PVC synchro trick question

[marshall]

Okay, so I'm cheap... REALLY cheap... The hardware store wanted $5 a piece for the two end caps I needed to close off some PVC pipe I bought for the vacuum gauge trick. So I said screw it and figured I'd just glue something flat up against the ends of the tube to create a seal - after all it's just vacuum, so it shouldn't be hard to seal.

But I was just thinking, does it really matter whether the PVC tube is a rigid volume, or can I just use something flexible like ceran-wrap on the ends, as long as it's sealed up? I don't know much about this pressure stuff, but it seems like the rigidity of the container isn't all that important. Could you use a couple empty 2L bottles of soda, for example?

Just curious - won't be hard to find something stiff and flat, like I was planning before.

Beautiful weather here in MI today, and the weekend is looking great. I've got the fever!!

Cheers,

Marshall

[DGXSER]

Hmmmm...well, the vacuum does tend to help the seal as opposed to pressure pushing it apart. However, it still is looking to pull air past it. So any nook or cranny it can pull air from, it will, and give you a false reading. How false? Well, the higher the vacuum your trying to pull, the more leakage you will get. So I think you want something to seal well.

As to saran wrap, well, if your building that rig, you have a gage and I can tell you My carbs run about 10" of vacuum at idle. So go put a tube on your vacuum gage and draw with your lungs for 10" of vacuum. Then think about saran wrap holding up to that amount of suction. My guess is you will get lots of leakage and a crinkled up piece of saran wrap trying to get pulled into your carbs. JMO

[fredintoon]

What pipe?

Okay, so I'm cheap... REALLY cheap... The hardware store wanted $5 a piece for the two end caps I needed to close off some PVC pipe I bought for the vacuum gauge trick. So I said screw it and figured I'd just glue something flat up against the ends of the tube to create a seal - after all it's just vacuum, so it shouldn't be hard to seal. - - -
Marshall

Hi Marshall,
got a link or a photo of this vacuum gauge? All the 4-cylider homebuilt vacuum gauges I've ever seen used lengths of aquarium tubing, two plastic Tees and a thin piece of wood.
OK, the posh ones used a yardstick.

[DGXSER]

C'mon Fred, its in the Tech tips. Figured on old timer like you would know of it. I built one from it to do my synching, only the aquarium valve I bought (the only one I could find for miles around) is such junk I tossed it and just go one cylinder at a time with the vacuum line.

[81xsproject]

I initially used the pvc plenum and the aquarium valve. However, I thing the vinyl tube one is superior and most certainly cheaper.

[marshall]

Thanks guys. I ended up cutting a couple circles out of my roommate's cheap plastic cutting board, then rubber-cementing them onto the ends of the PVC. Then I tested one cylinder at a time, and seems to have worked just fine. Really wasn't too difficult, and I'm glad the only thing I paid for was the $2.50 piece of PVC (already had a vacuum gauge lying around). Glad I didn't try ceran-wrap, you're right it wouldn't have been strong enough even with a couple layers. I could visibly see the plastic cutting board pieces deflecting inwards, but I realize now that this definitely has no effect on the readout of the gauge, since it's the pressure that counts - not the exact volume of the plenum. And even if the gauge was affected, synchronizing carbs is all relative - so basically, even if your setup was a little leaky, seems like it would still work for these simple comparison purposes.

My first ever carb synchro was a success! Now onto the air mixture screws...

[DGXSER]

Just so you don't miss out on any of the fun....After you get your mixtures set right, don't forget to resink the carbs.

Adjustign the mixture will effect the "power" that cylinder is creating at the same throttle position as you get to the sweet spot. Therefore, effecting how much vacuum it is pulling at that throttle position, and the need for a synch. It is an iteritive process. One I need to repeat myself sinc eI just changed my mixtures a bit today.

[adleeman]

faq vacuum guage

so i bit to haven't built it yet... will the vacuum collapse the aquarium tubing?

[marshall]

I don't know about acquarium tubing, but I used some ~3/16" clear vinyl tube at first, and it ended up collapsing where it attached to the carb holders. Those metal tubes get really hot, which made the vinyl get soft and collapse. I ended up using the thick rubber hose that came with my vacuum gauge. I just cut it into two smaller pieces.

[fredintoon]

... will the vacuum collapse the aquarium tubing?

Hi Tony,
No it ain't that much of a vacuum. It'd be more accurate to say "reduced pressure"

[b.walker5]

Hi Tony,
No it ain't that much of a vacuum. It'd be more accurate to say "reduced pressure"

With my OLD, VERY OLD, high school science hat, any reduction of pressure below atmospheric is regarded as a vacuum, although technically a vacuum doesnt actually exist. Atmospeheric pressure is measured at 760 torr, and a theoertic true vacuum is 0 torr (absolute 0), hence a "vacuum" is merely an absence of pressure, just like cold doesnt exist either, cold is merely the absence of heat and everything above absolute 0 (-273 degrees centigrade, and whatever that is in farenheiht) is a degree of heat. Told that to my wife once while skiing, just after she'd crashed her way down the mountain, and filled her top with fresh powder. She didn't believe me. Still doesnt.

[DGXSER]

The vacuum itself wont collapse the tubing, but the tubing may well collapse from the heat depending on how warm the engine is, how warm the air is, and how long it takes.

[planedick]

Collapsing

True, the vacuum doesn't crush the tube, it's the outside pressure pushing on the walls of the tube when there is less pressure within.....

[Rasputin]

True, the vacuum doesn't crush the tube, it's the outside pressure pushing on the walls of the tube when there is less pressure within.....

The heat makes the tubing more pliable and allows the pressure difference to act. While the tubing is colder it is less likely to yield under pressure.