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Hey Ryan,
As nateg & Jeff both said, you don't want to go with any stiffer spring than is necessary. In fact, on this part you want to head in the opposite direction as much as possible.
The DV's _ONLY_ purpose is to protect the turbo from back pressure. This duty not only keeps the impeller & bearings from being damaged, but also allows it to spin freely between shifts (eliminating shift-induced turbo lag). The way it does this is by engine-generated vacuum sucking the piston up off the inlet port when you lift off the throttle, & giving the pressurized air a new escape path when the throttle plate slams shut. If there was no escape path, the bosst pressure hits the closed throttle plate & "packs up" all the way back to the pressure source - the turbo - & then farther back beyond that. If this happens, the impeller gets pushed sideways in bearings that weren't meant to take that direction of load forces. If it happens often enough, the impeller starts to scrape against the turbo housing. And if THAT happens, the blades will probably shatter.
As you can see, this series of unfortunate events all occurs BEFORE the air charge ever reaches the engine. I think the fuel cutoff is rpm-based, isn't it? (Anyone??) In other words, the two "safety valves" (DV & fuel cutoff) are watching & acting upon different systems. Because of this, you can definitely do some turbo damage w/o the fuel cutoff ever kicking in.
Obviously, you need a spring which is strong enough keep the piston seated against whatever boost pressures you're running. If the spring's too weak, then you'll get boost leaking past the piston instead of going into the engine. (This is the "flutter" Jeff experienced with the yellow/green spring & resolved by using the blue spring.) Since you're relying on vacuum to suck & hold the piston up, you don't want a spring so strong that there's too short of a time duration (or worse, a failure) for that inlet port to be open.
So, the balance you're trying to achieve is a spring that's as weak as possible (allowing vacuum to snap open & hold the piston, giving the absolute quickest & most complete evacuation of boost back pressure), but as strong as necessary (keeping the piston in place under peak boost loads).
Boost leak could be causing the lower pressures you're seeing, but if you're already running the blue spring I wouldn't think it's being caused by too weak of a spring. Have you checked the piston nose/inlet port seat for any debris or irregularities which might prevent a good seal? (I haven't seen the insides of the FMDV004, so I don't know how similar it is to the FMDV007P.) How about leaks or gaps in other areas of the intake? (I don't know why you upgraded the DV, but if your old one was leaking & the new one doesn't it may have forced a new leak in a weak area.) Did you already say what it was boosting up to before this upgrade?
-Greg
posted by 146.82.1...
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