Vacuum gauge timing-gctid398524

[postie]

I have never owned a boat with vacuum gauges or had any experience with them. Can anyone tell me what they should read at optimum performance. I was running at about 3200 rpm on a pair of OMC 5.7s and they were reading 5. Also can anyone tell me what these motors should be timed at. Thanks.

[Guest]

That sounds roughly correct.

When reading a vacuum gauge, you can't read the number and compare it to a chart to give you the result. You need to fully understand what you are reading and interpret ALL the readings as a whole preferably combined with other engine diagnostic tools like a leakdown test, RPM drop test, compression test and good ol' just listening to the engine.

For starters, make sure you are reading INTAKE MANIFOLD vacuum and not some other vacuum source. Vacuum ports near the carburetor can be deceiving as some of them are PORT vacuum instead of MANIFOLD vacuum.

At idle, the vacuum gauge should read 18-20 inHg. The needle should be steady and not too shaky. If the vacuum reads too low

When you crank the throttle wide open and let the throttle snap back in place ... a healthy engine will drop the vacuum guage to near zero, then when the throttle snaps back in place the vacuum goes beyond 18-20inHg then comes back to 18-20inHg.

When you crakn the throttle open slowly to say, 3500 RPM, the vacuum gauge should drop ... when you hold a specific RPM the vacuum gauge should return slightly. The higher RPM you hold the lower the vacuum.

Use a quality oil filled vacuum gauge.

[2850Bounty]

makonnen wrote:
When you crank the throttle wide open and let the throttle snap back in place ... a healthy engine will drop the vacuum guage to near zero,

I agree with Makonnen, with exception to the above.

While RPM is returning from high to low, throttle plates are closed, therefor vacuum will increase momentarily.

However, it's rather moot, IMO, because a vacuum gauge for Marine use has little to offer other than for setting low speed fuel metering air screws.

The vacuum gauge perhaps found it's way into the Marine world back in the Old Trawler days, where the operator could back off on the throttle and actually see an increase in vacuum.

For your use, I'd not try to rely on it for any useful cruising information. While you are under power, your throttle plates will be open beyond what would produce a meaningful vacuum reading.... IMO.

*******************************

As for your Ignition Timing, best to see your OEM specs for this. One boo boo, or misinterpreted spec, and you may risk engine damage.

You'll be looking for an OEM Ignition Curve graph similar to this one below, but for your specific engines.

NOTE:

**Marine Ignition curves seldom include BASE advance in the Vertical Scale, so when doing the math, BASE must be added to our numbers.

**When strobing timing marks, BASE can't help but to always be included.

(example curve only)



Be sure that you not only set BASE/Initial, but that you also check the progressive advance, and the "Full-In" TA (total advance) RPM.

When our ignition distributors become old, worn, rusty or sticky, they may no longer offer the correct curve/limit at the correct RPM.

Not enough advance = poor performance and poor fuel economy.

Too much advance = risk of detonation and piston/valve damage.

https://"http://www.baylinerownerscl...e this thread.

.

[Guest]

Awesome information, thanks for posting!

[Guest]

I run a carbed 5.0 with Alpa G2 and would not be without my vac gauge.I get 10 inches at 3700,27 mph,that is kind of my baseline. If Im packing extra fuel,weight, vac will go down for same revs etc. Seems to be an excellent gauge of how things are performing,cleanliness of bottom,prop condition etc. Once I had dinged prop and not realized it, speed and vibration seemed normal but vac was way down(as would max revs have been if Id tried for that)Got looking around and found the defective prop.Your 5 sounds low to me.Would have thought youd be over 10 at those revs.Overpropped maybe.

One thing I have learned is that while a vacuum gauge is a form of fuel consumption meter it is not as straightforward as youd think. ie 10 inches at 3700 does NOT use the same fuel as 10 inches at say 3000.A correlation sheet ie graph, specific to your boat, could yield very good fuel cons nrs but would be cumbersome and difficult to make.

My opinions and experience.

[postie]

Thanks for the info on the vacuum gauges. I always thought that the only purpose they served was to try and keep you from opening up the secondary's on the four barrel in order to try and conserve fuel.

[Guest]

[QUOTE]2850Bounty wrote:
I agree with Makonnen, with exception to the above.

While RPM is returning from high to low, throttle plates are closed, therefor vacuum will increase momentarily.

However, it's rather moot, IMO, because a vacuum gauge for Marine use has little to offer other than for setting low speed fuel metering air screws.

