Adding by-pass valve (Turbo Injection) [1/2]
In case you don't know what a by-pass valve is here are some links that explains how it works:
Peugeot 505 Turbo Injection doesn't have a by-pass valve as standard equipment. However, the engine benefits from it especially if you're running it at higher than stock boost pressure and adding one is quite easy. A good and inexpensive valve is the one that's used in many Saabs and is manufactured by Bosch: Saab p/n 4441895 and Bosch p/n 0 280 142 110. It's widely available and costs only about 30 euros (36 USD).
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Here you can see the engine bay of a 1985 Turbo Injection (European model, series 1). The valve should be placed as close to the throttle plate as possible as it best prevents the forming of a shock wave in that location. There's a flexible hose connecting the intercooler and the intake manifold. The plan is to make a pipe connection for the by-pass valve there. You can use the original hose by cutting it half and adding a short pipe with a connection for the valve between the halves. You can also do as I did and replace the old hose completely.
To replace the old hose you'll need to buy or manufacture a suitable pipe with a connection for the by-pass valve and buy some new silicone hose. Here's the original flexible hose, the part made to replace it and some ugly colored new silicone hose. The intercooler has a smaller diameter pipe than the intake manifold so you should buy two different diameter hose. There are also transition silicone hoses available that allows you to adapt two different sizes of pipes. You can also probably find a size that fits to all parts good enough. I bought hose which inside diameter is 50 mm because that was the only usable size available at the moment.
The connecting piece is made of acid-proof steel and is designed so that straight pieces of hoses can be used in both ends. After the TIG welding was done I smoothed the seams inside and polished the part. The result is quite nice and fits almost perfectly. It could be done better by using different size of pipes that matches the intercooler and the intake manifold better etc. but it was made of the material that was available.