How much boost can a bosch bypass valve hold. Seems to be I might be losing boost. Ive pullied the s-trim smaller and the boost came in quicker, but seems to top out @ 10 psi. Im wondering if the bypass just cant hold anymore than that. Maybe I need to upgrade the bypass?

Its only a 350c.i. with le2 style heads and ive got it pullied to 7"/2.75". Spinning it about as fast as its rated @ 6k rpm.

 

Ive been researching the bosch bypass valves and found theres multiple p/n's for diff. applications. Seems the "710n" is a HD version of the older one and they say it holds more boost. the 0 128 142 102 I have looks like it may have been a weaker spring and diaphragm than the HD and I found somewhere its only good for about 10 psi, seems coincidental huh? Searched the 710N and think somwhere it said maybe 14 psi. Then came across this adjustable bypass with spring kit on ebay built with cnc parts:

http://cgi.ebay.com/ebaymotors/ws/eB...=STRK:MEWAX:IT

Seems like a much cheaper alternative to a mondo or racing bypass from procharger or vortech. Even If you had to use 2 of them.

 

In my app... (97ss 383-D1 MM6) I run 2 bosch style bypass valves. They would bypass with 7psi. I resized the spring (different compression spring with modded spring rate and pre-load shims).

Calculations were based on 14 to 15 psi and 3 to 13 psi of manifold vac during cruzing in bypass mode. Valves work quite well now. The original spring was a bad design for my app. I did a excell spread sheet to calculate the condtions of bypass and full/ partial boost load.
B.

 

So your agreeing the bypass I have is likely opening under boost and leaking once it hits 10. It seems as though when I stab it it will climb to 12 at times then drop, as if the valve let go and then stayed partially open.

Could you snap a shot of how you have the dual valves setup? I was pondering this last night, and couldnt think of how to do it neatly, need some creative inspiration. Also, which bosch p/n's do you have? I have a 0 128 142 102. Ive read the better updated one is 0 128 142 114.

 

My opinion is that off the shelf bypass valves do not work well since they are not designed for the individual application. The valve requires application mod to fit the specific boost level and manifold vac application. Manifold vac is affected by the intake plumbing , manifold induction, heads, and, exhaust. After some road data, I applied this data to an application spread sheet to size the valve spring rate and shims.

I run a Procharger D1 w/ pulley ratio 7.6" to 3.4" pd. MAF is on the intake side. Blower output is directed thru a large Spearco to the intake manf. I bypass @ the intercooler output to the blower intake side after the MAF sensor, so all the measured air is retained in the system.

I fabricated all the ducting (Intake & pressure side) with a simple 2 1/2" bypass valve manifold (from 2 1/2" exhust pipe). Two bosch style (alum) valves in parallel w/ output to the blower intake duct. I puchased these valves from evolution motor sports, but, I think they can be purchased from ebay.

Maybe you can see some of this @...
http://s306.photobucket.com/albums/nn244/mzgp5x/
If not, I shall take some additional photos when I put the car up for winter, since it will be subject to another year of mods. Hope this helps.
B.

 

If the valve is installed correctly, it should basically hold till the housing or diaphragm ruptures.

The outlet should be the side being capped by the piston/diaphragm. This way the spring is not fighting boost to keep the valve shut. Boost should enter the small vacuum port as well as the large side inlet port.

If you have 30psi on the vac port side of the diaphragm, and 30psi on the inlet/outlet side, the valve cannot physically open (aside from a materials failure) because the vac port side has a greater surface area.

How do you have it installed?

 

Mike, yer saying the port opposite the vacuum fitting should be in the charge pipe, and the other going to the recirc hose or the other way? Thinking about what you said, its makes sense. If I turn it around the other way the boost pressure will not be pushing the spring, it would be aiding in keeping it shut rather than fighting the momentary lack of pressure on the vacuum side as the charge pipe builds pressure first and could blow the valve open they way I have it. Ill look @ that later.

 

I have seen 13psi useing the bosch 102 valve. dont know if its the way its installed or if some just hold better than others.