Pinch valve question?
 

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? :confused:

Re: Pinch valve question?
 

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.

Re: Pinch valve question?
 

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.

Re: Pinch valve question?
 

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?

Re: Pinch valve question?
 

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

Re: Pinch valve question?
 

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...

Re: Pinch valve question?
 

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.

Re: Pinch valve question?
 

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.

Re: Pinch valve question?
 

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.

Re: Pinch valve question?
 

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.

Re: Pinch valve question?
 

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.

Re: Pinch valve question?
 

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....

Re: Pinch valve question?
 

I have a TPS sensor, logged in my racepak, and I have just looked back at 20+ runs on my Biondo stop, and it comes of the stop at exactly the same rate in each one. This is a small data set, admittedly, but I'm satisfied. I will keep an eye on it, because of this discussion, but I'm thinking is is a tempest in a teapot.

Re: Pinch valve question?
 

I have Rock and mine isn't (But what I don't understand yet is why?)

Re: Pinch valve question?
 

My guess would be that you have it opening up too slow. I had an issue with mine being inconsistant and spoke to Gary at 1 Stop and he had told me that. He told me how to set it properly because it needs to open back up within a specific amount of time, too slow and it will be enconsistant. Going off of memory (and that leaves a lot to be desired) I think he said it needs to be no slower than 6 tenths of a second. I hope this helps
Chuck

Re: Pinch valve question?
 

FWIW, here's five runs of ours. This is rpms coming off the stop with our Biondo stop. Only appreciable difference is the setting of the timer.

http://emdubracing.com/wp-content/up...09/offstop.jpg

Re: Pinch valve question?
 

Thanks CW that's the kind of info I am looking for mine is a lot slower than .6 actually it's in the .85-.87 range so per your experience and Gary's advice it sounds like I need to open the air control needle (i.e. pinch) valve to speed up the opening to the .5 to .6 range. I'll give it a try! again thanks.:D

Re: Pinch valve question?
 

sounds like you guys are on the right track.....bump the trace over so the lines are on top of each other...then you can see if it's acting the same.....just keep trying stuff till they lay on top of each other.....I have a pile of stops because of it........

Rock

Re: Pinch valve question?
 

I hate to tell you those are shyt.

Re: Pinch valve question?
 

Thank you for your thoughtful input.