What hose for vacuum gauge?

[Stanley]

Ordered a autometer z-series vac gauge that may go where the clock it, or maybe in a swivel bracket under the dash. Anyway, I'm not using the nylon tubing that comes with it, not the right look for my engine bay, and they say it kinks.

The meter has a 1/8" NPT male fitting I think, will see when it gets here. It will attach to an unused 1/8" BSPT vac fitting on the balance tube, using a 1/8" Male BSPT to 1/8" male NPT adapter, and a NPT Female (or M to F) elbow on the adapter.

Plan to use compressed air or air brake hose. "Made" hose with NPT fittings would be a plus. Looks like I need about five ft. to go from the balance tube, behind the motor, through the firewall under the glovebox and around to the clock. I guess hydraulic hose is out since I don't think it comes with NPT fitting, and it costs more for the NPT/AN adapters. I can get 1/4" yellow air hose with made 1/4" NPT ends at M-C for about $35 which is OK, would rather have something with a braided fire-resistant (not stainless) cover, mostly for looks. I could get the local hose place to make something, they mostly use AN with NPT adapters, though.

Any ideas? I saw this at hybridz, seems like someone's saying 1/4" ID is to big. http://forums.hybridz.org/topic/8312-vacuum-gauge-hose-size/

Edit: maybe it needs to be fuel or emissions rated since there's A/F mix in the balance tube. In which case those silicon rubber vacuum hoses might be wrong, unless they're formulated for emissions.

???

Edited May 5, 2015 by Stanley

[Stanley]

The gauge and pod showed up today. It's 2 1/16" diameter, and when I held it up next to the clock it looks to small to fit in there as is. No big deal though, it can chill under the dash, next to the adjusting knob for the trip odometer. Way easier than doing the clock thing. Now I'm glad it's small.

The 1/8" plastic tube that came with it is 1/8" O.D. Will toss it and use 1/8" (I.D.), 3/16" or 1/4" rubber or silicon vacuum line to a 1/4" BSPT/NPT adapter (from a previous unfinished project) on the balance tube. Might try a couple different hoses to see if there's a difference.

Still trying to understand that hybridz analysis of the performance of different hose sizes. I had to study pipe flow once to pass a test but I forgot all that stuff. Seems like the air in a large hose (consider an extremely large hose) would act as a buffer, slowing down response. For a very small hose, friction against the walls would slow down response. Could too large and too small hoses both slow down response? Seems unlikely, but maybe.

[FastWoman]

FAIW, on-board MAP sensors supposedly use those little tubes, and they respond very quickly to changing vacuum.  Diameter shouldn't be a significant issue.

 

One precautionary note:  Several of us have installed cheap fuel pressure gauges on our fuel rails, and these have commonly worn out.  (Mine still survives somehow.)  These have gear-driven needles, and the gears wear out with the frequent cycling of fuel pressure (which changes with manifold vacuum).

[Stanley]

I expect you're right, the reason I'm rejecting the tiny plastic tube is a number of online reviews for the Autometer vac gauge, that stated their gauges didn't work until they replaced the plastic with real vacuum hoses. Problem seems to be that they kink very easily, and can't be un-kinked.

I have a McDaniel industrial fuel pressure gauge in the cockpit, with a 1100 psi AN hose to the mechanical fuel pump, and a quick release so I can remove it in 30 seconds if the dragstrip tech crew has a beef with it (doubt if they would for a 16 second car). Looking at my new vac gauge, I have to say there's no comparison between industrial gauges and the automotive gauges that I've seen. Industrial gauges are much better quality. They have to be. They only cost a little more. I wanted the gauge to match the look of the stock gauges, maybe a dumb decision, we'll see.

[Stanley]

 

Here's a few shots. Used 7/32" vac line, found some smooth hose barbs to fit. Used a little Permatex "aviation" on everything. Put some wirecare braid sleeve on it for looks. Didn't bother hooking up the light.

 

Took a drive to check it out, car ran really crappy, my first thought was the gauge messed it up somehow. Had the symptoms of fouled plugs, though, so pulled the plugs and sure enough the back three were fouled. Cleaned them off and set the back carb a little leaner, took another drive and it ran well.

 

The vacuum gauge works fine, but I'd be afraid to put an automotive  mechanical fuel pressure gauge in the cockpit (industrial gauges are all metal, made in USA or Germany, repairable, with tempered glass faceplates good for 200 psi, bourdon tubes good for 3 times max pressure on gauge, etc.).  Maybe there are better automotive gauges I don't know about.

 

Vacuum really drops out on acceleration, guess the SU pistons are dropping as well. Learning experience.

[Jarvo2]

Awesome idea of putting it in the interior.  I would be worried about putting a fuel gauge inside though as it will require gasoline to pass through the firewall which is a big no-no.  

 

I ended up putting the vacuum gauge on the balance tube so it would help with dialing in the carbs.  I do have a video at the bottom of this blog page to show you what mine looks like.  Dropping down & then rebounding above normal & then returning to normal is what you're looking for.  This was before I rebuilt my engine & it reads even better now with the new rings, gaskets, and cleaning out 40 years of crud. 

 

http://jarvas240z.blogspot.com/2012/11/vacuum-gauge-installed.html

Edited May 14, 2015 by Jarvo2

[Stanley]

Good tips at the link you posted. With all those ports on the balance tube you could have a vac gauge on it, and one in the car without cutting into any essential hoses.

 

I'm thinking the gauge may be a help to carb tuning. Piston position in the carbs depends on vacuum (modified short-term by dashpot oil), and I expect it would be at the same height at the same vacuum at idle or under load. So if there's a lean condition confirmed by miss or other means at a certain steady-state rpm on the road, it might be possible to note the vac reading at that time and later see the piston position (at idle with the air cleaners off) at the rpm that gives the same vac reading. Then you'd know what part of the jet needles to modify. Unless I'm missing something.

 

Hooked up my F.P. gauge with regard to safety; I could probably pull a small trailer with it, the AN line is rated for tension. Wouldn't trust an inferior gauge or one with hose barb connections inside the car, though.

Found an unexpected benefit from  the gauge; when the gas tank gets real empty, the gauge shows a drop in fuel pressure before you run out of gas, so you know to stop at the next station.