Vacuum gauge instead of Scangauge? - Page 3 - Fuelly Forums

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Old 07-17-2007, 03:41 PM   #21
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Originally Posted by Rick Rae View Post
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.

Rick
Using the Scangauge for feed back I can tell that my 4 Runner does not cut the fuel in engine on coasting at all. Going down hill at 60mph in gear the fuel burn is .5-.6 gph, flip in neutral its .3-.4 gph at 60mph. The instant FE numbers take a jump higher when I flip in neutral as well. So for me its far better to coast in neutral.
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Old 07-17-2007, 03:57 PM   #22
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Originally Posted by Mentalic View Post
Using the Scangauge for feed back I can tell that my 4 Runner does not cut the fuel in engine on coasting at all. Going down hill at 60mph in gear the fuel burn is .5-.6 gph, flip in neutral its .3-.4 gph at 60mph. The instant FE numbers take a jump higher when I flip in neutral as well. So for me its far better to coast in neutral.
Which is an excellent example of why to use a Scangauge.
Far better than gut instinct, other gauge types, pretty much anything else, if you want improved FE.
Requires OBDII which started in '96 so not workable for older cars like mine...
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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.

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Old 07-18-2007, 03:10 AM   #23
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I don't want to spend the $$$ on a SG. I think my car does fuel cutoff.

Can any other Saturn owners here confirm that s-cars do fuel cutoff? Even if they do cutoff, it seems to me like the glide goes MUCH further out of gear than in gear.

In any event, I've changed my commuting route over the past couple days to one that seems to avoid long waits at left turns. Yay!

However, my MPG may suffer for a little while yet -- I changed my battery a couple weeks ago, and I think in so doing wiped out the computer's FE knowledge.
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Old 07-18-2007, 05:14 AM   #24
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I don't want to spend the $$$ on a SG.
Rather spend it on the extra gas you're burning from driving blind, eh?

Rick
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Old 07-18-2007, 05:40 AM   #25
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Seriously Jim, you drive a lot, the scangauge will pay for itself in 1/3 the time of a "normal" person.
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Old 07-18-2007, 07:31 AM   #26
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On the vacuum gauge, it works more for a carbed vehicle than a fuel injected one. A carb uses airflow across a venturi to create a vacuum at a specific point to mix fuel into the air, but under a situation where the carb is in a full vacuum condition doe to the throttle plate being closed it won't draw as much. On a fuel injected vehicle, the engine is going to put in fuel depending on the O2 sensor in closed loop, with input from the TPS, MAF and IAT with the primary reading coming from the O2. Keeping it in a high vacuum state doesn't make that much difference on fuel injection, at least it didn't when I tried it a few years ago. Pre-computer cars with engine economy gauges were just vacuum gauges.

On the P&G with slight throttle, I think there is something to that. I don't do P&G after trying it and seeing my mileage drop (my engine is too underpowered for the vehicle, takes way too much gas and time to get the speed back up) but I have noticed that if I have a very slight throttle on I can coast farther than I can with no throttle. I figure that with the throttle slightly open, the engine isn't trying to draw air from the tiny idle bleed hole, it's getting it from a much larger area so it isn't pulling nearly as hard.
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Old 07-18-2007, 11:55 AM   #27
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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?
Maybe I misunderstand what you are saying here but I can't see how this approach could be as good as coasting in neutral (at least for FE, safety is another question). I think what you are describing is giving the engine just enough gas to overcome the effect of engine braking so that your speed at any point on the hill will be equivalent to what it would be if you were coasting in neutral. I think that the amount of gas required to do this would be the same as what would be required to rev the engine to the appropriate rpm in neutral (i.e. there is no load on the engine except its own friction, and the power required to pump air, and whatever other losses are involved). Therefore you would burn the same amount of gas by this approach as coasting down the hill in neutral with the engine revved to the higher rpm and I don't think that would produce better FE then just coasting in neutral. I would actually like to be wrong about this because I would rather coast in gear then in neutral if the FE is the same.
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Old 07-18-2007, 12:50 PM   #28
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... I think what you are describing is giving the engine just enough gas to overcome the effect of engine braking so that your speed at any point on the hill will be equivalent to what it would be if you were coasting in neutral. I think that the amount of gas required to do this would be the same as what would be required to rev the engine to the appropriate rpm in neutral... Therefore you would burn the same amount of gas by this approach as coasting down the hill in neutral with the engine revved to the higher rpm ...
I think there's a significant difference between the fuel needed to overcome engine braking and the fuel needed to rev the engine to speed in neutral - because if you're going to coast in gear you'd be providing that fuel continuously during the coast - assuming you could find the exact correct pedal position to accomplish it. If you're coasting in neutral and need to spin the engine up before going back in gear you only need to do it once at the end of the coast. A huge difference, as long as you're coasting more than ten feet.

If a driver just doesn't find sufficient control with the engine in neutral I won't disagree with their own perception. However I think it would be difficult or impossible to show that you can coast in gear as well as you can coast in neutral. With or without fuel or wind or anything else. If you're in gear then the car's momentum is being used to spin the engine. If spinning the engine is of value to the driver then OK. But let's not fool ourselves into thinking it's going to get you as far on the same fuel compared with coasting in neutral.

