Mat,
my car started as a 2004 SV 140bhp k series 9 years ago
changed to k maps to “add a little bit”
no mot issues
then 4-2-1 exhaust “to add a bit more”
no mot issues
full vvc engine rebuild last year and now 217bhp ( have a look at my blog)
no mot issues
If you need to know how much things cost in the current market (last year) have a look at my blog
It make the upgrade route illogical, better to sell and upgrade the car
but, when did logic ever get in the way of emotional attachment to a car 😀
I had my exhaust coated Matt black by Zircotec in Abingdon in 2018 no stone chips
No idea if it improves performance, but i squeezed 216bhp out of a 1.8k series at Emerald.
Taken from my 1.8 2004 X power 140bhp engine as I have now changed to emerald
the immobiliser was switched off and it was set with “pops and bangs”
Seemed to make a difference to the feel of the car and was great on the over run
£200