Vacuum gauge instead of Scangauge? - Page 2 - Fuelly Forums

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Old 07-16-2007, 10:28 AM   #11
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BMac: as the rpm goes up, the power goes up, but the fuel efficiency goes down (fuel used for a given amount of power). So you wind up shifting before you get to the "sweet spot"

StanleyD: It seems obvious now that you mention it, but you must be correct. Turning the engine over the whole time has to be a significant penalty, even if it is running at max efficiency.

Scenario (more made up numbers): You are in 5th gear, trying to go a mile
effort one, you keep the engine lit, your engine turns 3000 revolutions
effort two, you coast half the distance, your engine turns 1500 revolutions.
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Old 07-16-2007, 11:08 AM   #12
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StanleyD, you've covered it very well I think.

Back to Jim's question re. the ScanGauge,...

There's nothing like real time data to let you know how you're doing. If you're not sure whether to take the hill at 45 or 50 or 55, your scangauge will tell you.

Looks like you've done some nice mods to the car.
Do another one that will really help with FE, get the ScanGauge.

I've seen about a gazillion posts on CleanMPG.com from people who have reported 10-20 percent increase just from putting one in their car.
I wish my '89 would support one, but they are based on OBDII which began in '96.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 07-16-2007, 12:49 PM   #13
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I have a vacuum gauge installed in my car, and have gained some knowledge from it however about all I use it for right now is to know when my engine is off/on. (no tach)

If you are using pulse and glide a vacuum gauge doesnt work, as the point is to keep your engine either loaded or off.

If you are trying to drive like a granny to achieve MPG then the vacuum gauge will work as long as you don't pay attention to it saying 2nd gear at 30 is better than 3rd as it does in my car. It wants me to rev my engine before I change gears cause if I change too soon there is actually load on the engine causing the vacuum to decrease.

From what I have gathered load and easy throttle would be better than next to no load and heavy throttle.

Of course most of this is just common sense that you would have figured out had you installed the gauge.

On that note, my opinion is if you can get one for cheap/borrow one it might be worth it. I wouldn't mind getting rid of mine at this point after having it in for a month or two. I have pretty much gleaned all the info I can from it as I use Pulse and Glide as my driving style.
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Old 07-16-2007, 05:28 PM   #14
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Quote:
Originally Posted by Jim Dunlop View Post
I was talking with my friend today and he said a great way to monitor fuel economy is hooking up a vacuum gauge to the intake manifold... Has anyone tried this and been able to train him/herself?
Yep; my brother, many decades ago. Check out his comments in this message from my intro thread.

(The same thread also has a photo of my dad with one of his Isettas, for anyone into Urklemobiles. )

Quote:
Originally Posted by Jim Dunlop View Post
I am still having difficulty understanding, from a physics equation standpoint, how P&G can be better than constant cruising speed... My gut instinct would be that constant speed trumps ANY amount of acceleration in terms of the amount of fuel to get from A to B.
You're not alone. I have the same difficulty, as does a PhD friend of mine I asked for an explanation. But I've seen it for myself, so while I'm still trying to get my head around the how and why, I do accept that it works... and I use it to my advantage.

(For what it's worth, probability and statistics don't respond to gut instinct either. )

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Originally Posted by StanleyD View Post
...just drive at 60 to top of a gradual incline and then record speed at bottom of hill. Then do the same experiment BUT put the car in neutral just before you start going downhill. I bet any dollar amount that speed at bottom of the hill in neutral will be HIGHER than when car was in gear.
I've seen this numerous times -- at many different speeds -- since I started doing NICE-ON P&G (I have an automatic but this part works the same as with a stick shift). If I DWL and hold the throttle constant, I'll see MPG climb up to 80 or so and my speed increases a little if the grade is steep enough. I used to think that was pretty cool ("Wow, 80MPG!"), until I realized the engine was actually holding the car back. If I drop it into neutral and take my foot off the gas, MPG more than doubles the other figure, and speed picks up nicely on even subtle downgrades.

In other words, free acceleration.

And it feels so velvety smooth!

Rick
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Old 07-16-2007, 06:21 PM   #15
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Gauge

This is what is installed in the Jelly Bean. Thought at first it was good only for looks. Thats changed, with only 3 cylinders its in the green way to much. So it took a while to learn its value. One is it buzzes when its into the power quite when its not. Very seldom do I see red, only down hill. Going up hill the help comes in by showing me how to stay out of the power. I have learned to lock my leg going up hill better. No chance for a scan gauge sooo we do with what we have. And its cheap like me!