The vacuum gauge perhaps found it's way into the Marine world back in the Old Trawler days, where the operator could back off on the throttle and actually see an increase in vacuum.

For your use, I'd not try to rely on it for any useful cruising information. While you are under power, your throttle plates will be open beyond what would produce a meaningful vacuum reading.... IMO.

*******************************

Rick,

Well, we continue to completely disagree on this one.

I'm not trying to by nasty here, but you seem to give the impression that all engines in "marine use" are in planing cruisers such as yours or of the original poster's. This is simply not true. There are many in use in displacement or semi displacement boats that can operate under a wide range of different loads in the same boat.

Vacuum gauges have two very important functions for any gas engine in any application. As was mentioned, they can be used as a diagnostic tool to analyze the internal condition of the engine and it's state of tune with regards to ignition or fuel tuning.

The other very import use is that it is an indicator of engine load. Contrary to what you seem to believe, this indication of load can be vital for efficient tuning and prop selection depending on what you are trying to accomplish.

As for the original question of what vacuum reading is optimum, it all depends on what you are trying to accomplish. It is generally accepted that for good engine life, continuous operation should not exceed about 80% of the engine's full rated HP RPM AND not run any lower than about 5" vacuum at sea level. 8" is better, 10" better yet.

[Guest]

+++1 Robert, the ideal way is note the response to various engine loads and revs and " WRITE IT DOWN AND KEEP IT AS A RECORD".

Then every now and then refer to it to keep a check on engine wear and tune.

However it does take time to learn and practice the full range of uses for the good old Vaccy gauge, but well worth the tme !

Tip, get a gauge with an electronic wire sender and not the old capillary type for convenient helm mounting.

All the best Joe

[2850Bounty]

Robert, I always enjoy reading what you have to say, and I never take offense to any disagreements that we have. I value your opinions very much.

We're good in that regard.

I do not totally disagree with the use of a vacuum gauge, and I certainly understand the dynamics between what occurs via larger cruisers -vs- smaller hulls re; engine manifold pressure.

In a small hull, there may very well be readings that could be useful.

In a larger hull, I believe that this diminishes.

As I recall, our original debate began with the use or no use of a vacuum diaphragm ignition advance system..... of which I still maintain has little to no value in a Marine application. We don't see this being used.

This excludes the vacuum sensors that we do see being used via some of the MPI systems, etc.

.

[Guest]

wHAT IS A GOOD gauge with an electronic wire sender AND WHERE CAN I GET ONE ?

TRIMIXJOE wrote:
+++1 Robert, the ideal way is note the response to various engine loads and revs and " WRITE IT DOWN AND KEEP IT AS A RECORD".

Then every now and then refer to it to keep a check on engine wear and tune.

However it does take time to learn and practice the full range of uses for the good old Vaccy gauge, but well worth the tme !

Tip, get a gauge with an electronic wire sender and not the old capillary type for convenient helm mounting.

All the best Joe

[Guest]

To me, the 5in at 3200 seems low. I'll be your wot is about 4k?? If this is true, either lighten the boat somehow, or if the weight won't be reduced anytime soon and you have money burning a hole in your pocked I'd reduce prop pitch.

On plane, my boat (although smaller than yours) runs at about 8-9 at 3500, and 5 at about 4k, wot = about 4.6k.

You can use your guage for efficiency. I try to achieve the lowest reduction in speed for the highest increase in vacuum. the vacuum is a great indication of load, and if we consider that the carb is working well and the air fuel ratio is close to fixed, then it's also an indication of fuel consumption. So, slightly lower speed with an increase in vacuum means better gallons per mile and I can buy another beer when I get there!!!

I wouldn't be without my guages either. I'm going to add them on my twin 2850 for sure. I go so far as to set my cruise by vacuum rather than by rpm, because my weights (mostly people) are always changing.

Chay

[2850Bounty]

gary1p wrote:
wHAT IS A GOOD gauge with an electronic wire sender AND WHERE CAN I GET ONE ?

Vac by wire? :kidding

[Guest]

I am in the U.K. so its not much point reccomending a supplier from here, just do an internet search, most race tuning shops of any repute should stock or able to order same.

There so cheap i have one wired to the dash and a capillary on a bracket on the engine for easy tuning [ can't see the dash one from the engine bay ], just makes life easy.

ever the customizer Joe

[Guest]

where did you put the sending unit?

[Chief_Alen]

http://www.secondchancegarage.com/public/186.cfm