If you want to improve on that, you cut the engine off, or install a fuel cut button. I rarely cut the engine on my auto trans car but I know that some standard drivers do it. You can bump start a stick shift car pretty quickly so it's not quite the same as with an auto.
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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 07-18-2007, 01:50 PM   #29
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Jim: my $0.02 worth.

A vacume gauge is not going to be as helpful as a scangauge, no question. However a vacume gauge beats the daylights out of nothing.

As far as cost goes, a lot of people who have bought a scangauge have found they have been able to make some significant improvements over what they had been able to do before. I don't think I've heard anyone who's said they didn't think they had gotten enough of a return from their scangauge, to make them feel it had been worthwhile to purchase.

However, I don't have one, largely because I don't have the extra cash for one. Consequently, I do the best I can, with what I have. Also I principaly drive a 1989 Honda and I can't use a Scangauge with it.

As far as coasting, engine on, in neutral, versus engine on, in gear, their are factors which are at play, that are not all the same. For example, by monitoring the Oxygen Sensor, I can tell that if the engine rpm is above about 1200 and the throttle is fully closed, the ecu quits firing the injectors, totally. Consequently, if I am in a position to coast, in gear, with no throttle position, then I get more effectiveness than if I put the car in neutral and coasted. If I could coast farther enough, in neutral, would it be better? Don't know, but that's where a scangauge might help.

I am probably going to try getting a fuel mileage display from an Acura and hook it up on my car, because although it's technically incorrect, the relative efficiencies are what I'm really after. Additionally, since other people on GasSavers have already figured out the wiring and so forth, this seems like a great possible, cost effective way to go, for me.
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Old 07-18-2007, 03:49 PM   #30
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Originally Posted by brucepick View Post
...I think it would be difficult or impossible to show that you can coast in gear as well as you can coast in neutral.
You could safely drop the "difficult or" in a vehicle without fuel cutoff (as mine appears to be).

Quote:
...let's not fool ourselves into thinking it's going to get you as far on the same fuel compared with coasting in neutral.
If someone suggested that, I missed it. I certainly don't think that's the case. You guys with stick shifts have it all over us automatics in that regard (as well as others).

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Originally Posted by viper75 View Post
Maybe I misunderstand what you are saying here but I can't see how this approach could be as good as coasting in neutral (at least for FE, safety is another question).
You didn't misunderstand, you merely overlooked.

FE-wise, I absolutely agree that coasting in gear can't match NICE-ON coasting... which in turn pales in comparison to EOC. The bit you might have overlooked is this:

Quote:
Originally Posted by me
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 ).
I can't safely EOC with my automatic. I've watched my transmission temperature climb unnervingly high during NICE-ON experiments, I sometimes think I hear disturbing sounds bouncing off of reflective surfaces when I drop it back in "D," and I don't have a definitive answer to whether NICE-ON coasting is safe for my vehicle. And my engine doesn't appear to cut fuel during in-gear idle coasts, so I can't trade off zero fuel use against engine braking.

That leaves trying to feather the accelerator into the "Goldilocks region" as one other option to explore, which is all I'm doing: exploring. I'm not offering it as a recommended or even useful technique at this point; I'm too new to all of this... crawl before you walk, and all that. I just gave it as another example of where you'd have to weigh this against that to determine which approach was better for a given situation (vehicle, driver, traffic, etc.) Apologies if I muddied the waters by mentioning it at that point.

Quote:
Originally Posted by viper75
I think what you are describing is giving the engine just enough gas to overcome the effect of engine braking so that your speed at any point on the hill will be equivalent to what it would be if you were coasting in neutral.
Correct.

Quote:
...you would burn the same amount of gas by this approach as coasting down the hill in neutral with the engine revved to the higher rpm...
Seems about right. The other effects are that A) you'll glide farther than you would if you took your foot off the gas and subjected yourself to engine breaking, and B) you'll use less gas than if you keep the throttle locked. Where's the sweet spot? Dunno, but playing around with it is one way to find out.

Quote:
I would actually like to be wrong about this because I would rather coast in gear then in neutral if the FE is the same.
I don't think you're wrong. No way would you come up with the same numbers. I doubt you could even get close and that's assuming perfect control of the gas, which brucepick and I both implied would be nigh onto impossible.

If I can get half the efficiency of a NICE-ON glide I'll be pleasantly surprised. As you both said, I'm giving the engine fuel to keep it at speed the whole time, after all (even if it is unloaded, assuming a perfect rev match).

Quote:
Originally Posted by brucepick
If a driver just doesn't find sufficient control with the engine in neutral I won't disagree with their own perception... If spinning the engine is of value to the driver then OK.
For the record, I don't have that perception, and the value to this driver is not destroying either of my transmission or my nerves.

Honestly, I want to NICE-ON coast (heck, I want to EOC glide!). I've been playing with it; I love the velvety feel, I love the MPG numbers. But until I feel confident it doesn't put excessive stress on my transmission or TC, I'm not going to push it. In the meantime, I'm playing with whatever other techniques I can think of to see how close I can get to the same fuel efficiency. Maybe I'll settle on coasting in neutral at low speeds and something else on the highway; who knows at this point. Like I said, I'm new.

I hope that clarifies things a bit?

Rick
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