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.

[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.

[Adger Smith]

Ray,
We are actually using the term pinch valve incorrectly in this throttle stop application. It is usually a needle valve used to control the air flow.
Needle Valves

The needle valve is used specifically for accurately controlling the flow of fluids at low flows. The valve is basically a globe valve without the disc. It is generally used provided in small sizes of up to 20mm bore..


This is a good example of a pinch valve:
Pinch Valves

The pinch valve is a theoretically ideal solution for fluid on-off duties. The valve is simple a length of pipe made from an elastomeric material with a mechanical system for squeezing the tube closed when a shut off is required. The valve is a true full bore valve - there are no mechanical parts in contact with the fluid- The operation of the valve is ideally simple- The valve can be easily engineered as a tight-shut off valve..

The valve is often supplied with the pinch tube contained within a outer pipe between the end flanges. This option provides a method of monitoring for tube leaks and provides a degree of secondary containment

The valve has similar limitations to the diaphragm valve. The diaphragm valve is really a variation on the pinch valve principles.. Pinch valves are supplied is for diameters 25 mm - 1000 mm, temperatures -50 C - +160 C, and pressures 0 - 100 bar.

Sorry can't get the pictures to copy.

[CBS]

Speculate all you want.....but put a linear potentiometer on the cylinder and connect it to your racepak......then you will know if it's consistent....