Twin BOV's anyone?

[Doug_MPS6]

Has anyone here found a need to run twin BOV's after modifying their induction, or adding an after-market TMIC or FMIC or turbo supplying more boost? If so:

1. What was the status of the mods when the problem issue appeared?
2. What was the issue that it was hoped the addition of a 2nd BOV would resolve?
3. Where was each BOV placed wrt the inter-cooler/turbo/TB?
4. Did adding a 2nd BOV resolve the issue?
5. How were the two BOV's set up wrt 100% Bypass, 50:50 or 100% VTA?

The more astute among you will see where I am going with this. But whatever, please try and stay close to topic - preferably first or, at worst, second hand experience only. And try to answer in numerical sequence as above, as far as possible - it's so much easier than trawling through a series of paragraphs.

I don't need to read a pile of nonsense or hearsay that one inevitably finds anywhere on the net as soon as one raises the issue of BOV's. I've never seen so much rubbish talked about them, and so little understanding of their purpose and function. I know how they work and how to set them up, but have a reason for asking and may be able to elaborate further depending on responses. Thanks in advance. Cheers :-)

[2XS]

I am still running the std BOV at full recirc.

I've run the HKS SSQ at full VTA and the only difference being the very minor hesitation between gears.

I'm looking into a 2nd BOV, 1 before the throttle and 1 before the intercooler. Originally I thought about a larger one, but with 2, I think I can stage the second one to only vent for +20psi boost, or WOT shifts.

This should reduce lag between gears, right?

The pressure buildup from the turbo not being able to escape when the throttle is closed would then go back towards the turbo and slow the turbo. Or am I wrong?

[Nexus]

Where I think it might be a good idea is where you have two discrete pressure areas. Say two intercoolers, two turbo's, twincharging.
for instance in turbo feeding supercharger you might have BPV on either side of the supercharger.

I heard of racers using a one way butterfly to store boost, say in the IC, when one lifts off. That is then keeping it on boost when you're into the next gear. That sounded complicated.

---------- Post added 18-06-2010 at 12:07 AM ---------- Previous post was 17-06-2010 at 11:32 PM ----------

I think you're right, 2XS

[Doug_MPS6]

2XS and NEXUS, thanks, you've both got it in one, pretty well. The issue for both my son and myself, is this:

1. In my son's case, he has been running a BIG supercharger with inter-cooler on a bored-out and worked V6 Holden engine. It's running about 350Kw. After fitting, he experienced a pronounced stumble when lifting off the throttle, such as when reducing power after accelerating and thus being "on boost", or on up-shift during acceleration. This of course was the back-pressure wave from the closed throttle acting on the supercharger vanes, and trying to stall the compressor. But with the vanes being positively belt-driven, they aren't easily stalled but this means that the pressure wave and resulting stall effects acted directly on the crankshaft and if the clutch was not disengaged, would be felt through the transmission as a stumble, by which I mean a noticeable break in power delivery but which immediately returns upon reapplication of throttle. What was happening is that the single BOV was not able to eliminate all the back-pressure being built up in the lines and on the pressure side of the inter-cooler and in the cooler itself - it's a long convoluted pipe too, and so has lots of air stored under pressure, the only place for it to try to escape being back to the compressor, with the effect of not only the sensation of the stumble but blowing oil seals and almost stripping the drive belt. He solved this by adding a second BOV, so he now has one on the throttle side of the inter-cooler, while the new 2nd BOV has been placed in the turbo discharge line TO to the inter-cooler.

