What's my vacuum gauge telling me?

[95metro]

This weekend's project consisted of installing a vacuum gauge in my Metro. It was a bit of an adventure and if I never have to remove my gauge panel again it will be too soon (I had this "brilliant" idea to try and place the gauge in or over top of the panel, but that was a bit over zealous).

Anyway, I plugged the vacuum hose inline with the MAP sensor hose, but I wasn't certain if it was the best idea (it was the only hose the same size as the fittings that came with the gauge). So, what is this gauge telling me? Can anyone with a vacuum and/or scan gauge help me begin to interpret what's going on?

At normal idle the needle is around 16 in. Hg. During acceleration from a stop it drops to 5 in. Hg and builds from there. Cruising at 60 km/h in 5th gear keeps it around 11 in. Hg.

However, I played with the gearing and the results are all over the place. I even revved the car up to 50 km/h in 2nd gear and the needle was hovering around 15 in. Hg...so, I can't figure out what's going on. Maybe using the MAP sensor vacuum isn't the correct thing to do?

[krousdb]

Ouch! only 16 in hg at idle? Unless suzuki engines are lots different than Hondas, you should expect over 20 in hg at idle. Maybe you have a vacuum leak?

What it tells you is how much load you are putting on the engine. It is best to use to target acceleration rates as it gives instant feedback on ho much you are asking the engine to do. It is a poor substitute for a scan guage but it is better than nothing. I removed mine after I installed the SuperMID.

[95metro]

In gear coasting drives the needle up to 21 in Hg. The Haynes manuals stated that when testing with a vacuum gauge readings around 17 in. Hg were normal at idle. They seemed to be referring to most engines in this case, not just the Metro's.

It could be that Honda's do read higher. They do have a tendency to get better fuel economy than most engines in the same liter and/or hp class.

Also, the little car needs a new exhaust. The cat is original and it's straight-piped (due to previous owner). There's no back-pressure from a muffler so the low-end powerband could be reduced a fair amount.

[95metro]

Oops...definitely shouldn't have used the MAP sensor hose. Found this information: http://www.centuryperformance.com/vacuum.asp

Connection of the gauge is to a simple "manifold" vacuum source. "This must NOT be "ported" vacuum that rises as RPM increases".

That's exactly what my gauge is doing. I was getting higher readings at high RPM. Guess I'll have to find another hose to use. I wish the instructions for the gauge had been clearer.

[SVOboy]

At the very least, it's telling me to get off my *** and install my own,

Have you seen Dan's thread about using his vacuum gauge for driving?

[JanGeo]

There are no other hoses to use - you are using the correct one on the MAP sensor. The vacuum numbers you are seeing are correct but sound a little low - idle vacuum should be a little higher but that depends upon your ignition timing - more advance will increase the vacuum. Higher RPM should get you higher vacuum but it also sounds like you have some intake restriction?

[95metro]

Thanks guys. I drove around after work last night and realized that it probably was giving me correct readings after all since it basically gives me a readout on my throttle response - vacuum increases with RPM as long as you keep a steady throttle. Push the pedal down and that needle drops pretty rapidly.

I've been trying to keep the needle from dipping below 5. This means I'm starting slower and actually shifting slightly later than usual. I have a lot of experimentation to do.

Oh, right...and I haven't seen Dan's vacuum gauge thread.

[JanGeo]

keep the vacuum high improves the fuel vaporization but increases pumping losses - on my Geo I did not see much of a difference between lugging it and reving it - friction increases at heavy loads on the rings and gearing so I think it cancels out.

[MetroMPG]

Originally Posted by JanGeo

on my Geo I did not see much of a difference between lugging it and reving it - friction increases at heavy loads on the rings and gearing so I think it cancels out.

