GFB DV+ TMS Diverter Valve (part T9301) - Manual (Page 2)

Spring Pre-Load Adjustment
The dv+ can be installed and used directly out-of-the-box without any adjustment to the
spring pre-load at all, regardless of the boost pressure your car runs.
So if you are not inclined to tinker, you can finish the installation here, or you can read on
for more tech info on the subject!
Unlike other aftermarket Bosch replacement valves on the market, the GFB dv+ DOES NOT require
spring pre-load adjustment to suit specific boost pressures. The pressure-balancing design of the dv+
means that under wide open throttle conditions there is equal boost pressure on both sides of the
piston, so it will stay shut under boost REGARDLESS of the spring setting or the boost pressure.
However, adjustments to the spring pre-load can often help improve throttle response and reduce
lag, so it can pay to experiment.
Adjusting the spring pre-load changes how easily the valve vents when the throttle is closed. The
best throttle response is typically found when the spring is set to the firmest setting possible that
does not cause compressor surge (turbo fluttering) at when the throttle is closed on a high
boost/high RPM gearshift.
To explain further, contrary to popular belief, venting as much air as possible to “let the turbo
freewheel” does not reduce lag. Perhaps 20 years ago it may have been true, but turbos these days
spool up very quickly, and the greater benefit comes from setting the valve up to keeps as much
pressure in the intercooler as possible during a gear-shift or brief throttle lift.
This is especially true when a large front-mount intercooler is fitted – venting all of the air means the
intercooler must be re-pressurised, and even though a turbo does shift a lot of air, it still takes a
measurable amount of time to fill an intercooler.
Here’s where adjusting the spring pre-load can help. Increasing the spring pre-load can help retain a
small amount of pressure in the intercooler during a gearshift, which leads to a faster return to peak
boost. The limiting factor in how much you can increase the spring pre-load is compressor surge
(turbo flutter). Once this starts to occur at high boost/RPM, there is no further gain to be had and for
the life expectancy of the turbo it is best avoided.
Note that it is common however for compressor surge to occur at low RPM/boost, even if it doesn’t
occur at high boost/RPM. This is not really a concern for the turbo because the shaft speed and loads
on the turbo at this point are much smaller, and the pressure spikes from compressor surge are
much lower than those experienced at peak boost.