The nose is sharp and acrid: pungent diesel, sour funk and a metallic, chemical bite, the raw 'chem' smell at its source. The effect is heavy and cerebral — a fast, pressing head-rush over a weighty body, potent and a little overwhelming for the unprepared.

It's loud, divisive and historically important, a connoisseur's cut more than a crowd-pleaser. For people chasing the roots of the diesel family, Chem 91 is where much of it begins.