Test timing meant that both this month’s long-term John Deere 6920S tractor and our earlier 6920 AP test tractor (profi 02/03) used the firm’s ‘old’ twovalve engines. Since then, Deere has introduced its new four-valve common rail (4V-CR) unit for the top-end 6020 tractors, promising a performance boost while complying with latest emission regs