MarkRobinson

Hi guys, My turbo 928 will only make about 3psi of boost, after spiking to 5psi. (still made 340/375rw) I'm unsure why this valve would do this, as I've had 2 944t's (both modified), plus my 911 turbo. I've played with & replaced these valves as long as I can remember. This particluar valve will hold 15psi boost on my buddie's 951, but only 3 on my car. My wife's BOV was blown, so couldn't test that.

I just dunno why they would allow sooooo much air to bleed off...seems wrong. Yes, I am pressurizing the side of the valve, where boost holds it closed. Another buddy's valve only yielded 1psi, maybe 3. Last weekend I checked the entire system & even R&R'd the intake to ensure all hoses & such were intact. Two weeks ago I R&R'd the 46mm tial WG & tested it to ensure the spring was holding up to 5.8psi, which it was.

Any thoughts?? What valve should I try as it looks like I need to purchase a new one.

evil 944t

Dumb question but is it in backwards?

MarkRobinson

No, if it were in backwards, I'd be pressurizing the bottom of the valve, opposite the nipple. I'm pressurizing the side of the valve, where the pressure holds the valve closed. If pressure was opposite of the nipple, it would want to push it open. This is the same way it's installed in my last 944t's and my current 911 turbo.

Mark.

daniel951

i had good luck with the HKS ssqv holding boost. I also heard the tial BOV works very well too

toddk911

EVO unit is nice.

evil 944t

Just asking, you never know. As far as a stock BOV not holding 5psi, something else is going on. I have used stock BOV's to 20-22 psi with no problems.

Are you sure, what ever is controlling your WG is not bleeding off the boost or are you just running off the spring?

MarkRobinson

I've tried it with & without the boost controller hooked up...3psi either way. I put a gauge in between an air line & my WG on Monday, it starts to open at 6psi, which is correct...

special tool

Dude - Is this a 993 part number, or 951?
The OEM 951 valve is notorius for diaphragm rupture - hence the new style 993 part.

You should get a Forge racing adjustable valve because of your unique situation. It will let you tune it for the needs of the turbo 928.

Tom M'Guinn

If your valve works on another car (and your buddy's valve does not work on your car), then something else must be wrong. Have you tried capping off the bypass valve ports completely to see if the problem remains? (Just let off boost gently with the ports capped so as not to stress the turbo.) If you are still losing boost with the ports capped, you can then rule out the valve itself. If if makes boost with the valve removed, then you can narrow it down to the valve or its plumbing.

I had a nearly invisible crack in a rubber line feeding the bypass valve that made it leak -- have you checked all the plumbing?

toddk911

ST is right. With all the time and money you spent on that motor, would be silly to waste time with a 40$ BOV.

The Forge or EVO is great.

toddk911

Oh, while doing some BOV reseasrch, I found an anser to the spring rate question.

"Stiffer will hold more boost, but will be less responsive, so it won't blow off so easily when you let off after part-throttle driving. Softer will be more responsive (better for driveability), but may leak air at idle or under boost. You basically want it as soft as possible while still holding the boost you want to run."

Porschefile

Mark, I'd definitely go with something like a Forge bov, Tial bov, or HKS ssqv. I tested my stock one, as well as 2 stock type billet bypass valves and they all leaked. One of the billet ones actually leaked at idle. I'd highly recommend an HKS ssqv, as it can be setup to recirculate if you wish, plus the design of it makes it basically impossible to leak.

evil 944t

Rather than trow money at it, trouble shoot the problem. After you find the problem, I would upgrade.

Guys, Telling him to go out and get a $125 valve isn't solving the problem. We are talking 5psi, a stock BOV is fine.

If a good valve is not working than something else is not. Are you sure you tried good working BOV's with the same bad result?

The BOV should not open unless its off boost. Where do you have the signal coming from?

toddk911

Here Mark. This might make you feel a little better

http://video.google.com/videoplay?do...6839&q=porsche

MarkRobinson

Yeah, I've had 2 SC 928's, fun, but not FUN! I'm turbo literate, afterall, I did build the only turbo/intercooled 928 32c in the country. I just don't know why I'd have issues like this...most have been reliable before. Vacuum actuates the vavle easily, & they appear to seal under low boost conditions (testing by mouth: about 2-3 psi at best). I need to try other "good" valves before I go more expensive. weight is important as the hose supports the valve, hence why I've not tried a HKS or Greddy BOV. My 965 runs makes 387/423rw on a factory BOV, & since they're inexpensive, I thought I'd go that route. Also, you cannot see it since it's just after the compressor housing. Though it's not the best placement for it performance wise, it's the best place for it spool & driveability wise: car drives as-stock until you're on boost.

Toddk: still planning on using you for chips once I get to that point: I have a AEM Supra tuner here (good buddy & ex-customer of mine) who will assist me in pulling timing measurements for a custom chip.