Poll - Vacuum Gauge Readings

[Administrator]

Vacuum gauge readings in neutral at idle (800 RPM)

[romantic comedy]

I hook up a vacuum gauge once in a while but have not installed a permanent one yet. It is on my list.

[sdemore]

On my list, but behind a bunch of other things. If only boating were cheap, I would have everything automated and instrumented!

[Dave Neptune]

Bill, I'm trying to recall for sure but can't swear to it.

800 RPM engine warmed a few minutes about 18", I do remember I would loose about 1" if the alt was working hard.

Dave Neptune

[ndutton]

I'll take readings tomorrow. Per Dave's suggestion I can provide a number with and without alternator load and can specify what that load is. The carb in place has a fresh rebuild too.

edit: test done, a hair over 17 in. Hg @800 RPM. Batteries fully charged so alternator load was negligible, like 4A.

[capnward]

I almost voted 'don't know,' and I'm far away from the boat until spring, but my best recollection is the needle was a little to the right of vertical, which is 16 or 17. In gear at 1800 rpm it reads about 8, as I recall. That number goes up with a clean bottom.

[roadnsky]

800 RPM - 19”
1500 - 13”
1700 - 11”
2000 - 7”

[Dave Neptune]

Jerry, those numbers other than idle are terrible if not in gear. If in gear their great for a direct drive.

Under load with a clean bottom and Indigo prop my numbers were close to yours posted. My "cruise" speed was set with the Vac gage and my RPM's would indicate how clean or out of tune the engine was. When all was good 8" of vac would give me 2100 and my temp was always rock solid at 130*. This would yield about 6.4 kts in calm water with my E-35MkII and I was always towing a 10' hard dinghy.

Dave Neptune

[joe_db]

Today at idle was 15 inches.

[roadnsky]

Jerry, those numbers other than idle are terrible if not in gear. If in gear their great for a direct drive.

Yeah, they were recorded while under way.
(I have to confess over a year ago)
I'm not currently at my boat. (Sitting in the FL Keys)

[joe_db]

I am feeling under-vacuumed How do I get my number up?

[GregH]

Bouncing right around 15 late summer - early fall when I first installed the gauge. Then I started fiddling and now can't even get an idle down to 800

only another 3-4 months before I can fiddle more... sigh

[ndutton]

Here is an excerpt of an article from Motor Magazine, August 2001 entitled Mastering the Basics - Reading a Vacuum Gauge.

Idle vacuum for most engines is about 18 to 22 in.-Hg, but some may produce only 15 to 17 inches at idle. If vacuum is steady and within these ranges, the engine and fuel and ignition systems are operating normally.

If vacuum is steady at idle but lower than normal, the ignition or valve timing may be retarded. Low compression, an intake leak or tight valves also can cause low vacuum at idle.

If the vacuum reading fluctuates within the normal range-the gauge needle bounces around a lot-uneven compression (broken rings or leaking valves or head gasket in one or two cylinders) is a likely culprit. An uneven air/fuel mix, erratic ignition timing, a misfire, misadjusted valves or a manifold leak near one or two cylinders also are possible causes.

If vacuum drops intermittently at idle, one or more valves may be sticking open or dragging. Higher-than-normal vacuum at idle is a common clue to overly advanced ignition timing, while low vacuum can indicate retarded timing.

Low vacuum also can be an immediate clue to a plugged exhaust. To check further, run the engine at about 2500 rpm for about 15 seconds. If vacuum drops during this period and does not increase when you close the throttle, you're almost certainly looking at a restricted exhaust.

Vacuum Fluctuations & Power Balance
Several of the guidelines in this article have distinguished between steady vacuum gauge readings and fluctuating readings, where the gauge needle bounces up and down erratically. This may seem secondary-almost inconsequential-but it's an important distinction. A steady but abnormal vacuum reading indicates a problem common to all cylinders. Things like incorrect ignition timing or an old, tired, high-mileage engine affect vacuum equally for all cylinders. A bouncing needle, however, usually indicates that the problem is localized to one or just a few cylinders.

[Dave Neptune]

Warning

Neil, that is a very informative post (#13) however it should come with a warning. Virtually all of the dancing of the gage is controlled by the vacuum and the dampening of the gage. Some gages are just to "dampened" to provide much more than a vacuum reading and some are designed for diagnostics with little dampening.

These dancing readings are mostly taken at very low RPM's. The dancing in a up & down pattern reading can be from plenum fluctuations at low speeds and will be symmetric in pattern if all is OK. A pattern of up & down that is not symmetric IE a big pulse is what you are looking for.

Also where the gage is hooked up can influence patterns,, fortunately on the A-4 we have a good access port via the scavenge tube on the manifold. This port reads directly from the plenum which is where you want to be for fluctuations.

When working on the dyno sometimes we would have a port on each intake runner to check for even flow through the manifold.

Dave Neptune

[joe_db]

My vacuum gauge jumped all over the place at first. I transitioned from 1/4" rubber line through a restrictive brass fitting to the clear tubing and then ran that to the gauge. The fluctuations went away. Do not use the clear tubing the gauges come with all the way to the engine, when it gets hot it gets soft and will sometimes come off the fitting or melt. I used fuel line for the first part of the run, it seems to hold up quite well.