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Re[2]: Improved S-Knob


On Thu, 19 Jun 1997, russ southerlin wrote:
> When we spoke on the phone a few months ago I mentioned that I was going 
> to try putting a check valve inline with the W/G vacuum line to prevent 
> engine vacuum from being applied to the W/G.
> At any rate with the check valve installed I think there is a noticable 
> difference in how fast the boost pressure comes up during gear shifts. I 
> have a mechanical boost gauge and can see that the pressure comes right 
> up to about 6 psi when shifting before it lags for a second or two and
> then takes off for 15 psi. Without the check valve the boost only comes
Hmm, I'm not sure a check valve is the right thing to do here.   I used a 
pressure regulator in the lline to the top of the wastegate, and that 
worked well.  

Maybe I didn't explain my setup well enough. I have the same pressure 
regulator to the top of the W/G (standard S-Knob) set for 15 psi boost 
pressure. While installing the pressure reg I noticed that when the 
engine was in a vacuum condition ie. between gear shifts there would be 
vacuum on top of the W/G which I initially thought would be working 
against the W/G spring and possibly even opening the W/G between gear 
shifts. After thinking about it more I think the vacuum is on both sides 
of the W/G which gives the net effect of only the spring keeping the W/G 

 The thing that makees me wonder though, is why does your boost sit at 
6psi and then rise to 25?   My car has alaways gone directly to the max 
pressure set point with out any hesitation at all. 

I have a 0-15 psi mechanical boost gage which is very sensitive. When I 
shift gears the boost initially falls to 0 when I lift off the throttle 
and then as I get back into the throttle it jumps to about 6 psi (2 psi 
without the check valve) and hesitates for a second or two before 
continuing up to the 15 psi set point. I have always assumed that this 
was normal turbo lag. You can't feel any noticable hesitation by the seat 
of your pants.

Occasionally it will rise one psi or so higher than the setpoint, but it 
generally gets to the setpoint very smoothly.

Mine acts similar. How fast does yours get to the set point when 
shifting? Mine goes to 6 psi real fast and the climbs to the set point.
> put a small pin hole in the hose to the W/G to prevent any pressure 
> spikes from gradually building up pressure on the W/G above what is 
> required for the 15 psi of boost.
Top side hose or bottom side hose?   I don't like leaks.   I'd rather 
tune the control system to work at the desired boost pressure than to 
fool the control system into providing something it isn't designed for.

The pin hole is between the check valve and the W/G. I put a temporary 
pressure gauge between the check valve and the W/G just so I could see 
what pressures were on top of the W/G. You don't need very much 
pressure from the regulator for 15 psi of boost, I was seeing 6 psi 
from the regulator for 14 psi of boost. Since the check valve only 
alows pressure to be applied to the W/G and not vacuum you would have 
no vacuum leaks (proved this with the gauge) only a small leak when the 
pressure is being applied to the W/G. Since this pressure is small ~6 
psi I don't think it affects the system. As evidence I have run with 
and without the check valve and have gotten essentially the same boost 
set point (15 psi) without adjusting the Knob.
Russ, do some testing and report back please.  Sounds interesting.

I'll try some timed tests although I think there are alot of variables 
associated with this that are hard to quattify.