I take it your Astra is either a Mk4 SRi/GSi Turbo or a Astra Coupe/Cabby Turbo (only models released with the Eco-tec Turbo) but shouldn't you have 184bhp rather than 170bhp as standard???
Let me know if you need anyhelp with making yours go faster as I will be more that willing to offer my services and theories
Chow for now and welcome to the site
BAD63R