2. Now to my case. At virtually the same time, I was experiencing much the same thing. Of course, we all know BOV's are fitted to all turbo engines to prevent compressor stall, and to maintain fuel-air balance (by by-passing some or all of the bled off pressure back to the turbo inlet side, as that air has all been through the MAF and the ECU has already figured out how much fuel is required to match that air). Also, by correctly setting up a BOV it is possible to reduce turbo lag and improve throttle response by ensuring that just the required amount of line pressure is dumped and no more, and thus reducing turbo lag when the throttle is opened again. I was experiencing a stumble too, but as the compressor drive to a turbocharger is not a positive mechanical drive as with a supercharger, I learned to drive to accommodate it by making sure that my clutch was disengaged a fraction of a second before lifting off the throttle, and hence on up-shifts I don't feel the stumble via the engine and transmission. This was fine but no use in traffic if applying even light boost at low engine rpm, for lifting off to reduce speed, such as in traffic, would induce a stumble that no amount of BOV adjustment or re-tuning can eliminate. Independently from my son, I reached the same conclusion, that while No. 1 BOV was working as intended on the TB side of the inter-cooler, it was not capable of preventing the back-pressure wave from the large volume of air under pressure in the long convoluted plumbing from the turbo outlet side of the inter-cooler and even the pressure of the air stored in the cooler itself, from feeding back to the compressor vanes.

Soooo....now I am in the throes of designing a new BOV setup. I have the room to position a second BOV right next to the first one. My only fiddle to consider is whether or not to 100% VTA the second BOV which is easier but risks initiating an Air-Fuel imbalance, or to make a new Y-pipe and feed both BOVs into the turbo-inlet. It's manageable, so I'll play with it for a bit.

I should add that I sent Corksport a query along these lines ages ago but they haven't seen fit to reply. In any event, I figured it out on my own. I find most Yank companies useless at replying to emails. They seem to be still wedded to faxes.

Hope this helps someone. Cheers.

[2XS]

Are you fitting a second one to the MPS? I noticed a hesitation from the throttle/BOV before the flash tune (throttle control too)

Have you thought about just getting a bigger one? Pros & Cons?

Do you think it could be a "Drive By Wire" thing, to do with the elec. throttle control?

I have noticed I can control throttle opening rate, but the throttle plate closes, either fully or at least alot, even on slight lift-off, causing a slight hesitation.

Eg: I drive at 75% throttle, I lift off to 50% (about half way), the throttle plate closes fully and reopens to the 50% mark. This is what it feels like to me.

This may be a totally different issue altogther, but, one I'd like to explore too.

[Nexus]

How hard is it to add a second bypass path? Presumably you would vent at the turbo side of the IC for a secondary to maximise the backpressure reduction on the turbo...only downside I can think of is recirculating hot air back into the turbo making it hotter still and I'm imagining the possibility of venting to recirculation too quickly creating a positive pressure at the turbo inlet that then goes back up the intake, maybe, maybe having a negative effect on the MAF. Maybe that's pre-empting a problem that wouldn't happen?

[Doug_MPS6]

NEXUS & 2XS Hi, I think I'll try a second BOV when I get time. My plumbing for both in and out of the IC are close side by side and there is space for the 2nd BOV next to the first, and I can splice it in easily at a join in the existing plumbing with the usual silicon connectors. Then my debate is whether or not to run the overflow to Bypass, in which case I will set up a Y junction off the existing metal bypass pipe which is part of the turbo inlet pipe. Initially however, I might try the OEM plastic BOV because I already have it on hand and just VTA. If there are adverse results to that, then I'll make up the Y junction and send it to Bypass, back to the turbo. You point about bypassing hot air to the turbo inlet is a good one, and may justify VTA if it doesn't mess up the AF ratios too much. However, given that this bypassed air is intermittent only, the temperature effects may be negligible. 2XS, interesting about your perceptions of throttle plate movement on throttle release. I don't know exactly what happens there so I can't shed much light on the matter. Thanks both of you.

[2XS]

Don't try venting the std BOV to atmo, the ECU gradually tries to tune around this and will eventually cause a miss. I've only noticed this with the std BOV, not any other BOV VTA. I think the std BOV has a bleed off valve (high pressure valve) of some sort, inbuilt.

[Doug_MPS6]

Hi 2XS, I've been told on reasonable authority that the OEM BOV is designed to bleed off boost above 15psi. That seems to be supported by some boost curves I've seen. I'm happy to go for an after-market one anyway.

[2XS]

From the horses mouth:

The factory BOV can hold over 22psi of boost. I've seen it first hand.