To add a bit more for comparison between the Delta S4 and the GP cars of the time, the pole time for the 1986 ETCC 500km Estoril was 1m 47.36, set by Walkinshaw / Percy in a TWR Rover SD1 Vitesse. The Rover was RWD, had about 340 BHP and weighed maybe 1200Kg In 1988 the pole time was 1m44.33 by Hahne / Allam in a Sierra RS500, RWD, 520BHP and 1100Kg. Downforce levels for both cars were likely a bit lower than the Delta S4, even for the whale tailed Sierra.
Personally I reckon the Delta with Henri at the wheel, with the longest gearing available to get top speed down the S/F straight (top end on the longest ratio maybe 140mph?), no co-driver and a tarmac setup, could have got into the late 1m30's in the dry (and not much slower in the wet!) Still an age from a dry '85-'86 F1 qualifying time, but would have been impressive to watch.
One way to approximate it would be for someone to set a laptime in a sim such as Rfactor - I think the S4 is available as a mod somewhere, but i'm not sure how accurate the physics are.
Thanks to Frank de Jongs excellent website for the ETCC car and time data
Edited by nmansellfan, 14 July 2014 - 12:20.