to about 8000rpm, which was hilarious away from the traffic lights.