The overboost thing is a real pain. The likely cause for this is that the stock twin exit design cannot flow enough exhaust gases to slow down the turbo... and hence boost goes wildly out of control.

The solution is to fit a replacement turbo - Turbo Technics S148 single exit wastegate variant. Direct bolt on, no hassles... then fit your huge exhaust etc etc

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Kris
95 ST205 - slept for 10 years, now waking up...
Avensis
http://www.gtfours.co.uk
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Turbocharging - the replacement for displacement