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Old 07-16-2007, 07:20 PM   #16
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OK thanks for the explanations; I really appreciate them instead of the usual "USE THE F-ING SEARCH FUNCTION" response on SaturnFans. I get it now -- I had been doing Pulse and In-Gear Glide instead of Pulse and Neutral, Engine-On Glide!

So: accelerating slowly but having engine-idle-at-highway-speeds sessions uses less fuel than a constant, semi-high speed. Won't help me much on my new commute; I do 55-60 the whole time!

Rick Rae, that's probably one of the reasons why Statistics and Probability are not considered Mathematics AFAIK.
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Old 07-17-2007, 04:33 AM   #17
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Originally Posted by Jim Dunlop View Post
I get it now -- I had been doing Pulse and In-Gear Glide instead of Pulse and Neutral, Engine-On Glide!
Well, in-the-know GasSavers tell me that above a certain RPM, below a certain load, in gear (did I get all the conditions right?) a lot of vehicles will cut fuel to the engine entirely, which puts you on par with EOC as far as fuel consumption is concerned. On those vehicles I guess it comes down to: Do you gain more using idle fuel over a long glide in neutral or using no fuel over a short glide due to engine braking? I'm still experimenting to find out which is the case with my vehicle... two tanks of in-gear "gliding," this one's in neutral.

Another approach I've played with a few times is to leave it in "D" and try to find the "Goldilocks zone" with the accelerator. By that I mean: If you just take your foot off the gas in gear, you start engine braking. If you leave your foot locked in place, your MPG value goes up somewhat because the load is reduced thanks to the downslope. Somewhere in between, there's a point where you're feeding just enough gas to rev-match the engine to how fast the road wants to turn the tranny. So you're not engine braking, but neither are you putting any load on the engine. If you tracked it perfectly you'd get the same glide distance as you would in neutral. Again it would come down to a similar question: Do you gain more running the engine at higher RPMs but with zero load over a long distance or in idle over a much shorter distance?

I wouldn't even bother with that, but A) I've yet to see evidence that my car does fuel cutoff, and B) throwing my automatic in and out of neutral at highway speeds makes me uneasy (when I think about it ). Basically, I'm still searching for what works best for my vehicle, my abilities, my routes. Fun stuff!

Quote:
So: accelerating slowly but having engine-idle-at-highway-speeds sessions uses less fuel than a constant, semi-high speed. Won't help me much on my new commute; I do 55-60 the whole time!
Pulse to 60, glide to 55. Is your terrain perfectly flat, or do you have some hills you could glide down without losing speeed? Even overpasses can be useful.

Bottom line, I think, is that P&G is just another available tool. Use whichever ones work best for you and your situation.

And that includes vacuum gauges.

Rick
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Old 07-17-2007, 05:56 AM   #18
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The more automated things are, the harder it is to get them to do what you want
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Old 07-17-2007, 11:14 AM   #19
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Quote:
Originally Posted by StanleyD View Post
Personally, I think that one of the biggest things that P&G overcomes is ENGINE DRAG.
An informal test occured to me on the way in, so I tried it:

at a light footed 50mph on a flat road, I was doing 2500 RPM @ 0.8 GPH
In neutral @ 2500 RPM I get 0.4GPH

So, roughly speaking, at 50mph, HALF my fuel is going into keeping the engine turning.
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Old 07-17-2007, 11:40 AM   #20
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Originally Posted by Rick Rae View Post
... Pulse to 60, glide to 55. ...
Lately I've been P&G'ing my auto-tranny car between 50-62 mph. Up to only 60 if I and the traffic can stand it. If I'm on a 40-50 mph limit road I'll P&G between 40-50 or 45-55. On a fast road with a steeper longer downgrade I'll goose it up to 70-75 when I'm on the downhill side, and let her coast longer faster. On upgrades I limit my throttle, really a drive-with-load technique. Charging up the hill seems to burn fuel quickly but I have no ScanGauge on this old car to know for sure.

For my car I coast in neutral with engine on. My tach has shown me that at highway speeds in Drive, rpms are about 1500 vs. 800-900 in neutral. I'm pretty sure my '89 doesn't have fuel cut for such coasting.

I do cut the engine for long slow coasts, upcoming red lights, etc. but only below 40 mph as that's the "flat towing" speed limit for my car. Many other auto trannies won't allow that.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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