Been discussed a few times, so i will put some links on for a read rather than re-post text

https://passionford.com/forum/ford-escort-rs-turbo/193139-turbo-timer.html
https://passionford.com/forum/ford-escort-rs-turbo/173190-please-help.html
https://passionford.com/forum/ford-escort-rs-turbo/63651-blitz-turbo-timer.html
https://passionford.com/forum/ford-escort-rs-turbo/60287-which-turbo-timer-is-best.html
https://passionford.com/forum/general-car-related-discussion/28924-turbo-timers.html

Hope they help.