[gr_diver]

Hello everybody!

At first I would like to apologize if this is irrelevant with the "Mechanical" section, but it didn't seem to fit in the "Electrical, Gauges, etc." section either, as there it is electrical talk.

I am interested in installing a mechanical vacuum gauge in my car so as to use it as an econometer.
I have the following question . Where should I connect it, to the ported tube on the carb (with a "T" to keep the distributor advance canister working), or directly on the intake manifold (where the valve cover vent-tube goes)?

I am interested in going economical, as gas prices have gone way up where I live...

Thank you in advance!

 

[soybean (R.I.P.)]

gr driver, Here's a thread that may help. http://www.opelgt.com/forums/showthread.php?t=2543&highlight=vacuum+gauge
Jarrell

 

[gr_diver]

Thanks Jarrell, I was actually thinking of using the intake manifold but wanted a second opinion!

 

[neuropel]

How do you use it?

What levels of vacuum tell you you're getting good economy? Gene Smith (BQS4) gave me a VDO vacuum gauge that fits nicely in the rallye gauge pod, but I've never bothered hooking it up (remember that Gene?). I refinished the bezel on it and it turned out pretty nice... might as well put it to use! I can't justify the cost of a new VDO clock to finish out the set (yet ).

Todd

 

[BQS4]

Dude;
At least it looks good in the car and in your avatar!! ROFLMAO!!

 

[gr_diver]

When it's not a dash gauge (thus calibrated for your car) you can assume that the highest the vacuum you get, the more economical you go (during cruising). After driving with it installed for a while, you will be able to say whether you are going good, or you can adapt your driving to get an even higher vacuum...

 

[kwilford]

When it's not a dash gauge (thus calibrated for your car) you can assume that the highest the vacuum you get, the more economical you go (during cruising). After driving with it installed for a while, you will be able to say whether you are going good, or you can adapt your driving to get an even higher vacuum...

Well, not always. Typically, low engine rpm (as in low friction losses) are the friend of economy, as is slow (or "gentle") acceleration. So the best "economy" (highest fuel consumption "efficiency") happens at different times for different reasons. And "usually" the highest manifold vacuum relates to the lowest rpm and hence best efficiency.

The notable exception is on acceleration. There is something called "pumping loss" in an Otto-cycle (and not the tekenaar Otto) that plays a part in efficiency. The engine has to "suck" air past the carb (or FI) butterfly, which requires work to be done that is not converted to motive force. This is the reason that a diesel engine is so efficient at an idle, whereas a spark engine is poor (and why big diesel trucks are left idling virtually all winter).

To minimize pumping losses, the best scenario is low rpm and as wide open a throttle (without kicking in extra fuel via a power circuit or enrichment jet) as can be achieved. It has been shown that the best "accelerating efficiency" is at nearly wide open throttle (virtually eliminates pumping losses) at the lowest rpm (lowest engine friction), with frequent shifts to keep the rpm low. And "wide open throttle" always shows the lowest vacuum, so that is a case where high manifold vacuum simply doesn't show the best economy.

JM2CW

 

[greensmurf20]

Keith:

In short your post can be taken as putting the pedal down nearly as far as you can get it without kicking in the secondary and accelerating quickly is sometimes more efficient than just slowly accelerating up to speed if you still shift and low RPM?

 

[kwilford]

Keith: In short your post can be taken as putting the pedal down nearly as far as you can get it without kicking in the secondary and accelerating quickly is sometimes more efficient than just slowly accelerating up to speed if you still shift and low RPM?

Yep. BMW (and another company I can't recall) did a bunch or testing about ten years ago, and proved that the best acceleration efficiency was at something over 3/4 throttle, and below a maximum 3000 rpm.
And for that matter, the best cruising efficiency is at the highest gear possible, for two reasons:
1) lowest engine rpm
2) lowest pumping loss

and I think you will find that manifold vacuum is lower at those conditions that at higher engine rpm (lower gear) with the throttle plate more closed.

 

[greensmurf20]

i ususally put the pedal down to where i can feel the secondary tension, so its not opening the secondary plate, and shift around 3,500 to 4,000 RPM. must explain why i still get 26.5 MPG with a weber and large camshaft. would probably get better if i'd set the carb up right, was right until dad fideled with it sayin "carbs can't be right straight from the box". well it seemed a heck of alot closer than it is now, lol.

 

[RallyBob]

Though this is entirely unscientific, I've found that the best mileage in my Opels has typically been at or around the peak torque rpm. This usually occurs (again unscientific) at the cruising rpm which has the loudest exhaust tone. Conversely, the mileage near the peak hp rpm (if jetted correctly) will often be as good or better as the peak torque mileage. Case in point, one particular car of mine got 32 mpg at 92 mph, but got 26 mpg at 65 mph. Go figure.

Bob

 

[greensmurf20]

Bob: go figure i drive about 65 or 70 MPH, maybe i need to drive FASTER!!!

explain that to the copper, "speeding cuz i get better gas mileage, i swear!" LOL

 

[gr_diver]

I connected the vacuum gauge today.

At first I had some problems, as I discovered that the port on the manifold was clogged on the inside*, but with the help of WD-40 and a very thin wire I managed to unclog it. I also replaced the hose from the valve cover to the intake manifold with a new one.

I have this issue though!

If I have only the vacuum gauge connected to the intake manifold's port I get a reading, which is about -18" Hg, and so far the thing works. But if I use the "T" adaptor and have both the valve cover hose and vacuum gauge connected to the intake manifold I get no vacuum reading.

