Hope this helps people to understand what BOV's and re-circulating valve's do.
Should I get blow-off valves or not? Do they really help?
The Great Blow Off Valve Debate:
First of all to understand the functionality and benefits of a blow off valve you have to have a complete understanding of how a turbocharger works. If you don't understand the basic mechanics of a turbocharger you can forget about trying to understand the benefits of a blow off valve. If you do understand the mechanics of a turbocharger, read on...
An Explanation of Compressor Surge:
The turbine in a turbocharger in its ideal mode is spinning at a very high rate and therefore generating lots of pressurized air that is fed into the OPEN (wide open throttle) intake manifold which translates into more power, but what happens when you take your foot off the gas when shifting between gears? The turbine in the turbo is still spinning at a high rate but now you've taken your foot off the gas and the intake manifold is CLOSED. Now the turbine spinning at a high rate and is trying to push out all this compressed air but since the manifold is closed, that compressed air has nowhere to go. What happens next is that the turbine slows down because of this pressure build-up between the compressor and the intake manifold. Since the pressurized air can't go forward into the engine it heads the wrong way back towards the turbo where it slows the turbine down (compressor surge). After you shift gears and you step on the gas again you want to have the turbine spinning fast so that its producing boost but since you took your foot off the gas and caused compressor surge, it slowed down, so you have to wait for the turbine to "spool up" (spin fast) again. This delay waiting for turbines to spool up is what causes some of the performance degradation that were trying to avoid.
The Blow off Valve:
What if we could figure out a way of eliminating compressor surge? Then the turbine in our turbo would have a much easier time keeping its speed up and we wouldn't get the lag in power waiting for the turbine to spool up between shifts. Enter the Blow off valve. At that moment in time between shifts when compressor surge is slowing the turbine down we'll just poke a hole in the tube running between the turbo and the intake manifold and let all that air leak out somewhere. The turbine can keep spinning fast & we'll have lots of boost when we step on the gas again. We'll also want to plug up that hole we made because we have a purpose for that pressurized air now, where as when we let off the gas we did not. The blow off valve is a gizmo that's constantly opening and closing giving you that cool sneeze sound when it "pokes" that hole in the intake manifold tube to keep the turbine spinning fast. We've eliminated compressor surge but we did confuse the engine just a bit since the ECU already told the injectors to issue fuel to be used for the air that we've now thrown out. Other than that sounds like a good idea right? so what if were running a bit rich for a moment, no big deal. Before you place your order read on...
The Re-circulating Valve... a better idea:
How about instead of poking a hole in the turbo to intake manifold tube and just letting the air leak out wherever it wants to, we could direct that pressurized air somewhere else where we could hold on to it until we need it again? The intake side of the compressor housing (between the intercooler and the turbo) would be a good place wouldn't it? that way we wouldn't slow the turbine down and we would have that pressurized air that would have otherwise been thrown out, ready in a really good place when we need it. On top of that eve got still got the correct air/fuel ratio going since we're not throwing and air out, just moving it from one place to another within the intake system. Instead of having to suck in air for the turbo to pressurize, we've got some pressurized air with the correct fuel amount already waiting. Now we've got all the benefits of a blow off valve and then some. That's a re-circulating valve. Let's all order re-circulating valves then and skip the blow off valves completely. They're probably gonna cost a bit more than the blow off valves since they work a little better right? Now the big surprise... Re-circulating valves were engineered into the design of your 90+ twin turbo z on the drawing board... you've already got them...
In conclusion: You could always disconnect your stock re-circulating valves (which you have to do to install blow off valves) and spend 500-600 bucks for blow off valves and what will you get? better performance? No, basically you do get the cool sneeze sound though you aren't going to be any faster than you were before (but you might sound like you are). If by chance you are running boost levels that exceed the airflow capacity of the re-circulating valves (~600+ hp?) then you've probably spent enough money and have enough experience and knowledge to know where the benefit reverses. For the rest of you who weren't in the know, I hope I've helped you save some time and money...
Paul
ok, thanks Dave, contacted them to see i they can make some new rims, but still use the center's from my Veilside wheels. Paul