Boost valve or Free Boost Upgrade? Which is best?
Figure 1 will help you locate where the boost will need attaching, along with the relevant pipes.
The red arrow in Figure 2 indicates the pipe which contains an inbuilt restrictor to control the amount of boost produced by the turbo. There are two options to achieve a huge power gain. The first is known as a Free Boost Upgrade (FBU) due to it costing virtually nothing. The restrictor pipe simply needs replacing with a normal pipe. This modification must be run in conjunction with a boost gauge due to the boost pressure being totally unrestricted. Too much boost (say, over 15 psi/1 bar) would make the turbo very susceptible to power spikes which could permanently damage it. This makes the second choice a much safer option.
A much more controlled option is to fit a boost (or bleed) valve (this particular one is an Apex Performance boost valve). It is a good idea to fit a boost gauge in this instance too. However it is not necessary, so long as you have a means to accurately measure boost (such as a Rolling Road session). Remove the T connector joining the restrictor pipe to the other two pipes shown in Figure 2. The restrictor pipe needs blocking using something suitable (a pencil is good!). Adjust the boost valve so that it is about half open then insert it between the other two pipes, as shown in Figure 3. It is best not to run boost higher than 15 psi/1 bar without upgrading other components. If the boost is too high it is quite likely that the engine will encounter fuel cut-out.
It is recommended that you upgrade your spark plugs to a colder rating such as 7 or 8.
I gained an additional 24 bhp from this modification, bringing my S14 200SX in line with the Japanese spec 220 bhp version.