Bosch Blow Off Valve


      To help reduce loss of boost between gearchanges I have fitted a Bosch blow off valve. Unfortunately until the ST205 was released the GT-Four never had one standard, and short ducting makes one hard to fit. This is the same one as fitted stock to the Sierra Cosworth and some Saabs. I chose this valve simply because it was cheap! It cost NZ$58 (about US$30).

Although small, the difference on hard 1-2 and 2-3 gearchanges can be felt. The piping is a decent size (25mm) and the experience of friends with Supras suggests it is OK up to 'around' 17 psi boost.

Another cheap(ish) option is the larger plastic one as fitted to the 1997 Subaru WRX, which appear to be more sturdy.

Although the photo is not clear you can see I've tapped off the silicone hose joining the turbo to the intercooler. An aluminium sleeve was machined up with a 90 25mm pipe coming off it towards the BOV. Although this sleeve is only bonded to the aluminium, the design means boost pressure results in a better seal. High pressure hose clamps (which won't break like normal hose clamps) hold everything tight. A one piece silicone hose with a smaller pipe moulded in would be much easier, but was unavailable locally.

A steel block with a hole bored through it acts as a connection to the low pressure side of the turbo. This block was brazed on to the sheet metal inlet duct. The BOV fits in between. As seen in the photo it is around backwards when compared to the stock Ford set up, but still works fine. It is a very tight fit under the bonnet though, hence a metal tube has been made up to allow the vacuum tube to exit parallel to the bonnet. If the tube exited straight out from the BOV (as intended), the tubing would be squashed. In its current position it has worn a hole in the underbonnet lining.

The metal parts took several hours to make but an easier option would be to just weld a 1" tube into the larger pipe.


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