It seemed no one answered you correctly, so I'll give it a shot.

There are a few reasons diverter valves are preferred on the Evo (and most other turbo applications).

1) The ecu expects all the air to be in the system. Lets say you build up to 28psi going wide open. Let off the gas, all of the air goes VTA (vent to atmosphere) and then you step on the gas again... you just hit a very rich condition because there isn't enough air in the system to be mixed in. If you have a diverter valve, all of the air stays in the system and is ready for use the next time you step on the gas. This is difficult to tune for on our systems.

2) You must build all that boost again. If you lose all the air and go to hammer on the gas pedal again, you just lost all of your air pressure (boost), so the system needs to rebuild it. It takes time to build up boost (though not a long time), but it still costs potential speed.

The reason tuners start using VTA blow off valves vs. diverter valves is because it allows the turbo turbine to be able to spin freely. This is only needed in extremely high measures of boost (45+ psi). Hope this helps.

 

Thanks for the info.

That makes a lot of sense, so a VTA system will make the car run very lean?

VR is claiming that a VTA BOV will possibly increase HP as it "will hold more boost?"

https://www.evolutionm.net/forums/se...-review-3.html

 

Try not to confuse VTA and holding more boost. AP's BOV hold's more boost because of design, not because it's VTA. Perrin's can theoretically hold infinite boost and it is a diverter valve. The stock one starts leaking around 26 psi because of the design. However, if you crush it you can get it to hold 30 psi without leaking.

 

Q) What does a BOV do?
A) Prevents compressor surge by releasing pressure when the throttle body plate closes.

The Evo X uses an upstream (before the turbo) mass air flow (MAF) sensor instead of a downstream (after the turbo) manifold air pressure (MAP) sensor to help calculate how much fuel to inject into the engine. A VTA BOV creates a rich condition not because of this venting, but because the ECU uses a different type of system to do these calculations. The MAF measures how much air is coming in - and this happens before the recirculation valve - the ECU uses this number. If you replace that valve with a VTA BOV, the number becomes inaccurate and causes a rich condition.

Since a MAP gets its numbers from the intake manifold AFTER the BOV, it doesn't matter if the BOV recirculates or simply vents to atmosphere.

And that's why a VTA does nothing for, and in fact hurts the Evo X's performance.

Listen to migs, he knows what he's talking about.

 

Well put

 

no- but it is no need too. Also the OEM is a safe bet . The aftermarket you can get a wrong type - bad designed etc. Also some of them needs to be calibrated, so why would you go for the grey area when you dont need to, specially if you dont know exatly what you doing? The open air intake gives great sound with an oem valve, if you looking for sound.

over the stock holding limit you must go for something else. that is the only time you need a new BOV.

Rob

 

Here's What Brett Turner (Go Fast Bits) said about this matter after I picked his brain a little:

Unfortunately you’ll never hear the end of conflicting opinions on this topic! You are right about the MR, because you don’t lift the BOV really doesn’t come into play for much of the time. One thing about the EVO X though, the factory valve does start to leak at 22psi and is leaking a LOT by 24psi, so to those who claim the factory one is fine at those levels are fooling themselves!

Flutter at low RPM is indeed compressor surge, however a bit below 3500RPM is nothing to be concerned about. You can reduce or eliminate it by softening the spring pre-load, however our testing shows that best throttle response is achieved with a firmer pre-load. The pre-load ultimately comes down to a combination of factors. Softer spring settings will reduce likelihood of surge, but when the valve is set to vent a portion of air to atmosphere, a softer spring will increase likelihood of stalling, backfiring, and generally boggy response when the throttle is modulated. Firmer pre-loads alleviate these issues when atmo venting, but often can result in low RPM flutter.

Compressor surge is really only something to be concerned about if it occurs on throttle lift from high boost and RPM. A turbo is more likely to surge at low RPM, because the compressor is operating closer to the surge limit. However, it is also spinning much slower and the resulting pressure spikes are really quite small – i.e. if surge occurs when lifting off at 5psi, the pressure spikes would be a couple of psi – way below the maximum pressure the turbo sees at full throttle, and therefore nothing to be concerned about.

So setting up your valve is a personal preference. If you want plenty of sound, you can go for a high atmosphere portion, and keep any possible negative effects at bay by running a firmer spring pre-load. If you want to keep it quiet, you can run a smaller atmo portion and soften the spring pre-load. Experimenting as you have already done is the best way to find what your car likes, and there’s really no harm that you can do to the car with the adjustments.

A final note on atmo venting, and this flies in the face of everything you’ll ever read about the subject. We’ve been doing some testing (need to go further with it though) on my STi, and there is a marked and measurable improvement in lag reduction when the valve is 100% atmo venting during gearshifts. This can be attributed to the effect of the brief rich mixture during the shift acting as sort of a “pre-throttle pump”, where the fuel is in the intake runners ready to go when the throttle is re-opened. The ECU will inject a larger than required amount of fuel during throttle opening (throttle pump) to compensate for the sudden inrush of air, but the atmo BOV actually works to inject fuel BEFORE the throttle opens. Of course, this effect is really only of benefit in the situation where the throttle is rapidly shut then re-opened, as is the case with a gearshift. But in our test car’s case, the time to return to peak boost is cut by at least 25-30%. We’ll be doing more testing to investigate this further.

 

VTA was solely created for the very reason that spool is faster without the back pressure of the recirculated air pressure. This is no secret. And ricers take it out of context, as they will never achieve the amount of boost this affects. It's very important on high PSI applications, such as drag cars because it prevents the turbo turbine from continuing the spin. With our normal applications and by design our turbo spools very quickly. It has negligible effects on our turbo when it is recirculated. I can't speak for the STi design of their turbo and intake system, so I won't.

As for the stock diverter valve, again, the amount of boost that yours holds before leaking will vary. Some have leaked at 21, some have leaked at 28 (mine). Crush them, and all of them will hold 28-30 psi without any leaking at all.

 

Wow, my head hurts!! lol. That is much more information than I expected to get. I appreciate all the feedback on this issue. You guys are awesome. Thanks.

 

I would only go with an aftermarket BOV above 26 psi. They tend to leak above that. Mine has been leaking for a while now. I've been meaning to get the AMS lower pipe with the Tial flange but I am almost positive I should tune again once I do.

 

Get a synchronic and be done with it.

 

My stock BOV leaked around 24-25psi so I crushed it a bit and my boost holds steady at 26 and tapers down at redline, but on average it is about 0.5-1psi higher at each RPM as it tapers. Stock BOV is great. Loud enough with an open intake too.

 

I have a turbosmart dualport that I need to get rid of. I used it for 2 days to get it adjusted properly before my tune but my stocker ended up holding fine for my boost level. It's 50/50 so you can run it full recirc, full vta, or 50/50. PM me if interested =)

 

"the factory valve does start to leak at 22psi and is leaking a LOT by 24psi, so to those who claim the factory one is fine at those levels are fooling themselves!"

http://www.youtube.com/watch?v=fXN2TkMZsgw

 

Not every diverter valve is created equally from . Some leak at 21, some leak at 28.