Can this be explained, because the vacuum created in the valve cover from the intake manifold is "satisfied" by air coming from the air-filter housing? I don't think this is correct because the oil vent system should be a closed circuit with stable negative pressure (at least at idle that I test), that should be displayed by the gauge...

Common sense would say that there is a leak in the system, but the hoses and "air-filter to carb" gasket seem air-tight.

What do you guys think?



*(this propably explained why I had the oil fumes coming in the air filter through the relative hose from the valve cover)

 

[ddoyle]

you wont get a reading on that hose if it is also hooked up to the valve cover. you are right, that vacuum gets "satisfied " in the valve cover. On my engine the large fitting for the brake booster has 2 small fittings on it, I use one of these for my vacuum guage, works great.
DAN

 

[gr_diver]

Well, I am unlucky then, because on my intake manifold there are only the port for the hose from the valve cover, and a single larger port (the type that gets "screwed" in the manifold) for the brake-booster hose. I will propably buy a new fitting for the brake booster hose that will be slightly longer (like an 1" extension) and have a machinery shop drill a hole and weld a small tube (like the one on the manifold for the valve cover's hose) to use as a port for the vacuum gauge...

 

[jvandyke]

retard port?

I bought a vacuum gauge off ebay (it was too cheap!) Hooked it up to the port that used to supply the vacuum retard on the distributor.

The gauge works but reads really low, idle or decel it shows 7 Hg, acceleration 0. Even though it was cheap and no big deal to ditch it, it's fun to fiddle with. I have yet to check those numbers with another gauge but I'm wondering if this is a decent place to hook it up?
Now this gauge is weird in that it has LED light(s), a harness, a box (voltage regulator I assume) and TWO pots, serious overkill just for a light! I hooked up the thing to a battery on the bench to see what the pots do, not sure what they're supposed to do (no documentation, it was a close-out item, for good reason probably!) They both seem to act as dimmers for the light. Maybe it's meant to work with other gauges in their series, different light colors, don't know, don't care really. Maybe one of the pots is a calibration control? (I haven't hooked it up to power in the car yet, just the vacuum line). Anyway, if it's worth keeping I'll make a little housing for it and hang under the dash.

 

[tekenaar]

Metered orifice = early PCV valve

you wont get a reading on that hose if it is also hooked up to the valve cover. you are right, that vacuum gets "satisfied " in the valve cover. On my engine the large fitting for the brake booster has 2 small fittings on it, I use one of these for my vacuum guage, works great.
DAN

Well, I am unlucky then, because on my intake manifold there are only the port for the hose from the valve cover, and a single larger port (the type that gets "screwed" in the manifold) for the brake-booster hose. I will propably buy a new fitting for the brake booster hose that will be slightly longer (like an 1" extension) and have a machinery shop drill a hole and weld a small tube (like the one on the manifold for the valve cover's hose) to use as a port for the vacuum gauge...

As ddoyle said, that particular port with the tiny hole can't be used for your gauge as it's used as an early solution for "metered" venting of the crankcase at the valve cover and is called "crankcase ventilation metered orifice" in every FSM! This later became the PCV valve - Positive Crankcase Ventilation - which may be more familiar to all.

This metered orifice is recommended to be cleaned every 6,000 miles in "Section B - Maintenance and Lubrication" of every FSM. This had probably never been done - I mean, who ever actually reads this section that closely anyway?! - and why you had to unplug it in the first place. :banghead:

 

[jvandyke]

I'm not attempting to tap into the small valve cover hose. (My intake manifold has that fitting near the head, inboard side of the carb). I'm using the vacuum retard line off intake manifold, near the brake booster hose, like ddoyl I assume. It too is a rather small port and thus the question: is it usable for a gauge or is it metered by it's size so as to be not the best choice? The supplied hose to the gauge is a very small gage too, probably 3/32" ID.

 

[MICAH1]

Vacuum Ports

I have a VDO vacuum gauge mounted in the dash where my clock died a respectful and "TIMELY" death. I ran the line from the gauge to the vacuum port on the intake manifold. I then relocated the metered orifice line from the valve cover to the vacuum advance port on my 32/36 weber with a "T" connector supporting the line from the distributor and the line from the valve cover.

Pictures below show the vacuum gauge line connection to the intake manifold, the "T" connection at the base of the carb, and the VDO gauge showing a respectable 18.5 Hg @ idle.... :veryhappy

 

[jvandyke]

That looks great. Super alternative to a busted clock. I thought that the vacuum advance port on the carb was "controlled" by the throttle plate; no vacuum at idle, throttle plate uncovers hole in carb; vacuum for advance on acceleration. So your valve cover isn't getting "sucked out" during idle when vacuum is highest?
I figured the retard port was simply a nipple on the intake, always seeing whatever vacuum is there and therefore a good place to hookup. But my readings are very low and it's either:
1) the gauge
2) that's all the vacuum there really is (yikes, I doubt it, I've seen more with another gauge last summer)
3) I've got a leak
4) I'm hooking up to a stupid place

I'll go hook up another gauge at various places and see what I get.

Addendum: hooked up another gauge, read 18 Hg, gauge is stupid, contacted someone else with same gauge, theirs wouldn't work at all.
It's the gauge. I'll put it on the end of the list to rip it apart and play with it. It is a nice looking gauge.