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Discussion Starter #1
Dunno if this has been done before but I searched and didn't find it.

I want to replace the fancy clock in my '68 XR7 with a vacuum gauge. Anybody done this already? Are there any problems associated with doing it? I'm assuming the right way to go would be to T the vacuum feed to the headlight system (upstream from the check valve) or upstream of the brake boost, but if that's not the best way to go, I would be happy to know the right way.

Also, I'd probably want to sell the clock to someone like WCCC to use as a core, but I guess that's a phonecall to them not a question for this thread.

If anybody's done it, or has suggestions as to how to best do it or why not to do it, I'm all ears! And if it's a good move like I think it is, gauge recommendations are greatly appreciated as well.

Getting the vac line through the firewall is the trickiest part, right?
 

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Sounds right. My intentions are to install a vacuum gauge in my 1968 XR7 too. Some day.
 

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If it were me, I'd run the line directly to a manifold fitting, preferably one that was dedicated to the gauge only. That'll provide the best measure of what's going on, engine wise, without interference with or diluting of the signal by accessories on the same vacuum "circuit."
I'd consider devising a suitable mounting bracket to correctly hold the gauge in the spot vacated by the clock to be the most challenging part. But perhaps you already have that figured out.

Just as an aside, does anyone remember the aftermarket "Performance meters" that were designed for dash/steering column mounting, much like an accessory tachometer? Really nothing more than a vacuum gauge, but with a reverse sweep of the needle, so that with lower vacuum, the needle would swing to the right, indicating a larger throttle opening and "more performance." I never bought one, but remember seeing them in places like the J.C. Whitney catalog and Sears automotive department.
 

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If it were me, I'd run the line directly to a manifold fitting, preferably one that was dedicated to the gauge only. That'll provide the best measure of what's going on, engine wise, without interference with or diluting of the signal by accessories on the same vacuum "circuit."
I'd consider devising a suitable mounting bracket to correctly hold the gauge in the spot vacated by the clock to be the most challenging part. But perhaps you already have that figured out.

Just as an aside, does anyone remember the aftermarket "Performance meters" that were designed for dash/steering column mounting, much like an accessory tachometer? Really nothing more than a vacuum gauge, but with a reverse sweep of the needle, so that with lower vacuum, the needle would swing to the right, indicating a larger throttle opening and "more performance." I never bought one, but remember seeing them in places like the J.C. Whitney catalog and Sears automotive department.
My one and only GM ('76 Monte Carlo), had a MPG guage from the factory, which was just a vacuum guage with red, yellow and green sections.
 

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I have a Autometer vacuum gauge mounted in a "$helby" pod on my console. It is connected directly to a manifold vacuum port.
 

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Discussion Starter #6
The gauge I'm liking so far is this one :

http://www.rockauto.com/catalog/racatalog.php?catalog=104&partnum=CP7978

There are lots of choices out there, I've found, with backlighting and all that good stuff.

Is it possible that there could be a "dedicated" manifold vacuum line? Isn't any access to manifold vacuum spatially coupled to all other access to manifold vacuum, with the pressure being distributed evenly across the available void area? I would have thought that any access to manifold vacuum that's not past a check valve would be an identical vacuum to all other manifold vacuum that's not past a check valve. Doesn't the pressure equalize itself automatically, and at a speed faster than a mechanical gauge needle can respond to?

And for mounting, since it's behind the console padding it doesn't have to be a pretty mount. Sheet of thin steel from Home Depot cut with a dremel tool, heavy-duty Velcro, heavy nylon string or safety tie-down wire, those are probably all goofy but workable ways to hold the gauge in place. Ideally I'll put together a CAD drawing and get my buddy with the crazy CNC machines to cut me a part from Aluminum. But there are plenty of "plan B" ways to do it, I'm sure.
 

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Is it possible that there could be a "dedicated" manifold vacuum line? Isn't any access to manifold vacuum spatially coupled to all other access to manifold vacuum, with the pressure being distributed evenly across the available void area? I would have thought that any access to manifold vacuum that's not past a check valve would be an identical vacuum to all other manifold vacuum that's not past a check valve. Doesn't the pressure equalize itself automatically, and at a speed faster than a mechanical gauge needle can respond to?
Its certainly possible that if the gauge feed was T-ed off of a hose that was large enough that the line itself, plus fittings, bends, etc. didn't create a restriction, the vacuum would equalize itself more or less instantaneously. But I've always heard that for applications where a "clean" vacuum signal is important, (such as for ignition advance or a turbo boost control) its best to have a line specifically for that purpose, feeding from as close to the manifold plenum as possible.
I always try to follow that practice when tuning with a vacuum gauge, but whether it makes any outside-the-margin-of-error difference, I really can't say.
If you're willing to experiment, I'd be interested in knowing, once your gauge is in place, whether you're able to detect any real-world difference in the readings utilizing different sources as a vacuum feed.
 

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Discussion Starter #8
Yep, I would definitely like to test it out, the difference in readings between various connection methods. Test with the brake booster disconnected, the headlight vac system fully disconnected-- or just the tank disconnected, both brake boost and headlight system disconnected, both connected, using a T or not, all that good stuff. I'm very curious to know the effects. From what I understand there's a direct link between strength of vacuum and performance/fuel economy, so it seems like it's worth knowing. I just need to make sure and execute the tests methodically and record the data right, not like last weekend where all I remember is the vacuum at idle was around 14 or was it 16. Lol. But that whole day was a bit of a blur as it wasn't my garage and time was of the essence.
 
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