PanTTher

I installed a vacuum - pressure gauge (one dial indicator) in series with the vacuum line hose on my Superd Duper (tm) Bosch bypass valve and the engine plumbing. The intent was to see how much vacuum the engine pulls.

I started the engine and watched the gauge. The dial read 12 inches of vacuum on a cold engine. The unit of measure is inches of Mercury. For reference remember the stock Bosch bypass valve cracks open at 5 Inches and is fully open at about 9 inches of Mercury. As the engine warmed up the dial slowly crept up to about 22 inches of Mercury of vacuum. As I gassed the engine gently the vacuum dropped to zero inches which was not a surprise.

Here is the surprising part, to me at least! When revving the engine hard the negative pressure or vacuum in the line turned into positive pressure or boost as the rpms climbed. With no load on the engine I got up to 5 psi of boost. Surprise, surprise!

What does this mean you might ask? So as you're driving the car, depending on the load requirements and the throttle position, the pressure in the vacuum line connected to your bypass valve varies from negative or vacuum to positive or boost all within a few hundreds of a second. As long as the difference between manifold pressure and the pressure in the vacuum line is within the force generated by the bypass valve's spring the bypass valve should remain closed (under its own spring force) under hard acceleration and not leak assuming it seals nicely beginning with.

If the pressure in the vacuum line leading to your bypass valve cannot go as high as the pressure in the manifold then the spring force in the bypass valve will not be able to overcome the high boost and it will start bypassing air into the air box prematurely. To confirm this I would have to extend the hoses from the bypass valve to inside of the car so I can drive and look at the pressure gauge at the same time. I'll have to leave that experiment for some other time.

Now what makes the engine act a little jerky or surgy sometimes when accelerating gently you might ask? I have a strong suspicion it's the bypass valve reacting to the quick pressure fluctuations in the vacuum line depending what your foot's commanding on the gas pedal. That is the jerkiness comes from when the pressure varies from vacuum to boost in a split second. How do you overcome this problem? Without having more data my first guess would be a stronger spring in the bypass valve should cure the problem. The stronger spring shouldn't let the valve respond to pressure fluctuations too quickly.

The next project today is to install my new bypass valve on the car. I vacuum checked it today and it cracks open at 17 inches (gauge) of Mercury, which is equivalent to 8.4 psig or 23 psi absolute. It's a mean looking valve!

raVen

Interesting observation........What brand/modeis your replacement ?

raVen

That would be model is

PanTTher

I haven't put it on yet but it's a Turbo-xs type H valve. I'll see if I can install it later today.