Pinch valve question?

[Bill Baer]

I have always used an inline Co2 cylinder with a pinch valve to slow the opening rate when the stop comes off and I recently read an article in a magazine that indicated that pinch valves could be a source of inconsistency in air cylinders and after reviewing my data it appears that it may be so. Does or has anybody else out there have data or experience on using and not using a pinch valve?

[GaryGoFast]

I have a dedenbear air on/air off under carb with 2 pinch valves, car has been pretty reliable over the years. Never had an issue where I could blame them and they repeat.

[Chris Williams]

I've used both the dedenbear plate and the biondo inline stops with control valves and you can put the racepak traces on top of one another and they are as consistent as you could ask for.

FWIW, I only control coming OFF the stop, not going on the stop. Helps a boatload with lousy tracks.

[Bill Baer]

Chris
I always pretty much considered pneumatics to simple and reliable but after reading this I started looking at the stroke times (i.e the time from the point the stop turns off until it reaches 100% throttle) and found that it varied +/-.02.
It's uses air (Co2) to stroke both ways
I'm thinking that the friction that needs to be overcome in the cylinder and the linkage could be a factor so I purchased the correct air cylinder lube and upped my lube frequency on the throttle shafts and linkage and will continue to monitor the data/stroke times to see if they become more consistent.
But what I'm wondering is if I need to consider doing some testing without the pinch valve? I shift on the stop and come off the stop in high gear so it's possible that the pinch valve and subsequent slow throttle opening may not be necessary to control wheel spin but as I said I have never tried it without one and was wondering if someone else has?

[Adger Smith]

I have always wondered if you use a pinch that is open to atmosphere if it is subject to Baro changes.

[Chris Williams]

I know enough people who spin the tires when coming off the stop that I've never considered trying it. I admit that you have a good point about maintenance. Never did it.

Wonder what Biondo says about this? They run more and sell more stops than a whole lot of people. Hmmm...

[Bill Baer]

Originally Posted by Adger Smith

I have always wondered if you use a pinch that is open to atmosphere if it is subject to Baro changes.

Well considering that the air pressure on the pressurized side is 80 psi and a change of 1" in the barometer is 0.486 psi it would take a barometer change of 2" to change the pressure differential 1% so while barometric pressure does make a difference it's not a significant one.

[Adger Smith]

Bill,
That is about the same conclusion I had when dealing with % of air pressure, but the actual air pressure coming out of the pinch is not the same as the 80 psi in the cylinder.
Also: How do you account for time and vol. as variables?
Or do you/should you?
Is this something that causes a quicker dump to be more consistant than a slow one?
I'm thinking it would be more consistant to pressure feed on both sides of the cylinder with regulated pressure differences.

[Bill Baer]

Originally Posted by Adger Smith

Bill,
That is about the same conclusion I had when dealing with % of air pressure, but the actual air pressure coming out of the pinch is not the same as the 80 psi in the cylinder.
Also: How do you account for time and vol. as variables?
Or do you/should you?
Is this something that causes a quicker dump to be more consistant than a slow one?
I'm thinking it would be more consistant to pressure feed on both sides of the cylinder with regulated pressure differences.

Adger
Actually the pressure coming out of the pinch valve is the same as the cylinger but the restriction provided by the pinch valve increases the amount of time it takes to fill the volume on that side of the cylinder and slows the throttle opening. There is little or no restriction in the throttle closing (Dumping) side.
The volume is fixed by the diameter and stroke of the air cylinder and the time to fill that volume thru a restriction should also be fixed if the pressure remains constant but, the data frm my throttle position sensor incicates that that latter time of strokeis not fixed and does vary somewhat.
I'm starting to question if the opening side might work more consistently if it had litle or no restriction.
I suppose you could try to feed different pressures to different sides of the cylinder but you have to keep the pressure high enough to work solidly So I don't think a lower pressure would do the mechanical work properly.

[ray sowers]

Originally Posted by Bill Baer

I have always used an inline Co2 cylinder with a pinch valve to slow the opening rate when the stop comes off and I recently read an article in a magazine that indicated that pinch valves could be a source of inconsistency in air cylinders and after reviewing my data it appears that it may be so. Does or has anybody else out there have data or experience on using and not using a pinch valve?

I have many years industrial experience with air and hydraulic operating systems, what is a pinch valve? I have always used flow control valves including on race cars.