RPM is a demonstrated FE killer under steady load: http://www.gassavers.org/forum_topic/rpm_gear_choice_vs_mpg_at_constant_speed.html

[95metro]

The pattern that I have become used to is from 1st to 2nd at 10 km/h, 2nd to 3rd at 30 km/h, 3rd to 4th at 40 km/h, and 4th to 5th at 50 km/h (unless I'm trying to hit 110 km/h on a merge lane for the highway...double those figures then).

What I've been trying with the vacuum gauge is simply adding 5 km/h to my shift points (15, 35, 45, 55). Maybe it will make a difference, maybe not. There's not going to be any definitive answer unless my mpg average shoots up (or down).

On the vacuum gauge it LOOKS like the engine is less strained and my ears tell me that the RPM difference is minimal.

The primary difference is going to be how fast I take off from stop signs/lights. I have always found the Metro's accelerator to be rather sensitive so now I'm really trying to feather it to keep the gauge needle from dipping too low.

[MetroMPG]

i shift pretty much at the same points you do.

[95metro]

That's good to know.

The vacuum gauge is pretty much doing what the ScanGauge's instant mpg reporting from what I can understand. Which seems to be tied into the instantaneous throttle response. As rpm increases, vacuum increases as long as the throttle remains in the same position.

If there were some way to capture and graph the instantaneous readings then I would be able to match the gauge readings to roughly extrapolate my instant mpg. Over time I may be able to start guess-timating and adjust my driving to suit.

[JanGeo]

RPM is a demonstrated FE killer under steady load

Yeah But I mean for acceleration - quick revs in the 2 lowest gears and pop it into 5th at 25mph instead of a little 1st 2nd 3rd 4th then 5th. Got to get the oil flowing a little.

[JanGeo]

adjust my driving to suit

Keep the Vacuum at a constant mid range and you maintain a constant vacuum advance on the timing which helps burn the fuel a little better, gets more power and constant vaporization of the fuel.

[MetroMPG]

Originally Posted by JanGeo

Yeah But I mean for acceleration - quick revs in the 2 lowest gears and pop it into 5th at 25mph instead of a little 1st 2nd 3rd 4th then 5th. Got to get the oil flowing a little.

ah, gotcha. have you tested it with your scangauge on the xb? what does it like in terms of acceleration?

i wish someone would do some controlled acceleration testing. i'm curious for some hard numbers. (and i have too much on my plate right now to do it myself. :O )

[95metro]

After driving around with my vacuum gauge installed I have noticed a few trends developing. Here are a few numbers for the curious observer:

Vacuum at idle is 18 in. Hg in warmer temps (13°C) with the engine fully warmed up.

The most common numbers during acceleration are 7 to 9 in. Hg...could be indicative of average mpg.

Exceedingly mild acceleration will yield 10 to 11 in. Hg, but I am subjected to dirty looks by drivers behind as they pull out and pass as soon as possible.

Cruising at 45 mph (65 km/h) seems to be yielding the best mpg - from 12 to 13 in. Hg on the gauge. Gauge will drop to 8 in. Hg on the mildest of inclines and idle or engine brake on the mildest of declines.

Wish I could do more pulse and glide driving, but there's too much traffic at most times.

[JanGeo]

The xB doesn't like lugging briskly, the Fuel Injection gets spastic if the engine is not reving in the 1500 to 2000 rpm range - the gearing is so low that the shifting time in each gear is only a few seconds and the computer has a hard time getting the settings right in that short of time so it feels mushy on the throttle response. I also notice that the MPG is pretty bad in the lower gears maybe they need breaking in more - the Geo would do the same thing usually the plugs needed gapping when that started happening. Best to keep the rpm low and sneak up on more throttle gradually and just wait for the speed to build. Adding a passenger yesterday and having damp weather sticky clutch and the seat a notch too far forward made for some really bad shifts and I really noticed the weight difference.

[95metro]

Bosch +2s or +4s are worth it on small cars simply because you don't have to gap them. The +4s worked out to $17 for my Metro. Not bad for top notch plugs.