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Old 07-27-2008, 01:21 AM   #1
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Default BOV idle/part throttle leak

OK so for starters I have read through these threads looking for the experiences of others on this topic:
https://www.miataturbo.net/forum/showthread.php?t=21657
https://www.miataturbo.net/forum/showthread.php?t=21522

Quickly covering the normal stuff:
- My BOV is a knockoff of this Forge valve
http://www.forgemotorsport.com/conte...oduct=FMCL007P
- It is connected at the exit of the compressor before the IC picture here
https://www.miataturbo.net/forum/sho...7&postcount=42
- I have tried all of the three springs that came with the BOV. Right now I have the stiffest spring installed with two shims.
- It works fine, opens up when the throttle is shut at boost, closes up with no leaks when the throttle transitions from idle to mid<->wide open.
- It does leak at idle, which for metering does not matter since I have a MS using MAP.

So I don't have a problem with the fact that it leaks at idle. What I would like to cure though is the light throttle leaks during cruise. Though I have yet to test this, it seems like leakage in this situation would amount to reduced efficiency. Energy is being bled off through the BOV that would otherwise pressurize the inlet piping/IC before the throttle. This would seem to counteract the potential of the turbo to offset throttling loss. I am not concerned with how much power I am making at light throttle of course, but am wondering about mileage effects.

So, I thought I might try removing the BOV, plugging the hole, and running a couple of tanks on my normal commute to see if the mileage improves; I track my mileage normally so I'll detect a change if it is more than 1-2 mpg. Since I am only running 5-7 psi right now it should not hurt the compressor, though I do dread the obnoxious gobble.

All of this is leading to two questions:
1. Anyone got any knowledge/experience with this? Is there any significant hit in efficiency with a light throttle BOV leak?
2. Anyone know of a BOV/recirc valve that will not open up at light throttle? From what I have read this is a typical characteristic of the device.

On question #2 one answer here is to keep increasing the spring pressure until it will not open up unless there is a BIG pressure differential across the BOV piston (compressor exit versus manifold) The downside of this of course is reduced opening of the BOV when the throttle is closed under full boost, so it would be less effective in its assigned task.

Thoughts?

I have one... Eva La Rue on CSI (on TV in the background) is hotness incarnate.
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Old 07-27-2008, 12:36 PM   #2
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You can always try turning around and running it backwards.

But really thats what happens when you buy a bootleg valve. I dont understand how you have that much spring in there and it leaks at part throttle, and that means it will leak at idle too.
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Old 07-27-2008, 02:18 PM   #3
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I don't think that it is a bootleg is the issue. It is functionally exactly the same as the Forge unit.

For the record I did not know it was a Forge bootleg when I bought it. See DIY BOV sig link for details. I also tried running it in the other direction. It does the same thing, but the noise is more obnoxious.

Last edited by ZX-Tex; 07-27-2008 at 06:48 PM.
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Old 07-28-2008, 11:13 AM   #4
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My previous BOV was a knockoff HKS SSQV that leak all the time except over 6-7psi. Mileage was good, spool was ok and it hit very hard when the BOV finally slammed shut.

Current BOV is the 'new style' non adjustable HKS SSQV. It definitely does not leak at idle and probably not at cruising load. Mileage is statistically the same as the super leaky BOV. I thought I would get better mileage, but it didn't happen. Spool is so much better.

BTW, I think your BOV sounds great!
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Old 07-28-2008, 12:41 PM   #5
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Interesting... So in your experience:
- The leaking at idle/cruise does not impact mileage.
- Spooling has improved with your authentic SSQV versus the knockoff. Presumably the authentic unit vents better than the knockoff.

And, how do you know how my BOV sounds? Is this Craig?
If so I'll reserve my SSQV questions until later.

Another interesting note. I looked into the Synapse BOV from these guys:
http://www.synapseengineering.com
It is a pull design like the HKS, but uses a piston instead of a diaphragm. Based on the youtube videos It looks like their valve stays open at idle and maybe at part cruise, and only shuts under boost. Based on some things I read that the Synapse designer/engineer/whoever posted in their forum it has something they call economy mode. And, if you add this little spring/piston device that prevents leaking at idle (for the MAF crowd), the economy mode does not work. So, reading between the lines, I think maybe they are indirectly claiming increased efficiency based on a cruise load BOV leak.

Soo... this ties into an alternate theory I have on efficiency. What if having the BOV open slightly at cruise is a GOOD thing? Think of it this way... The throttle, at a given cruise speed and load (say flat terrain) is ultimately determining the manifold vacuum. If pressure upstream of the throttle is increased (by a non-leaking BOV) then manifold vacuum is decreased, and the throttle closes further to compensate, returning the manifold to the steady-state vacuum required for the given cruise speed and load. This would not help the throttling loss, but would actually increase it. Plus the turbo is working harder since the flow rate is the same but the compressor outlet is at a higher pressure (upstream of the throttle). This equates to more restriction on the turbine side since it is also working harder, feeding power to the compressor.

So let's say that the BOV is allowed to leak a bit at cruise speed at the same given load. The post-compressor/pre-throttle pressure drops, so now at a given flow rate the compressor is not working as hard since the pressure rise is reduced. There is also a corresponding drop in turbine restriction. And, the throttle is open farther since there is less pressure upstream and it can open up farther for the target steady-state manifold vacuum. NOTE: The given flow rate assumption may not be true here, because the leak in the BOV means there is more flow across the compressor than just what the engine is ingesting. Whether it is significant flow through the BOV compared to the engine I do not know.

So, if I am not way off here, it may actually be beneficial to cruise mileage to have a slightly leaky BOV. I am assuming here that it is not a huge leak that would allow the turbo to overspool. And the BOV would immediately seal up when the throttle was opened up to produce more power (boost). The most efficient point of this transition from slightly leaky to sealed could be tricky. To be clear, I am talking about small leaks at cruise due to the position of the BOV, and not due to a bad seal.

Thoughts?

Last edited by ZX-Tex; 07-28-2008 at 01:09 PM.
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Old 07-28-2008, 01:27 PM   #6
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Quote:
Originally Posted by ZX-Tex View Post
Interesting... So in your experience:
- The leaking at idle/cruise does not impact mileage.
- Spooling has improved with your authentic SSQV versus the knockoff. Presumably the authentic unit vents better than the knockoff.

And, how do you know how my BOV sounds? Is this Craig?
If so I'll reserve my SSQV questions until later.
Right, for me no difference in mpg, maybe 1%-2% which is statistically insignificant, considering all the slight tuning changes that happen all the time.

The BOV leaking a lot when you are trying to build boost just sucks.

The HKS SSQV probably isn't any better or worse than any other BOV that works. Some people hate them, probably because of the sound. I heard an MR2 with one and thought it sounded a little like WRC, so that's why I went that way.

Yup it's me, so you'll hear the SSQV BOV soon enough!
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